Chapter 1
Introduction
Neutron research and detection goes back to the earliest days of nuclear physics and
currently it is still considered one of the most interesting and exciting research fields.
Great efforts are being made to improve reactor design and new techniques are being
developed as far as neutron detection and dosimetry is concerned. More specifically, it is
vital for state of the art instruments to be constructed which will detect and measure the
energies of different particles emitted from neutron interactions with different materials
[1].
Perhaps, the most important location where neutron detection is vital is in the workplace
but this has proved to be a difficult task due to the fact that the interactions of neutrons
with tissue cannot be simulated exactly by a computer program nor be experimentally
tested with detectors in exactly the same way as they occur within the human body. There
are different types of detectors that are currently used to measure thermal and fast
neutrons including a) scintillation detectors, b)proportional counters, c) track detectors, d)
proton recoil detectors, e) detectors that use a material like hydrogen to convert fast
neutrons to thermal (moderating detectors), f) semiconductor detectors and even
combinations of different detector types [1].
1
Neutron spectrometry is a basic tool for nuclear physics experiments and dosimetry is
directly connected to it, as it deals mostly with radiation protection issues. Areas where
spectrometry and dosimetry can be of significance include nuclear power plants,
spacecrafts, nuclear waste transport, reactor decommissioning and radiotherapy
treatments [2].
Chapter 2 will discuss the ways in which a neutron can be produced through various
nuclear reactions and the different energy ranges these neutrons might have when
emitted. The equations for conservation of kinetic energy and momentum are also
illustrated as well as the importance of gamma ray discrimination in neutron detectors.
Further, in Chapter 3 the basic characteristics of various neutron detectors are mentioned
for different energy ranges, including the importance of a material called a “moderator”
which is used in some fast neutron detectors to improve their performance.
In Chapter 4 several fast and slow neutron detectors used in industry today are
discussed, and details about the experimental procedure used are also illustrated in some
cases.
Chapter 5 describes how important neutron dosimetry is in the present day especially in
the workplace, and various neutron detector experiments are discussed. The current status
of neutron dosimetry is also mentioned in the end of this chapter along with some results
from current tests made on several neutron dosimeters.
2
Finally, Chapter 6 includes the conclusion of this literature review by referring to some
of the most important properties of the detectors mentioned (their efficiency, resolution,
etc..) and the differences between them as far as their performance is concerned can be
seen more clearly.
3
Chapter 2
Neutron production and interactions
Due to the fact that neutrons are uncharged particles and do not interact via the Coulomb
force, they can travel through several centimetres of material without interacting with
other particles and can remain undetected during this process as no charge reaches the
detector. When the neutron finally comes to the point of interacting with the nuclei of the
absorbing material it can have two fates: a) it may be captured by nuclei of the absorber
or b) it may undergo a major change in its energy and direction. If the neutron is captured
then secondary radiations such as gamma rays can be emitted and subsequently produce
fast electrons, whereas if it interacts with the absorber nuclei heavy charged particles
such as proton-recoil can be detected [4].
When it comes to detecting the neutrons, the detectors measure the energy absorbed by
those secondary charged particles. Since neutrons can have a wide range of energies, the
detection methods used are different for “slow” (low energy neutrons) and “fast” (higher
energy neutrons)[4].
This chapter describes what happens when slow and fast neutrons interact with matter
and also reviews some of the basic nuclear reactions used by detectors in order to detect
the neutrons emitted. Finally the importance of gamma ray discrimination for an efficient
neutron detector will be outlined.
4
a) Slow neutrons
These are neutrons which usually have energies around 1/40eV and the detectors
mentioned in a later section of this review are especially constructed to measure neutrons
with an energy range below the cadmium cutoff of around 0.5eV. Slow neutrons undergo
elastic scattering interactions with the nuclei of the absorber and a fraction of their energy
may be transferred to the nuclei they interact with. When such interactions take place the
energy transferred to the target nucleus is around half of the primary neutron energy and
this results in the recoil nucleus having very low energy. Collisions based on elastic
scattering are typical causing the neutrons to come to an equilibrium energy with the
material of the absorber and are then classified as being neutrons of the thermal region
(thermal neutrons corresponding to 0.025eV of energy at room temperature in air).
The interactions for the detection of the slow neutrons that are important are those where
charged particles are emitted from the compound nucleus formed following neutron
capture and release a significant (few MeV) amount of kinetic energy. The products of
these particles are ionizing particles. However for thermal neutrons, due to their very low
energy, the recoiling nucleus is not typically considered to be an ionizing particle. Some
nuclear reactions used for neutron detection are discussed further in this review [4,
chapter 14].
b) Fast neutrons
These types of neutrons have energies above 1 keV [4] and the detectors used for them
are similar to the ones for slow neutrons but with some modifications due to the higher
5
energy ranges. Fast neutron detectors take advantage of the fact that an important fraction
of the neutron’s kinetic energy can be transferred to the target nucleus producing an
energetic recoil nucleus. This will behave in a similar way to a heavy charged particle,
slowly losing energy while passing through the moderator (the moderator is the material
which slows down the fast neutrons). The usual moderator material is hydrogen due to
the fact that the fast neutrons can transfer all their energy even from a single interaction
with the hydrogen nucleus (proton). This is demonstrated below by using the following
equations and in Table.1 [4, chapter 15] .
For an elastic collision by conservation of momentum and kinetic energy, the energy of
the recoil nucleus is given by
ER = [4A / (1 + A)2] (cos2θ) En
(1)
In Equation (1), angle θ is the angle between the recoil and a target nucleus, En is the
energy of the incoming neutron before it interacts with the stationary target, and A is the
mass of the target nucleus/neutron mass. When the incoming neutron makes a head-on
collision with the target, the angle between them will be 0 degrees so the equation above
will be transformed to Equation (2) as follows [4, chapter15]
ERmax = [4A / (1 + A)2] En
(2)
6
Target nucleus
A(mass
of
target/neutron
(ER/En)max = 4A/(1+A)2
mass)
1
H
1
1
2
H
2
8/9 = 0.889
3
He
3
3/4 = 0.759
4
He
4
16/25 = 0.640
12
C
12
48/169 = 0.284
16
O
16
64/289 = 0.221
Table.1 The maximum fractional energy transfer occurring when neutrons are scattered elastically
following a collision with a variety of target nuclei. [4, chapter15]
2.1 NEUTRON SPECTROMETRY
New methods for detecting neutrons are continuously presented thanks to developing
technology and new computation modelling techniques such as the Monte Carlo
simulation which allows experimental data to be compared with the theoretical
description thus giving the opportunity for different procedures to be verified and
improved. Different detectors are being tested for various ranges of neutron energies as
well as combinations of them in order to achieve better energy resolution and detection
efficiencies [2,4].
These detectors have different performances for slow and fast neutrons with some being
preferred for fast neutron detection because of their good gamma ray discrimination
7
capabilities which is one of the most important aspects a neutron detector must have
along with high detection efficiency.[2,4]
2.2 NUCLEAR REACTIONS FOR NEUTRON DETECTORS
Neutron detectors are designed based on several nuclear reactions whose products are the
ones exploited in neutron detection. However in order to design and built a detector
capable of observing neutrons several considerations must be taken into account which
rely on the specifics of the nuclear reaction involved mentioned below [4, chapter 14]
a) Cross-section, which must have a high enough value in order for the detector to be as
small as possible (large detectors are costly and difficult to construct)
b) Target nuclide, must have high isotopic abundance also for the reason mentioned
above for the cross section
c) Q-value, must also be as large as possible because this maximizes the amount of
energy transferred to the reaction products and makes the discrimination between
neutrons and gamma rays easier as will be discussed further in this review in Section
1.4
d) The range of the reaction products, since the size of the active volume of the detector
depends on this.
The nuclear reactions which are of most importance for both slow and fast detectors are
given below [4, chapter 14]
8
1.
10
B+n
10B
(n,α) reaction
7
Li + α (ground state)
Q-value = 2.792MeV
Li* + α (excited state)
Q-value = 2.310MeV
7
This reaction has a cross section of 3840 barns at an energy of 0.1eV[4, chapter 14],
which drops to a lower value when the neutron energy gets higher and also depends
on neutron velocity as it is proportional to 1/v (v is the neutron velocity). Fig.1 shows
the different values for neutron cross-sections at different energies. [3]
Fig.1 Schematic graph of neutron cross section versus neutron energy [3]
9
2.
6
6Li
(n.α) reaction
3
Li + n
H + 4α
Q-value = 4.78MeV
Cross section is 940 barns in the neutron energy range of around 0.1eV as we can see
from Fig.1. This lower value compared with the Boron reaction in the previous page is
compensated by the larger Q-value [4, chapter14].
3.
3
3He
He + n
(n.p) reaction
3
H + 1p
Q-value = 0.764Mev
Cross section here is 5330 barns at the same energy range as the previous two reactions
but the smaller Q-value poses a major drawback as well as the high cost of obtaining He3 [4, chapter14].
4. The Gadolinium Neutron capture reaction
155
Gd + n
156
Gd*
γ-spectrum
conversion electron spectrum
The products of this reaction are directly ionizing and can be exploited as neutron
converters to produce fast electrons (specifically the 72keV conversion electron is the
10
most important one as it is produced in 39% of capture reactions [4, chapter 14]) in order
for the detector to respond to them. Here the cross section has a very large value of
255000 barns which makes it a good alternative for neutron detection [4, chapter 14].
5. Neutron Induced fission reactions
235
U+n
fission fragments + ~ 200MeV (Q-value)
Here as we can deduce from the very large Q-value excellent gamma ray discrimination
can be accomplished [4, chapter 14].
2.3 GAMMA-RAY DISCRIMINATION
An important feature of a neutron detector would be its good neutron/gamma
discrimination capabilities. When the neutron flux is low then discrimination does not
present a significant problem since one can arrange the appropriate pulse-shape time
constants from the electronics connected to the detector. If however, the neutron flux is
high, then pile-up effects can take place which will distort the true neutron spectrum and
it can become difficult to discriminate between gamma rays and high energy neutrons [5].
A large Q-value can help to separate fast (high energy) neutrons from gamma radiation
coming either from the source itself or the detector’s surroundings as was discussed in
Section 2.2. These properties differ between different types of detectors and will be
mentioned further in this report [4, chapter 14].
11
Chapter 3
3.1 SLOW NEUTRON DETECTORS
In this chapter I mention some of the basic characteristics of several slow and fast
neutron detectors that are used in industry today which are based on the nuclear reactions
described in the previous chapter. I also explain why they are used only in certain energy
regions as well as why some fast neutron detectors need an additional component in their
system called a “moderator” to convert high energy neutrons to thermal neutrons.
3.1.1 BF3 proportional counter
This type of detector consists of a substance in gas form called Boron Trifluoride. Due to
its high concentration in
10
B(96%)[5], as well as its double role as target and neutron
converter it is one of the most common types of slow neutron detectors as its efficiency is
around 91% for thermal neutrons with energies close to 0.025eV(for 30cm tube length
and pressure of 600torr). Unfortunately for energies above 100eV the efficiency
decreases to 3.8%[5]. The BF3 tube must be constructed with quite large dimensions so
that all the reactions occur at a far distance from the detector’s walls. If the reaction
products (such as the alpha particles), reach the detector walls then a pulse will appear in
the spectrum due to the so called “wall effect” [5].
Since the α and 7Li recoil particles move in opposite directions, if one of them hits the
detector wall the other will deposit its energy in the detector volume. Therefore it is clear
that the energy spectrum obtained from such a detector will depend on the size and
12
design of the BF3 volume and from a large size is preferred to obtain better results and a
stable operating point can be reached [5].
3.1.2 Lithium scintillator
More commonly, LiI (Lithium Iodide) scintillators are used for slow neutron
spectroscopy due to the fact that they are similar in chemical composition to NaI (Sodium
Iodide) and therefore exhibit high light output (around 35%). Moreover, 6Li has a large
Q-value which plays an important role in gamma ray discrimination and does not exhibit
the wall effect because the distances the particles travel are very short in comparison with
the size if the LiI crystal [6]. Finally, due to the fact that the LiI crystal is sensitive to
water vapor it must be sealed in a canning material for its protection [4, chapter 14].
3.1.3 3He proportional counter
Due to its higher value for its cross-section (5330 barns) [4, chapter 14] 3He can be used
instead of the BF3 (3840 barns) [4, chapter 14] detector for slow neutron observation. In
order to overcome the problem posed by the wall effect, one solution could be to make
the detector’s dimensions as large as possible, or even increase the pressure of the 3He
gas, which would consequently decrease the distance that the emitted charged particles
(3He and protons) will travel in the tube [7]. This is one of the most important
considerations for using this detector over the BF3, because the boron detector cannot be
operated at pressures higher than 0.5-1.0atm due to the poor gas performance of at higher
pressures [4, chapter 14].
13
Unfortunately due to the low Q-value of the 3He reaction, gamma ray discrimination
poses a significant problem for slow neutron detection which could be overcome by using
an additional gas to reduce the pile-up effects (e.g. CO2, Ar) [8].
3.1.4 Fission counters
A very important advantage of the fission induced reactions is the very high Q-value
(200MeV) which results in very low backgrounds, meaning that these detectors have
excellent neutron-γ discrimination capabilities. Fission counters are usually constructed
in the form of ionization chambers, with the deposition of some fissile material inside.
This often consists of a backing material which is placed at the opposite sides of a dual
chamber and helps in the detection of the fission fragments which move in opposite
directions. The size of the fission counter does not have to be very large because the
fragments can travel only half the distances 5MeV alpha particles can [9]
3.2 FAST NEUTRON DETECTORS
As we go higher in the neutron energy range, the probability that a neutron will come in
contact with the reaction components mentioned in previous section of this review,
becomes smaller. In this case, a material that will slow down (or “moderate”) the fast
neutrons is needed in order for our detector to be of useful efficiency. Hydrogen is often
used for this purpose (of slowing down neutrons), and fast neutrons undergo elastic
scattering while they are being slowed down.
14
The hydrogenous material used to slow down neutrons is called a moderator and usually
is several centimeters thickness and surrounds the detector. However one must be
extremely cautious about the moderator thickness due to the fact that neutrons may
sometimes be completely stopped inside the moderating material and not penetrate the
detector’s active volume. For neutrons in the intermediate energy range (keV) a couple
centimeters moderator thickness is required, but for higher ranges (MeV) it should be a
lot more (close to tens of centimeters) when the moderator material is polyethylene or
paraffin [4, chapter 15].
3.2.1 Fast neutron detectors using neutron converters
Bonner sphere
Bonner was the first to observe the effects that a small LiI scintillator had when placed in
the centre of polyethylene spheres of various diameters. Due To the fact that LiI has a
large Q-value (4.78MeV), its gamma ray discrimination capabilities are very good even
in very intense gamma environments and can therefore be used to measure neutrons at
energies of several MeV [10]. More information about the Bonner sphere is discussed in
Section 4.2.2. Moreover a 3He counter can also be used to make the whole detector
system less sensitive to gamma rays and can be surrounded by a layer of cadmium which
will improve the detector’s response to high energy neutrons as being a high Z element.
This configuration though, is not very efficient for energies in the MeV range [11].
15
Thermal neutron detector
Moderator
Figure.2 (This figure is modified from Ref.4 chapter 15, page 539) It shows two Bonner spheres
containing thermal neutron detectors in their centre and surrounded by moderating material.
3.2.2. Fast neutron detectors based on neutron induced reactions
There are several detectors which do not need neutron converters but rely their operation
on reactions such as 6Li (n,a) and 3He(n,p) and fast neutron scattering. First of all, some
of the detectors based on lithium reaction are: glass scintillators[12,13,14,15,16,17,18],
LiI scintillators [19,20] and sandwich spectrometers [21,22,23,24]. Lithium glass
scintillators are mostly used for time of flight measurements because of their relatively
fast detector response and easy construction over large areas. However, their light output
is quite low and that restricts their gamma discrimination capabilities. Additionally, the
sandwich spectrometer uses a material like LiF which is placed between two
semiconductor detectors and the reaction products are detected as they are moving in
16
opposite directions. Unfortunately, problems nay be caused by the energy lost due to the
foil thicknesses leading to a reduction in the detector’s efficiency [4, chapter 15].
Detectors that are based in the 3He reaction are: 3He proportional counters [25,26,27],
ionization chambers [28,29], which in fast neutron spectroscopy are considered to have
better resolution than other commonly used detectors due to their more improved pulse
height distribution. Moreover, in a 3He scintillator the purity of the gas is very important
if one wishes to obtain a good light output from this detector.
Finally, there are neutron detectors which rely on a procedure called fast neutron
scattering. Here the neutron is detected after it collides with light nuclei (such as
hydrogen) and a recoil nucleus results usually in a recoil proton. Such detectors are called
proton recoil scintillators and materials containing hydrogen such as anthracene and
stilbene are typically used. Stilbene has excellent discrimination properties and it is more
commonly used over anthracene but both crystals are very fragile and difficult to obtain
which makes them very expensive. So, mostly preferred in this case are plastic and liquid
scintillators which are far cheaper and can be easily made in large sizes and are discussed
in Section 4.2 in this review. [4, chapter 15]
17
Chapter 4
4.1 THERMAL NEUTRON DETECTORS
In this chapter I review the usefulness of several thermal and fast neutron detectors based
on experiments conducted the last years. I go into more detail about what sort of material
they include and what type of technical components they consist as well as their response
to certain energy regions. All the experiments described were proved successful and most
of these detectors are currently used in industry
According to the work of Oed [35], thermal neutrons are used for the analysis of the
structure of materials. However, the flux of neutrons from many sources can be quite
weak, so when trying to detect neutrons the beam aimed at the neutron source must have
a certain angle in order to obtain high intensity from the samples used. Unfortunately, this
fact leads to interference patterns so the detector must be placed at a certain distance from
the source, and consequently must have a large active area in order to achieve as high
efficiency as possible [35].
Reaction
Light fragment (l.fr)
Energy(MeV)
Heavy fragment(h.fr)
Energy(MeV)
n(3He,p)3He
p
0.57
3
0.19
n(6Li,α)3He
3
2.74
α
2.05
n(10B,α)7Li+γ
α
1.47
7
0.83
n(10B,α)
α
1.77
7
Li
1.01
n(235U,l.fr)h.fr
l.fr
<80
h.fr
<60
N(157Gd,Gd)e-
Conversion electron
0.07-0.182
He
He
Li
Table.2 Nuclear reactions used as a base for thermal neutron detectors (table taken from ref. 35)
18
Thermal neutrons can be measured through certain nuclear reactions (see Table2) which
lead to the subsequent emission of gamma rays, protons; alpha particles etc. Due to the
high velocities of these emitted particles the absorption material must be of a certain
thickness. For example, a neutron velocity of 2200m/sec requires 7.4cm thickness in 3He
detector. There is a wide range of different detector types used nowadays for thermal
neutron detection and some of the most common used in industry are described below
[35].
І GAS DETECTORS
In such detectors low Z elements such as Carbon (Z=6), Hydrogen (Z=1), Fluoride (Z=9)
insensitive to gamma rays are very important. Gases with a relatively high molecular
weight play the role of the stopping gas due to their high stopping power. Neutron
detectors must also have good energy resolution so that thermal neutron peaks will be
well separated ideally from the gamma ray background [35].
4.1.1 Cylindrical ionisation chamber filled with high pressure Xe+ 3He
gas mixture
In the work of Bolozdynya, Richards and other scientists [36] an ionisation chamber of
cylindrical geometry was used and filled with 0.35g/cm3 density Xe+3% 3He gas mixture
and detected simultaneously both gamma rays and neutrons. The detector was surrounded
by a 5cm thickness lead shield to protect it from background radiation and the source
used to irradiate the detector was an AmBe source [36]. The 511keV peak obtained when
19
using this source at a gas density of 0.07-0.35g/cm3 was of very similar width as the one
shown in Fig.3 which was obtained by using a 22Na source. This meant that the peak was
relatively unaffected by the gas density changes in the chamber.
Due to its very large cross-section (5333 barns) [4, chapter 14] for thermal neutron
absorption 3He is highly appropriate for detecting neutrons when combined with a noble
gas such as Xe. Unfortunately due to its cost it is relatively difficult to construct detectors
with large dimensions [36].
Fig.3 Pulse height spectrum displayed after a 22Na source was placed 50 cm from the detector and the
511keV peak width shows that’s its independent of gas density as described in reference 36.
In this experiment a detection efficiency of 85% was achieved, which was the highest
efficiency compared to other experiments in this review. [36]
20
4.1.2 Micro strip gas chamber (MSCG)
An experiment performed by Velletaz , Assaf and Oed [37] is the following one, in which
a two dimensional neutron detector containing a Micro Strip plate was used and based on
the nuclear reaction
3
He + 1n
3
H + 1H + 764keV
The triton and proton particles were emitted isotropically in the centre of mass and in the
opposite directions (to conserve linear momentum). They had different track lengths due
to their differing charges masses and kinetic energies. A very common error of detecting
the centre of gravity instead of the centre of the reaction was observed while performing
calculations, and in order to avoid it they tried to decrease the track lengths by using
some sort of gas. The gas used was 3He at a pressure of 3 bars and CF4 at a pressure of
1.5 bar [37] because it had high stopping power and also was composed by light elements
which lead to the fact that X-rays and gamma rays were not detected because of the low
absorption coefficient. The detection efficiency achieved in this particular experiment
was 46% for thermal neutron energies [37].
Џ SCINTILLATION DETECTORS
4.1.3 The Resonant Detector
In the work performed by Gorini and other researchers [38] a resonant detector was used
which consisted of a neutron analyzer foil which absorbed the scattered neutrons of
energy ranges close to the resonance energy and a photon counter which detected the
21
gamma rays emitted. For the foil the isotope used was
238
U because it gave out only a
limited number of strong absorption resonances. On the other hand, the type of photon
detector used was taken into more consideration, as a detector with low sensitivity to
gamma rays was preferred.
Different photon detectors had been used for this purpose such as NaI [39] which was
efficient but very sensitive to background radiation. Another detector used was the
Cadmium Zinc Telluride (CZT) semiconductor detector which has low leakage current
[40], and reasonable energy resolution (3-5% for Eγ=100keV) [41].
The major drawback of the CZT was that it gave low count rates due to its small size
even when no shielding was used [42], but it was quite sensitive to neutrons and had
good signal/background ratio [43]. So the RD is mostly used for low energy neutrons in
the range of 1-10eV but with some adjustments it can also be used for higher neutron
energies [38].
4.1.4 Silicon detectors
The design of silicon detectors permits the placement of material upon the face of the
detector itself which is an important feature that other detectors unfortunately do not
have. A slow neutron detector based on sol-gel glass doped on 6Li, 10B and 235U is a good
indication of the capabilities of this type of detectors and it is described in the work of
Wallace, Hiller and other scientists [44]. The Silicon Surface Barrier Detector measured
22
the thermal neutrons produced by a NUMEC PuBe source which was placed at the centre
of a moderated shielded container. [44]
Fig.4 Thermal neutron count rate by using the 10B doped sol-gel glass
Fig.5 Thermal neutron count rate by using the 6Li doped sol-gel glass.
The thermal cross-section of
10
B is around four times more that of 6Li as mentioned in
Section 2.2 previously. However, when the glass film used in this experiment was doped
with 6Li it had four times the count rate of the glass film doped with
10
B, meaning its
efficiency was much better. This occurred because of the detector’s abilities to detect the
products coming from the 6Li and
10
B nuclear reactions as mentioned in Section 2.2 of
this review. When 6Li absorbs a neutron, a triton and an alpha particle are emitted but
23
the10B reaction produces an alpha particle and a 7Li ion which has a shorter range from
the rest of the particles and therefore it is not able to reach the detector’s active volume
most of the time [45.] Figures 4 and 5 in the previous page show the difference in the
counts per second for both 10B and 6Li as obtained while performing this experiment [44].
4.1.5 Resistive plate chambers
These types of detectors are cheap, easy to apply at wide surfaces and easy to use in order
to detect neutrons for high energy ranges. In the experiment conducted by Abbrescia and
other researchers [46] it was considered that thermal neutrons could not be measured
directly with an RPC (Resistive Plate Chamber) because they were uncharged particles.
So a Gadolinium (Gd) converter was used in this procedure which consisted of 157Gd and
155
Gd. The two energy ranges measured by these researchers in this particular experiment
were 1eV-10eV and 11MeV-200MeV.
Also the efficiency to thermal neutrons of the RPCs was measured and found to be
around 10% [46]. Moreover, instead of Gd, Arnaldi and fellow scientists [47], used
Boron as a neutron converter which was preferred over Li due to its chemical properties,
and Monte Carlo simulations were conducted to verify the results obtained by this
experiment which showed that the RPCs can successfully be used as thermal neutron
detectors as the efficiency was found to be around 8% [47] which was quite similar to the
efficiency found before [47].
24
4.1.6 Inorganic thermal neutron scintillators
According to the work of Eijk, Bessiere and Dorenbos[48], combinations of different
inorganic scintillation detectors can also be used for thermal neutron detection based on
the nuclear reactions mentioned in Section 2.2 of this review. By combining inorganic
elements of low atomic number, these detectors are inefficient in detecting gamma rays
and very efficient for thermal neutron detection [48].
Some of the standard thermal neutron scintillators used in industry, are shown in the
Table.3 below taken from reference [48]
Host
Dopant (conc mol %)
Light yield photons per
Neutron
MeV gamma
~6000
~4000
6
Li glass
Ce
6
LiI
Eu
50000
12000
6
LiF/ZnS
Ag
160000
75000
LiBaF3
Ce,K
3500
5000
6
Ce
40000
25000
Cs6LiYCl6
Ce (0.1)
70000
22000
Cs6LiYBr6
Ce (1)
88000
23000
LidepGd(11BO3)3
Table.3 Thermal neutron scintillarors most commonly used and under test combinations [48]
LiF/ZnS:Ag is considered to have very good gamma ray discrimination properties, which
is a fact that has come to be one of the most important properties of a neutron detector,
despite its low efficiency [49,50,51]. Mostly 6Li and 10B based scintillators receive more
attention than the Gd detectors due to the fact that the Gd detector has very low efficiency
25
and lacks good discrimination capabilities. On the other hand, Ce-doped scintillators are
considered to have potential for fast response and good light yield [52].
A detector that contains a combinations of interesting materials with useful properties has
also been designed, namely the 6LidepGd (11BO3)3 :Ce scintillator. The use of 6Li offers a
high signal response and the compound can be quite efficient where light transport is
concerned due to its low refraction index of 1.66. This detector is often used when large
areas need to be covered by using a thin layer of scintillation [53]. Another scintillation
detector with good pulse shape discrimination is the Cs6LiYCl6 :Ce [54]. This is based
again on Ce-doped material but has the drawback that the material is hygroscopic and
that neutrons are absorbed both by Cs and Cl [54].
Until now the LiF/ZnS:Ag based scintillators are most frequently used for thermal
neutron detection but the other detectors also described above are still in experimental
stages and show great promise in the field of thermal neutron detection [48].
4.1.7 Thermoluminescence dosimeters
Another method for detecting neutrons was also tested by Hector Rene Vega-Corillo who
used two TLDs in a Bonner Sphere Spectrometer (BSS) along with a 6LiI(Eu) scintillator
and a 252Cf source [55] By using the BSS system wide energy ranges were measured and
the 6Li cross section combined with the detector’s small active volume made the whole
system very good for gamma ray discrimination. However, despite the BSSs advantages
concerning neutron detection there were several drawbacks including low energy
26
resolution, long time measurements and limitations while operating the Bonner Sphere
when it comes to strong neutron fields and different procedures were used in order to
overcome them [56,57,58].
Neutron source BSS with
Bare 252Cf
6
LiI(Eu)
D2O moderated 252Cf
TLD pairs
6
LiI(Eu)
TLD pairs
Neutron Flux (cm-2s-1)
244
247
219
218
Average neutron energy(MeV)
1.8
1.8
0.5
0.5
Neutron dose rate(Gy/s)
7.52x10-9
7.30x10-9
2.55x10-9
2.43x10-9
Neutron dose rate equivalent
6.83x10-8
6.59x10-8
1.68x10-8
1.56x10-8
(Sv/s)
Table.4 Neutron sources parameters calculated with the unfolded spectra. Table taken from ref [55]
By using the two TLDs in combination with the BSS some of the problems mentioned
can be overcome. The TLDs had a high content of 6Li and responded the same to gamma
rays. By using two different sources measurements were taken by using the TLDs and the
scintillator. The results distributed in Table.3 shows several parameters calculated from
the spectra obtained. Based on results from NCRP (1991) the
252
Cf average neutron
energy was 2MeV and the dose equivalent 6.8x10-8 Sv. Moreover, the D2O moderated
252
Cf had an average neutron energy of 0.4-0.6MeV and dose equivalent of 1.6x10-8Sv. If
we look at Table.3 we can see that the values from NCRP and the results from the
experiment for the same parameters are very similar. [55]
27
4.1.8 Silicon pin photodiode detector
In this experiment by Voythcer and his colleagues [59], a silicon PIN photodiode
detector was used as well as different thicknesses of a 6LiF neutron converter. The
detector was irradiated by an AmBe source and the background due to gamma radiation
was close to 1% due to the thin depletion layer (200-500µm) of the photodiode which
was thick enough to collect all the electron-hole pairs created by the charged particles
[59].
Fig.6 Experimental apparatus taken from ref [59]
As mentioned, different thicknesses for the neutron converter were used and results for
several distances were obtained. Fig.6 shows a schematic diagram of the apparatus used
and after all the experimental results were estimated, Monte Carlo simulations were
performed to check the validity of the experimental procedure. Fig 7, 8 show the results
with both Monte Carlo and experimental methods which proved to be in good agreement
and the optimal converter thickness was found to be 1-2µm [59].
28
Fig.7 Spectrum obtained by simulations when using 6LiF neutron converter taken from ref [59]
Fig.8 Measured experimental spectrum with 6LiF converter taken from ref [59]
4.2 FAST NEUTRON DETECTORS
4.2.1 Fibre-array neutron detector
In the experiment conducted by Zhang and other associates [60] a scintillating fibre array
detector was used to observe neutrons of energies of 2.5MeV and 14MeV. The fibrearray fast neutron detector showed also some exceptional properties such as a) high
sensitivity to gamma rays and b) a distinct peak at 14MeV in the spectrum obtained. The
29
detector apparatus is shown in Figure 9 .The detector was used both in current and pulse
mode and each mode has its advantages and disadvantages.
In pulse mode some counts were lost in lower channels but this did not occur in current
mode. However in pulse mode the individual pulses carried information which was not
seen in current mode. .
Fig.9 Schematic diagram of the experimental apparatus used in experiment taken from ref [60]
In general the fibre-array detector was successful in detecting fast neutrons at the
energies measured and had the advantage of one being able to use it both in current and
pulse mode which is quite unlikely to happen for other detectors [60].
30
4.2.2 Bonner sphere spectrometer
This spectrometer has been used frequently as a neutron detector due to its detection
capability over a wide energy range from thermal to fast neutrons. It uses a thermal
neutron sensor located at its centre which can give good gamma ray discrimination and
the sphere itself contains a moderating material such as water, which results in high
neutron sensitivity. These spheres vary in sizes according to the neutron energies to be
measured. For example for low energies not much moderation is needed so the sphere
can be small but for higher energies where the neutrons have a high escape probability,
the sphere must have a larger area rich and subsequently more moderation [61].
Different thermal neutron sensors can be used such as 6LiI(Eu) which responds well to
thermal neutrons but has discrimination problems when the gamma ray fluence becomes
greater than the neutron fluence [10] If larger crystals are used then the analysis of the
light output spectrum becomes more complicated so the LiI scintillator must be
connected to a PMT (PhotoMultiplier Tube) which can cause response problems due to
the light pipe’s connection [61] An alternative for LiI is a 3He proportional counter which
has excellent discrimination capability as well as relatively good sensitivity for neutrons
and can be more easily built in large sizes when better efficiency is needed.
Experimental results from Bonner spheres have been compared with Monte Carlo
simulations and the outcome by comparing the two methods has been quite satisfactory.
Continuing efforts are being made to improve the neutron energy resolution of the
spheres which will result in fewer errors while making measurements [61].
31
4.2.3 NE-213 Liquid scintillation detector
In the work of Davani and other researchers [62], an NE-213 detection system was used
for a neutron range of 1-30MeV as well as an AmBe neutron source. This system had
excellent (n,γ) discrimination properties and the factors that were taken in mind when
designing this detector were
a) efficiency
b) resolution
c) discrimination of neutrons and gamma rays
The resolution was considered one of the most important factors in this experiment and
the other two were based according to this. The two aspects that were recognized to affect
the performance of the resolution of the detector after several tests were made, were the
cell volume and the transport of photons inside the cell itself. It was observed that if the
detector’s volume was increased it would decrease the number of secondary electrons
produced leading to the wall effect, but at the same time it would increase the number of
scatterings within the detector. [62]
The NE-213 detector volume also determined its efficiency so an optimum size had to be
selected due to the fact that the resolution would be affected. Nevertheless, the detector
built in this experiment had a very good performance when detecting neutrons in the
ranges mentioned and the results were also tested with simulations in order to be verified.
32
This detector is now used for routine measurements in both detection as well as
dosimetry [62].
4.2.4 Solid state nuclear track detectors
Low fluences of fast neutrons were attempted to be measured by Lengar and colleagues
[63], who used a pair of CR-39 detector foils. By placing these two detectors very close
together and counting the coincidence tracks in the foils the background appearing while
measuring the fluences of the fast neutrons was indeed greatly reduced in comparison
with other tests made when the foils were not placed too close to each other. It was
observed that these kinds of detectors were perfect for low neutron flux measurements
but had the disadvantage of showing large backgrounds. This drawback was overcome by
placing the detectors very close to each other and observing the tracks made by the recoil
protons. The tracks taken into account were the ones found at the same point on the foil
surfaces consequently the ones left by the same nuclei [63].
Fig.10 Schematic diagram showing the two CR-39 detector foils in contact and recoil nuclear track
taken from ref [63]
33
The experimental apparatus is shown in Fig.10. The detector foils were pressed tightly
together to avoid having any air between them, and the tracks chosen as appropriate to
measure were the ones with a 15µm distance between them because of the angle of the
incident nuclei, the position of the microscope used, and the distinct shape and
orientation of the coincidence tracks. The rest of the tracks not chosen had an elliptical
shape and far from equal ellipse orientation. The experiment was proven successful as the
background was greatly reduced and these type of detectors have shown great potential
for fast neutron detection [63].
4.2.5 Semiconductor Germanium Detectors
An effort was made by Fehrenbacher and associates [64] to use a Germanium detector
both as a gamma spectrometer as well as a neutron monitor. The source they used was an
AmBe source and the main peak considered was the 692keV peak. The detector was
situated behind a lead shield of 5 centimetres thickness and results were taken for 80
different neutron energies [64]
By looking at the spectrum obtained by a combined photon and neutron environment they
were able to distinguish the points in the spectrum which were due to the neutron energy
contribution. Specifically, when taking into account fast neutron contribution, a
characteristic peak was shown which was broadened on its right side due to the process
of neutron inelastic scattering in the Germanium crystal [64].
34
Fig.11 Pulse height spectra for different energies showing the various shape of the 693keV peak. The
line represents the computational results and the points display the experimental measurements.
Figure.11 was taken from ref [64]
The results obtained by observing 80 different neutron energies from 0.7MeV to 6MeV
are shown in Fig.11 and were also verified with MC simulations.
As shown in the figure, for energies from 0.88-1.54MeV there is good agreement in the
computed and measured detector response. Above 1.54MeV the difference between the
two methods may be caused by the fact that neutrons may have undergone scattering in
the lab the experiment was performed, and this fact was not accounted for in the MC
simulation process [64]
35
Fig.12 Experimental apparatus taken from ref [65].
A similar experiment was also conducted by the same researchers to see the detector’s
response to fission neutrons by using this time a
252
Cf source. A 2 centimetre lead shield
was chosen this time because the efficiency of the detector might have been reduced
greatly if a thicker shield was used [65].
Fig.13 Detection efficiency of Germanium detector as obtained by MC simulations. The solid line
shows the efficiency of the detector while it was unshielded. Figure taken from ref [65]
36
The apparatus is shown in Fig.12 and the results obtained by the Monte Carlo
calculations, with and without the shield in use are shown in Fig.13. These show that by
using the shield the efficiency increased by 10%. Finally, as we can see in Fig.14
experimental results were in good agreement with the MC estimations.
Fig.14 Comparison of the 692keV peak response resulting from irradiation with neutrons from a
252Cf source. (line) and response from the spectral distribution (histogram) taken from ref [65].
By taking into account both of these experiments we can conclude that the Germanium
detector can be used to detect fast neutrons for energies 0.8-1.5MeV [65].
37
Chapter 5
Neutron Dosimetry
When it comes to measuring the dose delivered to the tissue by incident neutrons
different methods and dosimeters maybe used. For thermal neutrons and gamma radiation
an accurate discrimination can be achieved, but when it comes to high energy neutrons it
is somewhat more difficult to achieve a satisfactory separation scheme due to their higher
ranges, since the directly ionizing particles and secondary particles produced overlap
each other [66].
The energy deposited by a thermal neutron to the tissue is around half its initial energy
however for neutrons with energies above 10MeV, oxygen and carbon atoms maybe
produced as recoil particles which have much higher ranges than the recoil nuclei from
thermal neutrons. Based on this fact, when it comes to detecting such particles and
estimating the dose one must take into account the cross sections of the materials that the
detector is made of, as well as the tissue involved. In addition to this, the detector’s
specific composition must also be known for the results to be manipulated in the best
possible way. [66]
In this chapter I review some of the detectors used for slow and fast neutron dosimetry
and their technical components as well as their effectiveness to a wide energy range.
Finally I describe the current status that neutron dosimetry is today and the recent test
results from some of the most commonly used dosimeters.
38
5.1 Proportional counter
Two detectors were used in this particular attempt by Brady and Badhwar [66] to create a
neutron dosimeter. One detector had an atomic composition similar to the tissue [67]
(hydrogen-free), and the other was made of a material resembling the tissue. Their
gamma ray response was measured to be almost the same with the hydrogen-free detector
which was not able to observe neutrons below 10MeV. The tissue equivalent detector
was proved to be sensitive to recoil protons produced by the incident neutrons [66]
It was assumed by these researchers that an estimate of the dose could be made by
comparing the two sets of results from the two detectors and a dose estimate from higher
energy neutrons would require further considerations as far as the detectors elemental
composition was concerned [66].
5.2 Germanium detector
A germanium detector has also proved useful as a neutrons dosimeter, when surrounded
by a polyethylene moderator, in low neutron fields according to the work of Chao and
Niu [68]. Different sizes for the moderator were used in order to find the optimum one
and a
252
Cf source was placed in front of the detector. This particular source gives a
neutron spectrum similar to that expected from environmental neutrons (such as those
coming from outside the earth’s atmosphere) as shown in Fig.16. The experimental
apparatus is shown schematically in Fig.15. Measurements were taken under while the
source was bare and also when shielded with lead or moderated with a graphite material.
39
Fig.15 shows the experimental spectra observed by Chao and Niu from ref [68]
The most efficient of all these conditions for the measurement of fast neutrons, was the
system which was moderated. The main peak chosen to be observed throughout the
whole experiment was the one at energy of 596keV in the Ge(nth,γ) reaction. This was
shown clearly over the mixed n/γ spectrum and thus the germanium detector proved to be
relatively efficient for measurements in low neutron fields [68].
40
Fig.16 Spectrum obtained with apparatus from Fig.15. [23] The top diagram shows the spectra from
252
Cf. The bottom spectrum shows the measurements with environmental neutrons [68].
5.3 CRS neutron dosimeter
Lounis and other researchers [69] constructed a dosimeter capable to respond to fast
neutrons, which contained two CR-39 detectors, which were electrochemically etched
and irradiated with an AmBe and
252
Cf source. The main advantage of this type of
detectors is that they are insensitive to gamma rays at very low neutron energies, and
their cost was relatively low as well. Fig.17 shows their response to personal dose
equivalent [Hp(10)] for an energy range of 25keV to 66MeV as reported by Lounis and
his associates. The fitted line to the data is close to being linear which means that these
detectors can be reasonably efficient over a wide energy range [69].
41
When the detectors were irradiated under free-in-air conditions by 0.3-13mSv ambient
dose equivalent the results in Fig.18 showed that their response was relatively linear for
doses above 1mSv whereas, below this value the response was not very satisfactory
because of the very high backgrounds coming from the detectors surroundings [69].
Fig.17 Personal dose equivalent response of the dosimeter as function of incident neutron energy [68]
42
Fig.10 Dose equivalent versus ambient dose equivalent for neutron energies 1.2MeV, 5.3MeV, and
15.1MeV. The Figure is taken from ref [69]
5.4 Semiconductor detector
Sasaki and associates [70] created a semiconductor dosimeter to record the dose from
both slow and fast neutrons by using two p-type silicon semiconductor detectors with the
detector measuring slow neutrons containing boron to produce the necessary recoil
protons [70.] The energies measured were from 8.1keV to 22MeV and the results were
proven satisfactory. The dosimeter’s design is shown schematically in Fig.20
43
Fig.20 Diagrams of the slow and fast neutron detectors for personal dosimetry as reported by Sasaki
and associates in ref [70].
The slow neutron detector could observe neutrons with energies lower than 1MeV and
the fast one above this value. The silicon detectors also had very good discrimination
capabilities as the gamma radiation contribution was below 1% due to the appropriate
detector thicknesses. Their response to the measured neutron fields is shown in Fig.21
44
and was also confirmed by Monte Carlo simulations. These detectors are currently in use
in many neutron exposed facilities [70].
Fig.21 Si semiconductor dosimeter response to ambient dose equivalent. Figure taken from ref [70]
5.5 Current status of electronic personal dosimeters
In the work of Errico and other scientists [71] it has been proved that nowadays neutron
dosimeters in particular are not precise enough to follow the newly published radiation
safety procedures. Examples of their inadequacy come from reactor decommissioning
procedures, service attempt to nuclear power plants etc [71]. Some of the commercially
available dosimeters are displayed in Table.4.
45
Manufacturer
Type of sensor
Size (mm3)
Weight (g)
Dose range
One silicon detector,n
30x145x12
70
10µSv-0.1Sv
55x102x14.5
110
100µSv-1Sv
70x130x25
<200
1µSv-1Sv
63x85x19
110
10µSv-16Sv
and model
Aloka PDM-313
sensitive
Fuji electric EPD
Four silicon detectors
(NRN)
n/γ sensitive
Saphymo
One
Saphydose-n
detector, n sensitive
Siemens EPD-N
Three silicon detectors,
silicon
strip
n/γ sensitive
Table.4 taken from ref [71] and shows the different characteristics of several personal dosimeters
used in industry.
Neutron dosimeters are hard to construct due to their complexity of the neutrons
interactions over a wide range of energies with various materials. The dosimeters shown
in the Table.4 have been tested in laboratory environments and the results proved to be
quite surprising. For example the Fuji electric EPD was examined and proved to be
measuring fewer neutrons by a factor of 100 and the Aloka PDM-313 was proved to be
very inaccurate in the epithermal neutron energy region. Siemens EPD-N is only capable
of detecting thermal neutrons efficiently whereas for fast neutrons its estimates are off by
two orders of magnitude. However, the Saphydose-n showed that it responded very well
over a wide energy range but the silicon material it was made of proved to be quite
fragile. [71]
46
Chapter 6
Conclusion
This literature review examined the different detectors used for fast and slow neutron
spectroscopy. Slow neutron detectors proved to be far more efficient than fast neutron
detectors due to the fact that in lower energy regions, no additional material need to be
used to decrease the neutron energy whereas in fast neutron detectors a moderating
material has to be used to slow down the neutrons which at the same time may result in
limiting the detectors efficiency as it may completely stop the recoil particles from
reaching the detector.
Also the detector must be placed at an appropriate angle to the neutron source as the
recoil nuclei are emitted in different directions and at various angles after interacting with
the target nucleus. Finally a very important feature of both slow and fast neutron
detectors is their gamma ray discrimination capabilities that the detectors must have in
order to be able to distinguish between neutron peaks and gamma ray backgrounds.
For thermal neutron detectors, the cylindrical ionization chamber mentioned in Section
4.1.1 achieved the highest efficiency to thermal neutrons (85%) in comparison with the
rest of slow neutron detectors and had good discrimination capabilities at a certain gas
density. In Section 4.1.2 the Micro-Strip gas chamber proved to be very efficient too
(46%) and the Resonant detector described in Section 4.1.3 had a relatively good energy
resolution (3-5%) The Resistive Plate Chamber proved suitable for both fast and slow
neutron spectroscopy in Section 4.1.5 (with efficiency of 8-10% for thermal neutrons)
47
However, Thermoluminescence Dosimeters (TLDs) proved to have a low energy
resolution, one had to measure for a long time in order to be able to obtain satisfactory
results and TLDs were not able to operate in strong neutron fields (Section 4.1.7). On the
other hand the Silicon Pin Photodiode detector described in Section 4.1.8 had very low
backgrounds of 1%.
For fast neutron detectors the Fibre-Array detector (Section 4.2.1) could be operated in
both current and pulse mode which was a feature that no other detector had in this whole
review and the Bonner Sphere Spectrometer examined in Section 4.2.2 was efficient over
a wide energy range and had good neutron/gamma discrimination properties. Finally,
Solid State nuclear track detectors were able to achieve low backgrounds even in high
energy ranges (Section 4.2.4)
Another part on nuclear physics described in this literature review was neutron dosimetry
and the type of detectors that would prove more efficient in this particular field. It is more
difficult to choose detectors for measuring the neutron dose because the interactions of
neutrons with human tissue cannot be detected or even tested in a very accurate way due
to their complexity. Detectors such as proportional counters as discussed in Section 5.1
had a similar atomic number with the human tissue and showed some good results.
Germanium detectors (Section5.2) and CR-39 track detectors (Section 5.3) proved
efficient for low energy neutron fields. For higher energy regions of 8.1-22MeV,
semiconductor detectors (Section 5.4) distributed a background of less than 1%.
48
Dosemetrs in general are hard to construct and their capabilities may sometimes be
overestimated as was shown in Section 5.5
49
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