Lecture 7
Neutron Detection
22.106 Neutron Interactions and Applications
Spring 2010
Types of detectors
• Gas-filled detectors consist of a volume of gas
between two electrodes
• In scintillation detectors, the interaction of
ionizing radiation produces UV and/or visible
light
• Semiconductor detectors are especially pure
crystals of silicon, germanium, or other materials
to which trace amounts of impurity atoms have
been added so that they act as diodes
Types of detectors (cont.)
• Detectors may also be classified by the type of
information produced:
– Detectors, such as Geiger-Mueller (GM) detectors,
that indicate the number of interactions occurring in
the detector are called counters
– Detectors that yield information about the energy
distribution of the incident radiation, such as NaI
scintillation detectors, are called spectrometers
– Detectors that indicate the net amount of energy
deposited in the detector by multiple interactions are
called dosimeters
Modes of operation
• In pulse mode, the signal from each
interaction is processed individually
• In current mode, the electrical signals from
individual interactions are averaged
together, forming a net current signal
Interaction rate
• Main problem with detectors in pulse mode is
that two interactions must be separated by a
finite amount of time if they are to produce
distinct signals
– This interval is called the dead time of the system
• If a second interaction occurs in this interval, its
signal will be lost; if it occurs close enough to the
first interaction, it may distort the signal from the
first interaction
– Show figure p.120
Dead time
• Dead time of a detector system largely
determined by the component in the series with
the longest dead time
– Detector has longest dead time in GM counter
systems
– In multichannel analyzer systems the analog-to-digital
converter often has the longest dead time
• GM counters have dead times ranging from tens
to hundreds of microseconds, most other
systems have dead times of less than a few
microseconds
Paralyzable or nonparalyzable
• In a paralyzable system, an interaction
that occurs during the dead time after a
previous interaction extends the dead time
• In a nonparalyzable system, it does not
• At very high interaction rates, a
paralyzable system will be unable to
detect any interactions after the first,
causing the detector to indicate a count
rate of zero
τ
Dead Live
Paralyzable
Events in detector
Time
τ
Dead Live
Nonparalyzable
Image by MIT OpenCourseWare.
10,000
8000
Count Rate
6000
Ideal
Non-paralyzable
Paralyzable
4000
2000
0
0
10,000
20,000
30,000
40,000
50,000
Interaction Rate
Image by MIT OpenCourseWare.
Current mode operation
• In current mode, all information regarding
individual interactions is lost
• If the amount of electrical charge collected from
each interaction is proportional to the energy
deposited by that interaction, then the net
current is proportional to the dose rate in the
detector material
• Used for detectors subjected to very high
interaction rates
Spectroscopy
• Most spectrometers operate in pulse mode
• Amplitude of each pulse is proportional to the
energy deposited in the detector by the
interaction causing that pulse
• The energy deposited by an interaction is not
always the total energy of the incident particle or
photon
• A pulse height spectrum is usually depicted as a
graph of the number of interactions depositing a
particular amount of energy in the spectrometer
as a function of energy
Relative number of photons
100
662 keV
32 keV
0
100
200
300
400
500
600
700
Energy (keV)
Number of Interactions
32 keV
662 keV
0
100
200
300
400
500
600
700
Energy (keV)
Image by MIT OpenCourseWare.
Detection efficiency
• The efficiency (sensitivity) of a detector is
a measure of its ability to detect radiation
• Efficiency of a detection system operated
in pulse mode is defined as the probability
that a particle or photon emitted by a
source will be detected
Number detected
Efficiency =
Number emitted
Number reaching detector
×
Efficiency =
Number emitted
Number detected
Number reaching detector
Efficiency = Geometric efficiency × Intrinsic efficiency
A
Ω
a
S
d
Image by MIT OpenCourseWare.
Gas-filled detectors
• A gas-filled detector consists of a volume of gas
between two electrodes, with an electrical
potential difference (voltage) applied between
the electrodes
• Ionizing radiation produces ion pairs in the gas
• Positive ions (cations) attracted to negative
electrode (cathode); electrons or anions
attracted to positive electrode (anode)
• In most detectors, cathode is the wall of the
container that holds the gas and anode is a wire
inside the container
Incident
charged
particle
+
+
+ + + - -
Meter
+
Anode (+)
Battery or power supply
Cathode (-)
Image by MIT OpenCourseWare.
•
•
Outer shell is a metal cylinder made of either Steel or Aluminum
– Filled with a gas
Inner wire is gold plated tungsten
– Tungsten provides strength to the thin wire
– Gold provides improved conductivity
Types of gas-filled detectors
• Three types of gas-filled detectors in common
use:
– Ionization chambers
– Proportional counters
– Geiger-Mueller (GM) counters
• Type determined primarily by the voltage applied
between the two electrodes
• Ionization chambers have wider range of
physical shape (parallel plates, concentric
cylinders, etc.)
• Proportional counters and GM counters must
have thin wire anode
Ionization Counter
• Ionization region
1010
Pulse Height (Arbitrary Units)
– All electrons are
collected
– Charge collected is
proportional to the
energy deposited
– Called ion chambers
1012
108
106
GeigerMuller
Region
Recombination
Region
Ionization Chamber
Region
104
102
0
Proportional Region
500
1000
Applied Voltage
Image by MIT OpenCourseWare.
Ionization chambers
• If gas is air and walls of chamber are of a
material whose effective atomic number is
similar to air, the amount of current produced is
proportional to the exposure rate
• Air-filled ion chambers are used in portable
survey meters, for performing QA testing of
diagnostic and therapeutic x-ray machines, and
are the detectors in most x-ray machine
phototimers
• Low intrinsic efficiencies because of low
densities of gases and low atomic numbers of
most gases
Proportional Counter
• Proportional region
• Avalanche ionization
– Charge collected is
linearly proportional to
energy deposited
1010
Pulse Height (Arbitrary Units)
– Electric field is strong
enough to create
secondary ionization
– Further increases
create more ionization
in the gas
1012
108
106
GeigerMuller
Region
Recombination
Region
Ionization Chamber
Region
104
102
0
Proportional Region
500
1000
Applied Voltage
Image by MIT OpenCourseWare.
Avalanche Ionization
25µm
Anode wire
Image by MIT OpenCourseWare.
GM Counter
• Geiger-Mueller region
– Saturation of the
anode wire
– Avalanche over the
entire wire
– Cannot be used for
high count rate
applications
1012
Pulse Height (Arbitrary Units)
1010
108
106
GeigerMuller
Region
Recombination
Region
Ionization Chamber
Region
104
Cathode
102
e
U
Proportional Region
ot on
V Ph
Anode Wire
UV Photon
0
500
Applied Voltage
Individual
avalanches
Images by MIT OpenCourseWare.
Cathode
1000
GM counters
• GM counters also must contain gases with
specific properties
• Gas amplification produces billions of ion pairs
after an interaction – signal from detector
requires little amplification
• Often used for inexpensive survey meters
• In general, GM survey meters are inefficient
detectors of x-rays and gamma rays
• Over-response to low energy x-rays – partially
corrected by placing a thin layer of higher atomic
number material around the detector
GM counters (cont.)
• GM detectors suffer from extremely long dead
times – seldom used when accurate
measurements are required of count rates
greater than a few hundred counts per second
• Portable GM survey meter may become
paralyzed in a very high radiation field – should
always use ionization chamber instruments for
measuring such fields
Neutron Detection
• Neutral particles
– Detection is based on indirect interactions
• Two basic interactions
– Scattering
• Recoiling nucleus ionizes surrounding material
• Only effective with light nucleus (Hydrogen,
Helium)
– Nuclear reactions
• Products of the reaction can be detected (gamma,
proton, alpha, fission fragments)
Neutron Detection
• Since x.s. in most materials are strongly
dependent on neutron energy, different
techniques exist for different regions:
– Slow neutrons (or thermal neutrons), below
the Cadmium cutoff
– Fast neutrons
105
Cross Section (b)
104
103
102
101
100
10-1
10-9
Image by MIT OpenCourseWare.
10-8
10-7
10-6
10-5
10-4
10-3
Energy (MeV)
10-2
10-1
100
101
Thermal Neutron Detectors
• Two important aspects to consider
– Find a material with large thermal x.s.
– Find an arrangement that allows you to
distinguish from gamma rays
• High Q-value of neutron capture will make it easier
Cross Section (barns)
104
He-3 (n, p)
B-10 (n, α)
Li-6 (n, α)
103
102
10
1
10-1 -2
10
10-1
1
101
102
103
104
Neutron Energy (eV)
105
106
107
Image by MIT OpenCourseWare.
10B(n,alpha)
• Equations
• 94% of the time in excited state
– Q-value of 2.310 MeV
– Other 6%, Q-value of 2.792 MeV
Alpha kinetic energy = 1.78 MeV
Alpha kinetic energy = 1.47 MeV
Li-7 kinetic energy = 1.02 MeV
Li-7 kinetic energy = 0.84 MeV
α
α
n
B-10
Li-7
B-10
n
*Li-7
(excited)
Image by MIT OpenCourseWare.
BF3 Tube
Li-7
B-10
The alpha particle deposits all its kinetic
energy in the gas while the Li-7
deposits only a fraction of its
energy.
α
α
n
B-10
Li-7
2.31 MeV
n
Gamma ray
pulse, noise, etc.
Image by MIT OpenCourseWare.
# Pulses
The Li-7 deposits all its kinetic energy
in the gas while the alpha particle
deposits only a fraction of its
energy.
0.84 MeV
1.47 MeV
2.79 MeV
Pulse Size (energy deposited in detector)
Image by MIT OpenCourseWare.
Efficiency of BF3 Tube
• Intrinsic efficiency
– Eff(E) = 1- exp(-Sigma(E) * L)
6Li(n,alpha)
• Equation
• Only a ground state exists with Q-value of
4.78 MeV
– EHe-3 = 2.73 MeV and Ealpha = 2.05 MeV
• Other possible reactions
– 3He(n,p)
– 157Gd
Gamma-ray sensitivity
Thermal Detectors
Interaction Probability
Thermal Neutron
1 - MeV Gamma Ray
0.77
0.0001
Ar (2.5 cm diam, 2 atm)
0.0
0.0005
BF3 (5.0 cm diam, 0.66 atm)
0.29
0.0006
Al tube wall (0.8 mm)
0.0
0.014
3He
(2.5 cm diam, 4 atm)
Fast Detectors
Interaction Probability
1 - MeV Neutron
1 - MeV Gamma Ray
0.01
0.001
Al tube wall (0.8 mm)
0.0
0.014
Scintillator (5.0 cm thick)
0.78
0.26
4He
(5.0 cm diam, 18 atm)
Neutron and gamma-ray interaction probabilities in typical gas proportional counters
and scintillators
Image by MIT OpenCourseWare.
Fission chambers
• Inner surface of gas tube is coated with a fissile
material (U-235)
– Current mode in reactor operation
– Pulse mode elsewhere
• Efficiency of about 0.5% at thermal energies
• Combine fissile and fertile material to avoid loss
of sensitivity (regenerative chambers)
• Memory effect when used in high flux regions
– Decay of FPs
Fission Chambers
104
103
103
N(E)
N(E)
104
102
102
10
0
20
40
60
80
100
10
120
20
40
60
80
100
120
Energy (MeV)
Energy (MeV)
Deposit thickness = 0.0286 µm - 0.0312
0
mg
cm2
Deposit thickness = 0.714 µm - 0.778
mg
cm2
Image by MIT OpenCourseWare.
Neutron Spectroscopy
• Bonner Sphere
2"
3"
5"
8"
10"
12"
12" + Pb
12" + Pb
internal part
Image by MIT OpenCourseWare.
Long Counter
• Flat response for neutrons of all energy
20"
0.018" Cd cap
15" diameter
1"
9" diameter
8 holes on a 31/2"
diameter pitch
circle 6" deep
1" diameter
8" diameter
14"
2.5"
BF3_ Counter (12.2" active length)
Paraffin
1.0
Relative Sensitivity
0.9
0.8
0"
0.5"
B2O3
Counter Position
1.0"
0.7
0.6
0.5
0.4
0.02
3.0"
0.04 0.06
0.1
0.2
0.4
0.6 0.81.0
2
4 MeV 6
E
Image by MIT OpenCourseWare.
Fast-Neutron induced reaction
detectors
6
• Li(n,alpha)
3 4 5 6 78 9
3
He [π, ρ]
6
2
Li [π, α]
1 x 10
-1
2
1
3 4 5 6789
Cross-section [barns]
1 x 101
– Q-value of 4.78 MeV
– Peaks appear at Q-value + neutron kinetic
energy
– Lithium based scintillators
1 x 104
Image by MIT OpenCourseWare.
2
3
4 5 6 7 89
1 x 105
2
3
4 5 6 78 9
1 x 106
Energy [eV]
2
3
4 5 6 7 89
1 x 107
• 3He(n,p)
– Energy range 20 keV to 2 MeV
• Smooth, nearly flat x.s.
• Tube is wrapped in Cadmium and Boron layersto
reduce contribution from thermal neutrons
• Lead shield is also added to reduce impact of
photons
• Intrinsic efficiency is very low (0.1%)
• Full energy peak at En + 765 keV
• Thermal neutron capture peak at 765 keV
• Elastic scattering spectrum with a max at 0.75En
Epithermal
peak
dN
dE
Full-energy
peak
Recoil
distribution
0.764 MeV
0.75 En
En + 0.764 MeV
E
En
Image by MIT OpenCourseWare.
Fast Neutron Recoil Detectors
• Most useful reaction is elastic
scattering
• Large elastic scattering x.s
• Well known x.s.
• Neutron can transfer all of its energy
in one collision
– Q-value is zero
• Information can be found about the
incoming neutron energy
100
Fission spectrum
shape
Scattering cross section (barns)
– Recoil nucleus will ionize medium
– Most popular target is hydrogen
10
4
He
1
H
1
0.1
0
1
2
3
4
Neutron energy (MeV)
5
6
Image by MIT OpenCourseWare.
Gas recoil detectors
Incident Neutron Energy (MeV)
20
0.8
0.4
1.0
0.6
1.2
1.6
10
4
He Weight/MeV
(~ Counts/Channel)
30
2.0
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4
4
He Recoil Pulse Height, MeV
1.6
0.8
He Weight/MeV (~ Counts/Channel)
1.2
1.0
0.8
Incident Neutron Energy (MeV)
4.0
0.6
6.0
0.4
8.0
4
He Weight/MeV (~ Counts/Channel)
1.4
0
4
0.2
0
1
2
4
3
4
5
He Recoil Pulse Height, MeV
6
0.7
0.6
0.5
0.4
Incident Neutron Energy (MeV)
0.3
0.2
10.0
12.0
0.1
0
0
1
2
3
4
5
6
7
8
14.0
9
4
He Recoil Pulse Height, MeV
Image by MIT OpenCourseWare.
Scintillators
• Desirable properties:
–
–
–
–
High conversion efficiency
Decay times of excited states should be short
Material transparent to its own emissions
Color of emitted light should match spectral sensitivity
of the light receptor
– For x-ray and gamma-ray detectors, μ should be large
– high detection efficiencies
– Rugged, unaffected by moisture, and inexpensive to
manufacture
Scintillators (cont.)
• Amount of light emitted after an interaction
increases with energy deposited by the
interaction
• May be operated in pulse mode as
spectrometers
• High conversion efficiency produces
superior energy resolution
Materials
• Sodium iodide activated with thallium
[NaI(Tl)], coupled to PMTs and operated in
pulse mode, is used for most nuclear
medicine applications
– Fragile and hygroscopic
• Bismuth germanate (BGO) is coupled to
PMTs and used in pulse mode as
detectors in most PET scanners
Photomultiplier tubes
• PMTs perform two functions:
– Conversion of ultraviolet and visible light
photons into an electrical signal
– Signal amplification, on the order of millions to
billions
• Consists of an evacuated glass tube
containing a photocathode, typically 10 to
12 electrodes called dynodes, and an
anode
Glass tube
+100 volts
+300 volts
Visible light
photon
Signal to
preamp
+200 volts
Photocathode
0 volts
Dynodes
+400 volts
Anode
+500 volts
Image by MIT OpenCourseWare.
Dynodes
• Electrons emitted by the photocathode are
attracted to the first dynode and are accelerated
to kinetic energies equal to the potential
difference between the photocathode and the
first dynode
• When these electrons strike the first dynode,
about 5 electrons are ejected from the dynode
for each electron hitting it
• These electrons are attracted to the second
dynode, and so on, finally reaching the anode
PMT amplification
• Total amplification of the PMT is the product of
the individual amplifications at each dynode
• If a PMT has ten dynodes and the amplification
at each stage is 5, the total amplification will be
approximately 10,000,000
• Amplification can be adjusted by changing the
voltage applied to the PMT
MIT OpenCourseWare
http://ocw.mit.edu
22.106 Neutron Interactions and Applications
Spring 2010
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.