Printed in: Nucl. Instr. & Meth A454 (2000) pp. 26-39
Advances in gas avalanche radiation detectors for biomedical
applications夽
A. Breskin*
Department of Particle Physics, The Weizmann Institute of Science, 76100 Rehovot, Israel
Abstract
Gas avalanche detectors are instruments of choice for radiation detection and localization in numerous "elds of basic
and applied research. Recent advances in detection techniques, involving multiplication and detection of single or a few
charges deposited in gas media, or emitted from solid converters into gas, are described. The properties of radiation
converters and associated advanced gas multipliers are discussed, with an accent on the recently introduced gas
avalanche imaging photomultipliers. Applications in the "elds of radiation damage studies to DNA, digital mammography and early detection of cancer tumors are presented.
1. Introduction
Gas avalanche radiation detectors have been
massively employed over the past decades, mostly
in particle physics. The `modern eraa in this "eld
started in the late 1960 s, with the introduction by
Charpak of Multiwire Proportional Chambers often named `Wire Chambersa [1]. For the "rst time,
it was made possible to localize charged particles,
X-ray photons and thermal neutrons with submillimeter accuracies, over detection areas exceeding a square meter and at very high repetition rates.
This has revolutionized many "elds of science,
particularly particle physics. Modern High-Energy
夽
Invited talk at SAMBA, Symposium on Applications of
Radiation Detectors in Medicine, Biology and Astrophysics.
Siegen, Germany, October 6}8, 1999.
* Corresponding author. Tel.: #972-8-934-2645; fax: #9728-934-2611.
E-mail address: fnbresk@wisemail.weizmann.ac.il (A.
Breskin).
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Physics Experiments massively employ such
detectors, which have contributed to numerous
important discoveries.
The "rst attempts of employing Wire Chambers
for digital medical radiography were made by
Perez Mendez already in the early seventies [2].
Ever since, gas avalanche radiation detectors, either
Wire Chambers [3] or more recent advanced
Micro-pattern Detectors [4,5], have been widely
employed in biology and medicine, as will be bie#y
summarized below.
Though there exist numerous other detection
techniques, comprising scintillators, scintillating
"bers, a variety of solid-state devices and others,
gas avalanche detectors persist and improve
impressively, due to some important inherent
properties:
E large sensitive area and #exible geometries,
E relatively simple and economic manufacture,
E variety of real-time readout techniques: charge
integration or pulse counting,
E
E
E
E
E
high multiplication factors: 10}10,
sensitivity: down to a single electron,
localization accuracy: down to 30 lm,
time resolution: down to 100 ps,
counting rates: up to the MHz/mm scale.
On the other hand, gaseous detectors have usually poor energy resolution due to relatively low
primary ionization statistics and #uctuations in the
avalanche process, low detection e$ciency for energetic X-ray or gamma photons, they often age under long-term operation at high radiation #ux and
su!er some gain limitations in such conditions.
Gaseous detectors are mostly custom-made, are
rarely sealed and therefore generally require gas
circulation systems and often well-trained operators.
In medical diagnostics, gaseous detectors are
currently employed in digital X-ray radiography
[6] and angiography [7]. In X-ray radiography,
large-volume xenon-"lled detectors, with wires
oriented towards a narrow fanned beam, have
shown to successfully compete with traditional
"lm-screen imagers. These line-scanning devices,
operating in photon-counting or current-integration modes, permit a considerable reduction of the
radiation dose to patients [8]. In digital intravenous coronary angiography, patients are scanned
simultaneously at two monochromatic X-ray energies with a high-pressure ionization chamber. The
photon energies are chosen above and below the
K-edge of iodine contrast agent. Logarithmic subtraction of the two data sets provides high-quality
angiographic images with a high dynamic range
[9].
High-pressure gas ionization chambers have
been also investigated, such as X-ray sensors in
Computerized Tomography (CT). Having rather
limited sensitivity to energetic gamma photons,
methods were found to couple position-sensitive
gas avalanche detectors to solid converters. This is
the case in gamma cameras, equipped with thick
metal-grid converter [10]. Such devices were
routinely applied for medical inspection and more
recently, in a Positron Emission Tomography
(PET) mode, for high-resolution 3D small-animal
imaging [11]. Small-animal PET cameras were also
developed, where UV-photons from BaF crystals
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are detected in wire chambers operated with
a photosensitive (TMAE) gas [12]. Current e!orts
to develop large-area gas avalanche photomultipliers [13], to cope with Gamma scintillators, will be
discussed in this work.
In the last decades there has been considerable
R&D activity in real-time 2D imaging detectors for
X-ray di!raction, particularly due to newly installed intense synchrotron radiation facilities [14].
Large-area gas avalanche detectors, capable of providing di!raction images of complex organic molecules (e.g. in protein crystallography) in a few
seconds are currently operational [15,16] or under
advanced development stages [17]. Also here, wire
chambers [15,16,18] are being replaced by wire-less
detectors like the micro-CAT chamber , consisting
of "ne-grid multiplication element followed by avalanche localization with an advanced 2D resistive
readout system [17]. Many other potential multipliers will be reviewed. New, secondary electron
emission (SEE) soft X-ray imaging detectors with
high localization resolution and ns time response,
will be described below [19]. Similarly. SEE detectors. combining novel composite thermal-neutron coverters with advanced gaseous multipliers
[20], could be advantageously employed in neutron di!raction experiments in the "eld of biology,
in future intense spallation sources.
Gas avalanche imagers are currently employed
in autoradiography [21,22], for real-time analysis
of beta-labeled biomedical samples or electrophoretic gels. Some of the techniques permit to
resolve details in tissue sections traced with lowenergy electron emitters, at the 30 lm level [21]
(Fig. 1). There are numerous other applications of
gaseous detecors in biomedicine, e.g. in beam
monitoring in radiotherapy [23], in various "elds
of radiation dosimetry, etc. New applications in
nanodosimetry [24] will be discussed below.
2. Wireless electron multipliers
The electron multiplier has the important role of
converting the radiation-induced charges within
the detector volume, into detectable signals. It is
expected, in most cases, to provide high charge
gain and the fastest possible response, to cope with
Fig. 1. Autoradiographic images of Tc-labeled anatomic section (20 lm thick) of rabbit's kidney. The emitted low-energy
electrons were measured with (a) a high-resolution (30 lm) gasavalanche (Beta Imager 2000 of Biospace Mesures, France) and
(b) an autoradiographic "lm [21]. Courtesy of N. Barthe.
operation under very large radiation #ux (repetition rates) and to allow for the accurate determination of the radiation impact location.
As mentioned above, the current tendency in the
"eld of gaseous detectors is replacing wire chambers by advanced micro-pattern electron multipliers, o!ering an order of magnitude improvement
in spatial accuracy and counting-rate capability.
Such multipliers consist, on one hand, of miniature
anode and cathode strips or other electrode patterns deposited by micro-lithographic techniques
on insulating substrates. In this family, one may
recall Microstrip Gas Chambers (MSGC) [25],
Micro-Gap Chambers (MGC) [26], Micro-Dot
Chambers (MDOT [27,28]) and other variants of
these techniques (reviewed in Refs. [4,5,29]). Due to
the small anode-to-cathode distances, typically
50}200 lm, these multipliers o!er inherently good
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localization accuracy, of a few tens of microns.
Moreover, the rapid avalanche-ion collection by
the near-by cathode patterns considerably reduces
space charge buildup, responsible for counting-rate
limitations typically observed in wire chambers.
However, gain limitations and long-term instabilities often appear in these devices, due to the insulating nature of the substrates, gas pollution and to
micro-discharges, which damage the electrode patterns [29]. Passivating the electrode edges and
other technical improvements in electrode production can solve the latter. An interesting but more
complex solution is the Microgap wire chamber,
having anode wires located, through thin insulator,
on top of narrow cathode strips [30]. Except for the
MSGC and the recently introduced Small-Gap
chamber [31], all other multipliers in this family
provide 2D localization in a single-detector element. The MDOT device; produced in silicon
technology, is a true pixelized device; due to the
symmetric `circulara pixel geometry, it o!ers good
stability at very high gain [27,28].
More robust electron multipliers are the MICROMEGAS [32], Micro-CAT [17], MicroGroove (MGD) [33] and GEM [34]. The MICROMEGAS, developed at Saclay, is a thin gap
(50}100 lm) parallel-plate device, in which radiation-induced electrons drifting through a very thin
dense mesh are multiplied; signals are collected on
a strip anode. The micro-CAT is an expanded form
of the CAT (`compteur a trousa) [35], in which
electrons drifting from a conversion volume are
multiplied within a hole in a metallic foil with
an anode at the bottom. The Siegen University
Detector Group has demonstrated that the microCAT, equipped with an interesting pixelized 2Dresistive anode readout, has good potential for
X-ray di!raction and radiography applications (see
Fig. 2) [36]. The Micro-Groove, proposed by the
Pisa Group, could also be considered as a further
derivative of the CAT. Here, electrons are multiplied within thin grooves in a metallized kapton
foil, having anode strips running in an orthogonal
direction at the bottom of the grooves (other face of
the same foil). Recording the signals from the top
and bottom strips [33] provides a 2D sensitivity.
One of the most interesting developments in gas
avalanche detectors is no doubt the Gas Electron
and increased lifetime. It could be of a particular
advantage as a preamplifying element in front of
another micro-pattern multiplier like a MSGC
[41]. It has been demonstrated that high-resolution
X-ray imaging could be performed with a cascade
of two GEM elements, coupled to a 2D readout
board, as shown in Fig. 3 [42]. Stable operation at
Fig. 2. An example of a 2D-di!raction pattern from a DSPC
lipid sample, recorded with the Micro-CAT X-ray-imaging detector of the University of Siegen. The observed pattern is due to
small individual resistive anode pads, providing an interpolating
position encoding at high counting rates [17,36].
Multiplier (GEM), proposed by Sauli [34]. It consists of a thin insulating foil (usually 50 lm thick
kapton), metal-clad on both sides, perforated by
a regular dense matrix of holes (typically 50}80 lm
in diameter, 100}200 lm apart). Upon the application of a potential across the foil (typically
400}500 V), a high dipole "eld develops within the
holes. Radiation-induced electrons are focussed
into the holes, where multiplication occurs under
the very high electric "eld. A large fraction of the
multiplied charge is transferred either to a collection electrode or to an additional electron multiplier. Multiplication factors reaching 10 are
attainable with a single GEM element, as well as
high-rate capability and good localization accuracy
[37,38]; long-term stability under high radiation
#ux has been recently demonstrated [39]. Compared to the other multipliers discussed above, the
GEM has the unique advantage of being able to act
as a preampli"er, namely to preamplify the primary
ionization electrons and to transfer them to a further multiplier. This idea of a multi-step avalanche
multiplier [40], permits reducing the gain of each
multiplication element, resulting in higher stability
-4-
Fig. 3. A schematic view of a double-GEM X-ray-imaging detector equipped with a 2D readout board. Photography of a
bat's claw and its X-ray radiographic image, taken at 8 keV, are
shown. The localization resolution is of the order of 0.1 mm [42].
gains above 10 was reached in this con"guration.
We will discuss below gas avalanche photomultipliers based on multi-GEM structure [43].
All gas avalanche multipliers discussed above
can operate over a broad pressure range, from
several atmospheres down to a few Torrs. The
latter has been demonstrated in MSGC [44],
MDOT [45] and GEM [46] multipliers. The lowpressure operation is usually characterized by very
fast response, high-rate capability and high gains;
the latter permits ultimate sensitivity down to
single electrons. Factors governing the gain limitations of micro-pattern detectors are reviewed in
Refs. [47,48].
3. New applications in dosimetry
Gaseous detectors, operating in both ionization
and proportional modes, have been employed for
many decades for radiation dosimetry. Their usual
role has been in assessing radiation e!ects to the
living tissue, by mesuring radiation-induced energy
deposits in expanded tissue-equivalent gas models.
Such measurements have strong relevance to
radioprotection and radiotherapy.
While current dosimetric techniques are limited
to measuring global or integral radiation e!ects,
contemporary radiobiological concepts advocate
the di!erential measurements. Indeed, because of
the highly #uctuating stochastic nature of the energy deposits, it is important to measure not only
the deposited energy by an event, within a given
tissue-equivalent volume, but also the spatial distribution of the ionization pattern. Most signi"cant
damage occurs at the sub-cell level, more precisely
to the DNA molecules [49]. It is currently established that irreversible radiation damage to a living
cell occurs when its DNA is considerably upset.
This occurs when a signi"cant energy quantum is
deposited within a small DNA segment, typically
30 bases long, causing multiple breaks in both
DNA strands [50]. In such events the cellular repair mechanism fails to correctly repair the damage, which results in cell mutations or death.
While ionization deposits at the cell nucleus
scale (10 lm), or at the chromosome level (1 lm)
can be studied with `microdosimetrica tools like
-5-
tissue-equivalent proportional counters (TEPC),
the sub-micron scale, namely that of the chromatin
"ber (25 nm) and, moreover, the DNA molecule
(2 nm) require new `nanodosimetrica approaches.
The evaluation of the lethality of a given radiation
"eld depends indeed on the precise knowledge
of the energy-loss distribution within the
relevant (2 nm in diameter, 10}20 nm long) DNA
segment [50].
Miniature TEPCs operating at very low gas
pressures [51] could in principle reach sensitivities
at the 5 nm scale, but they su!er from unwanted
radiation interactions with the cell walls. A few
other nanodosimeter techniques, capable of reliably assessing single ionization events, are currently being developed; they cover the sensitivity range
from the chromosome down to the sub-DNA scale.
One technique consists of recording the "ne
structure of ionization patterns induced by particle
tracks traversing the low-pressure (10}40 Torr) gas
volume of a Time Projection Chamber (TPC). The
ionization electrons deposited along each track are
collected and multiplied, inducing avalanches, of
which the emitted light is recorded with an intensi"ed CCD camera and a set of photomultipliers
[52]. This technique permits recording ionization
patterns induced by charged particles, photons and
neutrons, in a few lm-size tissue-equivalent sensitive volumes, with a resolution of a few tens of
nanometers. The local deposited energy is proportional to the light inensity recorded by the CCD
camera, as shown in Fig. 4. The bright spots, like
those seen at the end point of a stopping deltaelectron, correspond to large local ionization clusters, which are indeed those of high-potential
lethality to DNA. The `optical TPCa is not sensitive to sinlge ionization charges, but rather to clusters of several charges. Its resolution is limited by
electron di!usion, a!ecting both the ionization pattern structure and the avalanche-induced `lightspota size.
Single-charge sensitivity, of prime importane for
low-ionization pattern investigations, cannot be
reached with detectors based on charge integration
mode. The pulse-height resolution of such `proportionala detectors is seriously a!ected by the statistical #uctuations in the avalanche growth, which
makes the di!erentiation of a single-electron event
Fig. 4. Images of ionizing particle track patterns recorded in an optical avalanche microdosimeter, on a tissue-equivalent scale shown in
the "gure. The 5 MeV protons and 19 MeV alpha particles traverse a gas volume at a pressure of 20 Torr. The potentially lethal to the
DNA track spots are that of high local ionization, like the endpoints of the delta-electron trails, clearly observed in the "gure [52].
from that of a few-electron cluster, an impossible
task.
We have developed a new approach for the differential recording of small radiation-induced energy deposits in gas, based on single-charge
counting techniques [53,54]. The idea, derived
from early works on relativistic-particle identi"cation by electron-cluster counting techniques
[55}57], is illustrated schematically in Fig. 5; it
consists of extracting ionization charges, electrons
or ions, radiation induced in a small gas-sensitive
volume, followed by their individual multiplication
and counting. The wall-less sensitive volume is de"ned by the charge extraction e$ciency; its size,
which is the most critical element of the
nanodosimeter, is a function of the extraction-slit
size, the gas type and pressure and the electric "eld
geometry.
In the single-electron counting nanodosimeter
[58], electrons are extracted through a small aperture into an electron multiplier (see Section 2),
operating in the same gas pressure. The sensitive
volume is strongly a!ected by electron di!usion,
which sets a lower-pressure limit of a few Torr;
below this pressure, the electron extraction e$ciency diminishes due to a quasi-ballistic electron
transport. The electron-counting technique is presently limited to tissue-equivalent sensitive volumes
of the order of 20}30 nm [59], namely to the
chromatin scale.
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In the single-ion counting nanodosimeter [24], the
ions are more e$ciently extracted into vacuum,
with very small di!usion losses. The pressure can be
reduced to a fraction of a Torr, leading to possible
sub-nanometer tissue-equivalent sensitive volumes.
The ions are accelerated into a vacuum-operated
secondary-electron multiplier, yielding fast pulse
trails. It has been demonstrated that the ion-counting technique provides alpha-particle ion density
distribution spectra in a cubic nanometer equivalent gas volume, compatible with those theoretically predicted [60,61]. A nanodosimeter built
according to this principle is presently being tested
at the Loma Linda proton synchrotron accelerator.
It permits attaining biologically relevant sensitive
propane volumes on the order of 2 nm in diameter
and 25 nm long [62]. Other types of ion nanodosimeters are developed, based on ionization
measurements in gas jets [63].
The ion-counting technique, in which ions
formed in gas are multiplied in vacuum, has
the additional advantage of being able to operate
with any gas. The equivalent nanometer resolution
in condensed matter could have numerous
applications beyond those discussed above. An
important example is the study of radiation
damage to advanced nanoelectronics, e.g. using
silane gas to simulate silicon, etc. It could have
important implications in accelerator physics and
space science.
Fig. 6. The principle of the gas avalanche photomultiplier.
A photon-induced electron is emitted from a solid photocathode
into the gas. Avalanche multiplication takes place in the electron
multiplier, close to an anode of a micropattern device. In this
con"guration, most avalanche-induced ions are collected on the
neighboring cathodes and some drift to the photocathode.
Fig. 5. (a) A schematic view of the nanodosimeter concept:
ionization charges (electrons or ions), deposited by the primary
radiation in a low-pressure gas volume, are extracted by an
electric "eld through a small aperture. The extraction e$ciency
de"nes a wall-less sensitive volume. The charges are individually
multiplied, deteced and counted: electrons are detected with
a gaseous avalanche electron multiplier, providing pulse-trials
shown in (b), while ions are detected with a vacuum-operated
detector, resulting in similar pulse-trials shown in (c).
4. Gas avalanche imaging photomultipliers
Photomultiplier tubes (PMT) are currently and
massively used in medical diagnostics instrumentation, for recording light from large scintillator arrays mostly in gamma cameras and CT apparatus.
Standard vacuum PMTs are slowly being replaced,
in small gamma camera systems, by position-sensitive PMTs [64] or hybrid photodiodes (HPD) [65],
of which the cost is rather prohibitive.
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An alternative and probably more economic
solution for light recording from large scintillator
or scintillating-"ber arrays would be the use of gas
avalanche imaging photomultipliers, combining
a thin solid photocathode with a gaseous electron
multiplier (Fig. 6). Such photomultipliers, recently
reviewed in Ref. [13], have been developed over the
last decade, particularly for single UV-photon localization in Ring Imaging Cherenkov (RICH)
counters used for relativistic particle identi"cation
[66]. These fast photon detectors are gradually
replacing the much slower wire chambers "lled
with UV-photosensitive `gaseous photocathodesa
(TEA, TMAE) [67], also employed for PET in
combination with BaF
crystals [12]. The
quantum e$ciency (QE) spectra of some UVphotocathodes, relatively stable in air, are shown in
Fig. 7. CsI photocathodes, reviewed in Ref. [68],
are widely employed for RICH, where MWPCbased photon detectors, reaching square meter dimensions, are under construction for future particle
physics experiments [66].
The gas avalanche photomultipliers, operating at
atmospheric pressure, can be made 10}20 mm thin,
Fig. 7. Typical quantum e$ciency spectra (in vacuum) of annealed CsI [68] annealed CsBr [72] and hydrogenated CVD
diamond [73] photocathodes, suited for operation under gas
multiplication.
employing modern gas electron multipliers (see
Section 2). They are sensitive to single photons and
can operate at photon #ux reaching a MHz/mm.
Unlike vacuum PMTs, they can operate at intense
magnetic "elds [66]. They are usually equipped
with highly pixelized readout electronics, developed for particle physics applications [69].
The CsI photocathode, having its red boundary
cuto! around 210 nm (Fig. 7) has very low sensitivity to the fast component of BaF scintillation.
It could be employed in combination with other,
less e$cient UV-scintillators, e.g. KMgF [70] or
with high-pressue or liquid xenon scintillators
(peak emission around 170 nm) [71], which has an
interesting potential in medical imaging. CsBr [72]
and the very robust CVD-diamond [73] "lms are
interesting solar-blind photocathodes, provided
some e$cient crystals can match their spectral
range
It is obvious that the most important applications of gas avalanche photomultipliers are in the
visible spectral range, in which there is a large
variety of scintillating crystals. Unlike UV-photocathodes, those sensitive in the visible range, e.g. the
currently employed alkali- or bi-alkali-antimonides, are very reactive to even minute amounts of
impurities. Therefore, "rst attempts to operate gaseous photomultipliers with such photocathodes
[74,75] were not pursued.
-8-
Fig. 8. The evolution of the absolute quantum e$ciency of
K}Cs}Sb photocathodes exposed to oxygen. Shown are the
results at a wavelength of 312 nm, for bare and coated photocathodes (200 and 250 As CsI), as function of the residual oxygen
pressure. Each data point represents 5 min of exposure to oxygen followed by quantum e$ciency measurement in vacum [78].
Visible photocathodes could be proected by thin
deposited "lm [76]. Only recently, it has been demonstrated that thin alkali}halide "lms (CsI, CsBr)
deposited on Cs Sb, and K}Cs}Sb photocathodes
e$ciently protect them against exposure to large
amounts of oxygen (Fig. 8) [77,78]. The protective
"lm prevents the photocathode from having contact with gas impurities, but at the expense of
a reduction by about a factor of 6 in QE due to
photoelectron losses. Protected K}Cs}Sb photocathodes reach typically QE values of the order of
5% at 350 nm, as shown in Fig. 9; this is a viable
QE in many applications.
We are currently developing gas avalanche imaging photomultipliers for visible light, equipped with
MSGC electron multipliers, for X-ray mammography [79]. They will localize X-ray-induced
photons, being directly coupled to an appropriate
converter.
As discussed in Secton 2 and in Ref. [13], the
electron multiplier plays an important role in this
type of application, where high sensitivity to single
photoelectrons is of prime importance. Our recent
developments in this "eld indicate that multi-GEM
photomultipliers, in which three GEMs in cascade
are coupled to a photocathode (Fig. 10), could
provide a solution of choice [43,80]. In such
back to the photocathode. This permits, for the "rst
time, the operation of gas avalanche proportional
detectors at gains exceeding 10 in pure noble-gas
mixtures [43,80] (Fig. 11). This is an important fact
that could pave the way towards the operation of
gas avalanche photomultipliers with non-protected
visible photocathodes. E!orts are presently being
directed at the development of other novel gas-stable
photocathodes, with an extended spectral range.
5. X-ray and thermal neutron secondary electron
emission detectors
Fig. 9. Typical absolute quantum e$ciency spectra of K}Cs}Sb
photocathodes, bare and coated with 300 As thick CsBr and
250 As thick CsI "lms [78].
Fig. 10. The multi-GEM photomultiplier concept: 3 GEMs in
cascade are coupled to a photocathode. Each GEM operates at
a low gain, resulting in a high total gain. The avalanche-induced
pulses are recorded on a printed-circuit board, arranged to
provide 2D localization.
devices, the GEM elements e$ciently screen the
photocathode from avalanche-induced photonfeedback e!ects and in addition reduce ion feed-
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An important research tool in structural biology
is X-ray and neutron scattering from complex molecules. High-luminosity Synchrotron Radiation
Facilities and intense neutron Spallation Sources
require advanced real-time imaging detectors capable of coping with the high radiation #ux. Modern applications, involving dynamic studies of
rapidly evolving processes, are requesting very fast
((100 ns) detector response.
Though a variety of detectors based on radiation
conversion and multiplication in gas, are successfully operating and others being under advanced
developments, their time resolution, localization
accuracy and rate capability are often limited due
to phenomena related to electron transport in gas.
The gas conversion gap induces localization parallax errors under angular incidence, often requiring
special costly detector geometries [18,81].
Secondary Electron Emission (SEE) detectors
[82}84], based on a similar principle to that of gas
avalanche photomultipliers discussed above, have
been developed for soft X-ray [19] and thermal
neutron [20] localization. In such SEE detectors
shown in Fig. 12, radiation is converted in a thin
foil, producing multiple low-energy (eV) electrons
emitted into gas. The secondary electrons initiate
multiplication, in a parallel-plate avalanche mode,
at their emission location; the electron avalanche is
transmitted into a second multiplication stage,
where fast timing and 2D-localization is measured.
The second multiplier can be a wire-chamber or
one of the micro-pattern devices described in Section 2. The radiation converter is selected upon the
application.
Fig. 11. Gain vs. voltage characteristics of a 3-GEM photomultiplier with a CsI photocathode, in di!erent gas mixtures. For some
mixtures having secondary scintillation e!ects in the photocathode-to-GEM gas gap, leading to a larger (but slower) total signal, both
`fasta (primary) and `totala (primary plus photon-mediated) gain curves are shown. For details see Ref [43].
The surface conversion, followed by surface
emission and multiplication, make the detector indepenent of the radiation incidence angle, leading
to high-resolution parallax-free imaging. The fast
avalanche process, which follows the surface emission, leads to ns time resolutions.
5.1. X-ray imaging detectors
The best-known soft X-ray (E(10 keV) converter is CsI, previously discussed as a UV-photocathode. Compared to other "lms, it has relatively
high conversion e$ciency (Z"54) and moreover
a large secondary electron escape length, of the
order of 20 nm. The latter results in good emission
properties, typically of 20}30 secondary electrons
per 6 keV photon [83}85]. The converter thickness
is selected according to the X-ray energy [19]. The
electron escape length limits the e!ective converter
thickness, and consequently the conversion e$ciency, unless employed in geometries where radiation
impinges the converter under a small grazing angle
[19,82]. A SEE detector of 200;200 mm, with
delay-line readout, was investigated with Synchrotron Radiation, under high photon #ux [86]. An
example of a X-ray scattering pattern from collagen
-10-
is shown in Fig. 13, recorded at a photon rate
approaching a MHz/mm.
Such detectors, which can operate at low or
atmospheric gas pressures, are rather unique tools
for time-resolved studies of fast-evolving phenomena on a ns or even sub-ns time scale. They
could play an important role in protein crystallography, where the sample properties evolve under
intense irradiation. Their present drawback is the
relatively low conversion e$ciency, of the order of
5% at 6 keV under normal incidence [19]. E!orts
are being made towards the design of detectors
with more e$cient converters. One could think of
`columnara CsI converters, in which secondary
electrons emitted from `needle likea converters are
focussed within the space between the needles into
a multiplier [87]. One could also employ multilayer converter}detector systems [88], of which an
elegant solution based on a CsI-coated multi-GEM
system was recently proposed [89].
5.2. Neutron imaging detectors
Thermal neutrons can be localized by gaseous
detectors, via detection of charged particles
resulting from their nuclear reaction with gas
Fig. 13. An example of a X-ray image recorded with the SEE
detector shown in Fig. 12a, representing small-angle scattering
from collagen in rat tail. The image was recorded at an intense
synchrotron radiation beam at ESRF-Grenoble, duing 20 s, at
a rate of 850 kHz [86].
Fig. 12. The operation principle of secondary electron emission
(SEE) X-ray and thermal neutron gas avalanche imaging detectors. (a) In the X-ray detector, each photon converted in
a thin solid "lm (usually CsI) induces the emission of multiple
low-energy (eV) secondary electrons. They start multiplication
at their emission location and are further multiplied and localized in a second multiplier (e.g. here a wire chamber). (b) In the
neutron detector, nuclear reactions between the incident neutron and a solid converter (here Li) result in charged particle
emission (here alpha or H). These emit multiple low-energy
secondary electrons when crossing a CsI "lm deposited on the
converter surface. Multiplication of these electrons occurs in two
steps, like in (a); the second stage here is, e.g. a GEM coupled to
a readout board.
molecules [90,91] or with solid converters coupled
to the multiplier [92]. Details about potential converters and cross-sections are given elsewhere [20].
While gas conversion results in the emission of
long-range charged particles, which does not permit an accurate localization, solid converters like
Gd or Li immediately followed by an avalanche
multiplier, provide better accuracy. This is due to
the exponential avalanche development, leading to
-11-
a higher sensitivity of the multiplier to the ionization electrons deposited by the energetic charged
particles in the gas, close to the converter surface
[92]. However, the particle emission being isotropic, tracks emitted under large angle (to the
normal) still induce image smearing.
A more advanced method consists of employing
a composite converter, namely, a neutron conversion foil (e.g. Li, Gd, etc.) coated with a secondary
electron emitter (e.g. CsI); the converter is coupled
to a low-pressure gas avalanche multiplier [20].
A neutron absorbed in the converter emits an energetic charged particle (electron, alpha, triton, etc.)
which emits a cloud of eV secondary electrons upon
crossing the CsI layer. These secondary electrons
initiate a multiplication process at the location of
the charged particle emission from the converter.
Due to the relatively low ionization induced by this
particle in the low-pressure gas (10}20 Torr) and to
the exponential nature of the avalanche process, the
detector has very low sensitivity to the charged
particles themselves. It therefore provides good localization (Fig. 14), independent of the neutron incidence angle or that of the emitted particle. Similarly
to the SEE X-ray detectors, the avalanche following
surface emission results in intrinsic time resolution
in the ns range.
Detectors with CsI-coated natural Gd and Li
foils, optimized in thickness, provided localization
resolution of the order of 0.4 mm (FWHM) [20].
Calculations indicate an average SEE yield of 60
Fig. 14. Comparative radiographic images of a small (25 mm in
diameter) metal ball bearing, made with thermal neutrons
(lambda 0.2 mm) with: (a) a photographic "lm preceded by
a Li/ZnS converter; (b) an SEE detector equipped with a Li/CsI
converter The images indicate clearly the presence (top) or the
absence (bottom) of grease in the bearing [83,84].
are proposed, with high sensitivities, down to
a single charge: an electron or an ion induced by
radiation in gas or solid media. Such fast, real-time
imaging detectors, equipped with advanced micropattern electron multipliers, can e$ciently detect
light emitted from scintillator arrays, localize Xrays and neutrons scattered from large complex
molecules or traversing objects under radiographic
conditions, to help assessing radiation damage to
the living cell, to monitor radiation, etc.
The steady progress in detector science is strongly motivated by the variety of interesting problems
and advanced applications. Some of them, like for
example the detection of cancerous tumors at the
early stages of their formation are very di$cult
ones and require, in addition to precise detectors,
better or multiple imaging modalities.
Presently, the detection threshold for small tumors, at maximal permissible radiation doses, is
de"ned not only by the detector's performance but
also mainly by the di!erence in X-ray attenuation
secondary electrons per neutron-induced triton
crossing the CsI "lm; the SEE yield from coated Gd
is about 10 folds lower. Multipliers placed on both
sides of Li and Gd converters coated with CsI
emitters, are expected of providing respective detection e$ciencies of the order of 30% and 45% for
0.25 nm wavelength neutrons [20,93]. The complex
technique of preparing Li converters has been
recently mastered [94], paving the way towards the
construction of fast, large-area thermal neutron
imaging detectors. Such devices, equipped with advanced micro-pattern multipliers, are projected by
us and by others [95] for neutron scattering experiments and radiography.
6. Concluding remarks
Gas avalanche detecors, conceived and heavily
employed in particle physics, have numerous applications in life sciences. New detection concepts
-12-
Fig. 15. Film-screen radiography of a Pt}CMdex}Z treated
mouse (top) and of a normal mouse (bottom). The liver of the
treated mouse, loaded with about 80 ppm of Pt, is clearly delineated and its border is easily outlined, while that of the normal
mouse is indistinguishable from the gastrointestinal system.
Note the darker area of the bladder, due to partial clearance of
the contrast agent [97].
between malignant and normal tissue, which is very
small.
A possible solution would consist of increasing
the detection contrast by modifying the tumor
characteristics prior to mammography or radiography, making it more `opaquea to X-ray
radiation [96]. This is achieved by targeting a
signi"cant amount of an e$cient contrast agent
into the cancerous tumor, in a selective way, similar
to that practiced in `speci"c drug-deliverya in
chemotherapy. The speci"city of the delivery relies
on physiological di!erences between normal and
cancerous
tissue.
This
`tumor-speci"c
radiographya technique is independent on the
detector type and complementary to detector
development. An example of an enhanced radiographic visualization of mouse liver, following
speci"c delivery of 80 ppm of platinum bound to
a targeting polymer, is shown in Fig. 15. E!orts are
currently made to develop e$cient delivery agents
into tumors and associated radiographic modalities
[97].
Acknowledgements
I would like to thank Dr. Rachel Chechik and
Guy Garty for their assistance in preparing this
manuscript.
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