Tsoulfanidis N. Measurement and detection of radiation
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258
MEASUREMENT
AND DETECTION OF RADIATION
7.5.4 Germanium Lithium-Drifted [Ge(Li)
1
Detectors
Ge(Li) detectors are not made anymore; they have been replaced by pure
germanium crystals. Historically, Ge(Li) detectors dominated the gamma detec-
tion field for about 15 years (until about 1985). Since there may still be some
Ge(Li)'s operating, a brief discussion is presented in this section.
Ge(Li) detectors are made from horizontally grown or pulled single crystals
of germanium.
As
the crystal is grown, it is doped with acceptor impurities such
as indium, gallium, or boron, and becomes a p-type semiconductor. Germanium
crystals may be cut to length and shaped by a variety of means, including the use
of diamond wheels or band saws. In these mechanical operations, great care
must be taken not to fracture the brittle material.
Lithium drifting in germanium follows the same approach as in silicon. The
deposition and diffusion of lithium are accomplished by one of the methods
discussed in the previous section. The ohmic contacts are made by electrolytic
deposition of gold,13 by using gallium-indium14 or mercury-indium,15 or by ion
implantation.16 The drifting process itself takes place at a lower temperature
(<
60"
C)
than for silicon, with the germanium diode in air" or immersed in a
liquid maintained at its boiling point.ls
After the drifting process has been completed, the detector is mounted on a
cryostat and is always kept at a low temperature (liquid nitrogen temperature
-
77 K). Keeping the Ge(Li) detector at a low temperature is much more
critical than for a Si(Li) detector. The mobility of the lithium atoms in germa-
nium is so high at room temperature that the detector will be ruined if brought
to room temperature even for a short period of time. If this happens, the
detector may be redrifted, but at a considerable cost.
7.5.5 Germanium (Gel Detectors
The production of high-purity germanium (HPGe) with an impurity concentra-
tion of 1016 atoms/cm3 or less has made possible the construction of detectors
without lithium
drifting.1g-21 These detectors are now designated as Ge, not
HPGe, and are simply formed by applying a voltage across
a
piece of germa-
nium. The sensitive depth of the detector depends on the impurity concentration
and the voltage applied, as shown in Fig. 7.23.
The major advantage of Ge versus
Ge(Li) detectors is that the former can
be stored at room temperature and cooled to liquid nitrogen temperature (77 K)
only
when in use. Cooling the detector, when in use, is necessary because
germanium has a relatively narrow energy gap, and at room or higher tempera-
tures a leakage current due to thermally generated charge carriers induces such
noise that the energy resolution of the device is destroyed.
Germanium detectors are fabricated in many different geometries, thus
offering devices that can be tailored to the specific needs of the measurement.
Two examples, the coaxial and the well-type detector, are shown in Fig. 7.24.
SEMICONDUCTOR DETECTORS
259
11
I
1
I
1
o8
1
o9
1ol0
loll
Impurity concentration, at/cm3
Figure
7.23
Depletion depth as a function of impurity concentration and applied voltage for planar
diodes of high-purity germanium (from Ref. 21).
More details about these detectors are presented in Chap.
12
in connection with
y-ray spectroscopy.
7.5.6
CdTe and
HgI,
Detectors
The major disadvantage of lithium-drifted detectors
is
the requirement for
continuous cooling. In the case of Ge detectors, the requirement for cooling
incident
radiation
-
incident
radiation
Figure
7.24
Two examples of geometries used for Ge Detectors;
(a)
coaxial;
(b)
well type (from
Canberra Nuclear,
Edition Nine Instruments Catalog).
260
MEASUREMENT
AND
DETECTION
OF
RADIATION
during operation is also a disadvantage. Cooling requires a cryostat, which
makes the counter bulky and thus impossible to use in cases where only a small
space is available; another disadvantage is the cost of continuously buying liquid
nitrogen. There is a great incentive, therefore, to develop semiconductor detec-
tors that can be stored and operated at room temperature. Two materials that
have been studied and show great promise for the construction of such detectors
are CdTe and
HgI,.22-35
A
comprehensive review of the state-of-the-art (until
1978) for both materials can be found in Ref.
36.
Successful detectors using CdTe or HgI,
have been constructed with
thickness up to 0.7 mm and area 100 mm2 (as of
1978).~' These detectors are
small in size, compared to
Si(Li) or Ge(Li) detectors, but the required detector
volume depends on the application. For CdTe and HgI,, the favored applica-
tions are those that require a small detector volume: monitoring in
measurement of activity in nuclear power plants,38 medical portable scanning,39
or medical imaging devices.40 Although the detector volume is small, efficiency is
considerable because of the high atomic number of the elements involved
(Table
7.2).
The energy needed for the production of an electron-hole pair is
larger for CdTe and HgI, than it is for Si and Ge; as a result, the energy
resolution of the former is inferior to that of the latter (see also Chap.
12).
But
CdTe and HgI, detectors are used in measurements where their energy resolu-
tion is adequate while, at the same time, their small volume and, in particular,
their room-temperature operation offers a distinct advantage over Si(Li) and
Ge(Li) detectors.
7.6
RADIATION DAMAGE TO SEMICONDUCTOR DETECTORS
The fabrication and operation of a semiconductor detector are based on the
premise that one starts with a perfect crystal containing a known amount of
impurities. Even if this is true at the beginning, a semiconductor detector will
suffer damage after being exposed to radiation. The principal type of radiation
damage is caused by the collision of an incident particle with an atom.
As
a
result of the collision, the atom may be displaced into an interstitial position,
thus creating an interstitial-vacancy pair known as the
Frenkel
defect.
A
recoiling
Table
7.2
Properties of Si,
Ge,
CdTe, and
HgI,
Atomic Energy
Material number gap (eV)
Energy needed
to form the
pair (eV)
Si 14
1.106
(300K) 3.65 (300 K)
Ge 32 0.67 (77 K)
2.96
(77
K)
CdTe 48 and 52
1.47
(300K) 4.43 (300 K)
HgI, 80 and 53
2.13 (300 K) 4.22 (300 K)
SEMICONDUCTOR
DETECTORS
261
Table
7.3
Particle Fluence That Causes Significant Radiation Damage
Heavy ions
Alphas Fast neutrons
(particles/m2) (a/m2)
(n/m3)
Junction detectors 1012
Si(Li)
Ge(Li) or Ge
atom may have enough energy to displace other atoms; therefore an incident
particle may produce many Frenkel defects.
Crystal defects affect the performance of the detector because they may act
as trapping centers for electrons and holes or they may create new donor or
acceptor states. New trapping centers and new energy states change the charge
collection efficiency, the leakage current, the pulse risetime, the energy resolu-
tion, and other properties of the detector. The changes are gradual, but the final
result is shortening of the detector lifetime.
Electrons and photons cause negligible radiation damage compared to
charged particles and neutrons. Heavier and more energetic charged particles
cause more damage than lighter and less energetic
particle^.^,^'
Also, the
damage is not the same for all detector types. Table
7.3
gives the fluences that
cause considerable radiation damage for different detectors and bombarding
particles.
Ge detectors are not affected
by
gammas, but they are damaged by the
neutrons in a mixed n-y field.
PROBLEMS
7.1 What is the probability that an electron energy state in Ge will be occupied at temperature
T
=
300
K
if the energy state is greater than the Fermi energy by
2
eV?
7.2 Repeat Prob. 7.1 for
T
=
77 K.
7.3
The energy gap for diamond is
7
eV. What temperature will provide thermal energy
(kT)
equal
to that amount?
7.4
What should be the maximum thickness of the gold layer covering the front face of a surface
barrier detector used for the measurement of 10-MeV alphas, if the energy loss of the alphas
traversing the layer should be less than
0.1
percent of the kinetic energy?
7.5 Repeat Prob. 7.4 for 6-MeV electrons.
7.6
The thickness of the gold layer covering the front face of a semiconductor detector may be
measured by detecting particles entering the detector at two different angles. Calculate that
thickness if alphas that enter in a direction perpendicular to the front face register as having energy
4.98 MeV, but those that enter at a
45' angle register as having energy 4.92 MeV.
7.7
What is the average distance traveled in Si by a 50-KeV gamma before it has an interaction?
What is the corresponding distance in Ge?
7.8 Lithium has been drifted in germanium at
50"
C
under a reverse bias of 500 V for
2
weeks. What
is your estimate of the drifting depth?
[
gLi
=
1.5
X
m2/(v s)]
262
MEASUREMENT
AND
DETECTION
OF RADIATION
7.9
A parallel beam of 0.5-MeV gammas is normally incident upon 2-mm-thick-crystals of Si, Ge,
CdTe, and HgI,. What fraction of photons will interact at least once in each crystal?
BIBLIOGRAPHY
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G. L.,
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&
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and Haiman,
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Press, New York, 1969.
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G.
F.,
Radiation Detection and Measurement,
2nd ed., Wiley, New York, 1989.
Price, W.
J.,
Nuclear Radiation Detection,
McGraw-Hill, New York, 1964.
Tait, W. H.,
Radiation Detection,
Butterworth, London, 1980.
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A.
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Semiconductor Detectors,
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Helenberg, H. W., and Mann, H. M.,
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SEMICONDUCTOR DETECTORS
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CHAPTER
EIGHT
RELATIVE
AND
ABSOLUTE MEASUREMENTS
8.1
INTRODUCTION
An
absolute measurement
is one in which the exact number of particles emitted
or the exact number of events taking place is determined, for example,
1.
Determination of the activity of
a
radioactive source, i.e., measurement of the
number of particles emitted by the source per second
2.
Determination of the neutron
flux
(neutrons per square meter per second) at
a certain point in a reactor
3.
Measurement of the number of neutrons emitted per fission
4.
Measurement of the first cross section for a nuclear interaction
A
relative measurement
is one in which the exact number of particles
emitted or the exact number of events taking place is not determined. Instead, a
"relative" number of particles or events is measured, a number that has a fixed,
but not necessarily known, relationship to the exact number, for example,
1.
Determination of the
G-M
plateau. The relative change of the number of
particles counted versus
HV
is measured. The exact number of particles
emitted by the source is not determined; in fact, it is not needed.
2.
Determination of half-life by counting the decaying activity of an isotope.
The relative change of the number of atoms versus time is measured. The
exact number of nuclei decaying per second is not needed.
266
MEASUREMENT AND DETECTION OF RADIATION
3.
Measurement of the fission cross section for 239~~, based on the known
fission cross section for
235~.
4. Determination of the variation of the neutron
flux
along the axis of a
cylindrical reactor. The relative change of the
flux
from point to point along
the axis of the reactor is measured, and not the exact number of neutrons per
square meter per second.
Relative measurements are, in most cases, easier than absolute measure-
ments. For this reason, investigators tend to perform the very minimum of
absolute measurements and use their results in subsequent relative measure-
ments. One of the most characteristic examples is the determination of the
value of nuclear cross sections. Absolute measurements have been performed
for very few cross sections. After certain cross sections have been measured,
most of the others may be determined relative to the known ones.
This chapter discusses the factors that should be taken into account in
performing relative and absolute measurements. Assume that there is a source
of particles placed a certain distance away from a detector (Fig. 8.1) and that the
detector is connected to a pulse-type counting system. The source may be
located outside the detector as shown in Fig.
8.1,
or it may be inside the detector
(e.g., liquid-scintillation counting and internal-gas counting), and may be isotropic
(e.g., particles emitted with equal probability in all directions) or anisotropic
(e.g., parallel beam of particles). Both cases will be examined. Let
S
=
number of particles per second emitted by the source
r
=
number of particles per second recorded by the scaler
It is assumed that the counting rate
r
has been corrected for dead time and
background, if such corrections are necessary. The measured rate
r
is related to
S
by
r
=
fifif3
.a.
frJ
(8.1)
where the
f
factors represent the effects of the experimental setup on the
measurement. These factors may be grouped into three categories, to be
discussed in detail in the following sections.
Detector aperture
or window
Figure
8.1
A
point isotropic source counted
by
a
pulse-type counting system.
RELATIVE
AND
ABSOLUTE MEASUREMENTS
267
1.
Geometry effects.
The term
geometry
refers to size and shape of source (point,
parallel beam, disk, rectangular), size and shape of detector aperture (cylin-
drical, rectangular, etc.), and distance between source and detector.
2.
Source effects.
The size and, in particular, the way the source is made may
have an effect on the measurement. Whether the source is a solid material or
a thin deposit evaporated on a metal foil may make a difference. The effect
of source thickness is different on charged particles, gammas, and neutrons.
3.
Detector effects.
The detector may affect the measurement in two ways. First,
the size and thickness of the detector window (Fig. 8.1) determine how many
particles enter the detector and how much energy they lose, as they traverse
the window. Second, particles entering the detector will not necessarily be
counted. The fraction of particles that is recorded depends on the efficiency
of the detector (see Sec. 8.4.2).
8.2
GEOMETRY EFFECTS
The geometry may affect the measurement in two ways. First, the medium
between the source and the detector may scatter and may also absorb some
particles. Second, the size and shape of the source and the detector and the
distance between them determine what fraction of particles will enter the
detector and have a chance to be counted.
8.2.1
The Effect of the Medium between Source and Detector
Consider a source and a detector separated by a distance
d
(Fig. 8.2). Normally,
the medium between the source and detector is air, a medium of low density.
For measurements of photons and neutrons, the air has no effect. If the source
emits charged particles, however, all the particles suffer some energy loss, and
some of them may be scattered in or out of the detector (Fig. 8.2). If this effect
I'
Detector
Figure
8.2
The medium between the source
and
the
detector may scatter and/or absorb particles emitted by
the source.