Chapter 7

APPLIED RADIATION MEASUREMENTS

John M. Keller

Oak Ridge National Laboratory

INTRODUCTION

The four major radiation measurements systems found in most
radioanalytical chemistry laboratories include gas-flow proportional
counters for alpha and beta counting, gamma spectrometry systems, alpha
spectrometry systems, and liquid scintillation counters. The practical
application of each of these counting systems will be discussed in
detail. Specialized counting equipment for the measurement of neutrons
and x-rays will be introduced along some non-electronic techniques used
for radiation measurements.

DETECTOR SELECTION

The types of radiation detection systems employed by radioanalytical
chemistry laboratories have changed considerably over the last sixty
years and there has been significant improvements observed since the
early 80\'s. Advancements in the areas of material science, electronics,
and computer technology have resulted in more sensitive, reliable, and
user friendly laboratory instruments. The main discussions in this
section will focus on four primary radiation measurement systems
considered to be absolutely necessary for any modern counting
laboratory. These basic components of a counting laboratory include
gas-flow proportional counters, liquid scintillation counters (LSC),
gamma spectrometry systems and alpha spectrometry systems. These four
counting systems provide the tools to measure and identify most forms of
radiation from alpha and beta decay, including both charged particles
and gamma photons.

MEASUREMENT OF CHARGED PARTICLES

The most useful charged particles for radiochemical measurements include
the following particles emitted from the nucleus; negatrons (negative
electrons or ^-^ particles), positrons (positive electrons or ^+^
particles), and alpha particles which consist of two protons and two
neutrons or the nucleus of a ^4^He atom. There are other sources of
charged particles that an analyst needs to be aware of because of the
potential impact on radiochemical measurements. For example, charged
particles such as conversion and Auger electrons would be difficult to
distinguish from ^-^ particles with most beta counting systems and
without proper correction could bias final activity measurements

An important distinction between electrons emitted from the nucleus and
alpha particles is the energy distribution of the charged particles
emitted during the decay process. The energy from both ^-^ and ^+^
particles are distributed over an energy continuum ranging from 0 KeV to
some maximum energy, usually referred to as E~max~. In theory, the value
for E~max~ from a ^-^ or ^^ energy spectrum could be used to identify a
radionuclide, however, in practice it is difficult to get an accurate
enough measurement of E~max~ to be used for conclusive identification.
In contrast, the alpha particles are emitted at discrete energies that
are characteristic of the parent radionuclide and are much easier to use
in practice for radionuclide identification.

GAS-FLOW PROPORTIONAL COUNTERS

There are many different types of radiation detectors that use a
counting gas but all gas-filled or gas-flow counters are based upon a
similar design. All of these detector systems apply a high voltage
across a volume of gas in some type of containment system. The gas
containment systems for these types of counters are available as either
sealed detectors or can be designed to allow continuous flow of the gas.
When the high voltage is applied across the gas an electrical field is
formed between two electrodes. Then when radiation passes through this
electrical field with enough energy (\~30-35 eV) to ionize the counting
gas a flow of current is created between the electrodes. Based upon
detector design, this electrical signal can be measured as either a
current, voltage pulse, or accumulated charge which can be related back
to the incident radiation.

There are three basic types of gas-filled detectors

+-----------------------+--------------+-------------------------------+
| **Detector**          | **Applied**  | **Characteristics**           |
|                       |              |                               |
| **Type**              | *            |                               |
|                       | *Potential** |                               |
|                       |              |                               |
|                       | **(Volts)**  |                               |
+-----------------------+--------------+-------------------------------+
| Ionization            | \~250-500    | no gas amplification          |
|                       |              |                               |
| Chamber               |              | pulse height energy           |
|                       |              |                               |
|                       |              | useful for , , and            |
|                       |              |                               |
|                       |              | simple, low cost              |
|                       |              |                               |
|                       |              | e.g., pocket dosimeter        |
+-----------------------+--------------+-------------------------------+
| Proportional          | \~500-750    | gas amplification, 10^3^ -    |
|                       |              | 10^4^                         |
| Counter               |              |                               |
|                       |              | pulse height energy           |
|                       |              |                               |
|                       |              | useful for and                |
|                       |              |                               |
|                       |              | relatively simple, higher     |
|                       |              | cost                          |
|                       |              |                               |
|                       |              | e.g., laboratory instruments  |
+-----------------------+--------------+-------------------------------+
| Geiger-Müller         | \~1000-1500  | gas amplification,            |
|                       |              |                               |
| Counter               |              | no energy discrimination      |
|                       |              |                               |
|                       |              | useful for , , and            |
|                       |              |                               |
|                       |              | simple, low cost              |
|                       |              |                               |
|                       |              | e.g., survey meters           |
+-----------------------+--------------+-------------------------------+

Although, almost any gas will work with an ionization chamber, the
counting gas for a proportional counter must have specific
characteristics for proper operation. The most commonly used gas for a
proportional counter is a mixture of 90% Argon with 10% methane. This
counting gas mixture is referred to as P-10 counting gas and is readily
available at a reasonable cost. For a gas-flow Geiger-Müller or G-M
counter another standard counting gas used is called Q-gas, which is a
mixture of 97% helium and 3% isobutene.

The objective of this chapter is to concentrate on the application of
laboratory counting systems and although there are some useful
applications for ionization chambers and Geiger-Müller counters in the
laboratory, the real laboratory workhorse is the proportional counter;
specifically the gas-flow proportional counter. There are several types
of gas-flow proportional counters frequently used in modern counting
laboratories. The first is a classical 2 gas-flow proportional counter
that is usually available with a thin Mylar window or as a windowless
detector. These 2 counters are typically shielded with a few inches of
lead and may use guard detectors to reduce background from cosmic
radiation. The next type of system usually available is similar to the 2
counter but also includes an automatic sample changer. This type of
stack counter is normally used to count samples that do not require a
long counting time (i.e., 30 min. to a few hours) and are good for
counting plates, filters and smears up to about two inches in diameter.
A third type, which is more expensive and may only be found in larger
laboratories, uses multiple detectors to simultaneously count multiple
samples. The standard configurations available commercially have a
modular design with groups of four detectors arranged in a slide out
drawer. These multiple detector counting systems can be obtained with as
few as four detectors and some can be expanded up to 64 detectors,
however, due to the high cost most systems are sold with 8--16
detectors. This type of counting system is very useful for low-level
samples that require a long count time (i.e., 24-48 hours per sample),
however, the laboratory needs a fairly high sample load to justify the
high instrument cost.

The selection of an operating voltage is critical for proper operation
of a gas-flow proportional counter. A gas-flow proportional counter can
be operated at a bias voltage (\~600-800V with P-10 gas) where only
alpha particles are detected, which is commonly referred to as "alpha
only" mode. If the bias voltage is increased to \~1500 V with P-10 gas
the counter will respond to both alpha and beta particles. Operation at
this higher voltage is referred to as "simultaneous" or "+" mode. The
optimal operating voltage for each mode is determined from the
alpha/beta plateau curves which are plots of counting rate versus bias
voltage with an alpha or beta source. Although an alpha/beta plateau
curve can be generated with a mixed source containing both alpha and
beta activity, there are advantages to generating two sets of data with
both a "pure" alpha emitter (^238^Pu, ^244^Cm, etc.) and a "pure" beta
emitter (i.e., ^63^Ni, ^90^Sr, ^99^Tc, etc.). When the two sets of data
are plotted on the same graph the analyst can easily choose a bias
voltage for "alpha only" mode that is below the point where beta
particles just start to be detected. Selection of the bias voltage for
counting alpha particles only by this technique will minimize beta cross
talk. Selection of the bias voltage for + counting is not as critical
and should be about the midpoint of the + plateau.

Once the bias voltages are selected the next step is to determine the
counting efficiency for alpha and beta measurements. The alpha counting
efficiency with a proportional counter is essentially constant over the
energy range of 4-6 MeV which includes most of the alpha emitting
radionuclides of interest to most radiochemistry laboratories. *(Note:
This energy range includes all the naturally occurring alpha emitting
isotopes of radium, thorium, and uranium, along with the plutonium and
americium present environmentally from the global fallout of nuclear
weapon testing. There are also a few naturally occurring rare earth
elements (e.g., ^147^Sm, ^144^Nd, ^152^Gd, etc.) that emit alpha
particles in the 2-3 MeV range, however, these radionuclides have very
long half-lives and are not generally of much interest to most
radiochemistry laboratories.)* Since the alpha particle will deposit
most of its energy within the active volume of a proportional counter
the alpha efficiency is a function of the source-detector geometry minus
any losses due to mass absorption from the detector windows. The typical
counting efficiency, which is the ratio of count rate to disintegration
rate (cpm/dpm), for alpha counting with a windowless 2 gas-flow
proportional counter is \~51.5%. The additional 1.5% over the expected
50% from the 2 geometry is due to backscattering of the alpha particle
from the source mounting material, which for many laboratories is simply
a stainless steel plate. The alpha efficiency for a similar proportional
counter with a thin Mylar^™^ window would be in the range of 30-40% due
to mass absorption by the window material.

Beta particles are more penetrating than alpha particles and are less
affected by mass absorption. Beta particles may also escape the active
volume of a proportional counter before all the energy from the particle
is deposited within the detector. In addition, there is a higher
probability of backscatter with beta particles which, depending upon the
source backing material, can have a significant impact on the overall
counting response with a proportional counter. Due to both the energy
distribution of beta emissions and the interactions of the beta
particles with the counting gas, the beta counting efficiency for a
proportional counter can be fairly complex. In general, the beta
counting efficiency is more of a function of the [average]{.underline}
beta energy and can range from 50-90% with a windowless gas-flow
proportional counter.

Since the alpha efficiency is fairly constant over the range of 4-6 MeV,
the activity of any alpha emitting radionuclide in this energy range is
frequently calculated with an estimate of the counting efficiency
determined with ^241^Am (5.50 MeV). In other words, radionuclides
ranging from ^238^U (4.02 MeV) to ^244^Cm (5.80 MeV) would all have
about the same alpha counting efficiency with a gas-flow proportional
counter. However, in contrast to the alpha emitting radionuclides, the
beta counting efficiency with a gas-flow proportional counter is
significantly different for each beta emitting radionuclide. Therefore,
to quantify the activity for ^63^Ni (66.9 KeV) or ^99^Tc (292 KeV) would
require a separate standard of each radionuclide be available for the
determination of the respective counting efficiencies.

Two measurements routinely performed by radiochemistry laboratories on a
gas-flow proportional counter are gross alpha and gross beta. These
measurements are performed on a sample solution after the initial sample
preparation but before any chemical separations. The gross alpha and
beta results are routinely reported in units of activity with requires a
known counting efficiency. This is not a problem for gross alpha
measurements since the alpha counting efficiency is fairly constant over
a fairly wide energy range, however, as discussed previously the beta
counting efficiency is a function of the average beta energy so there is
no single beta counting efficiency to use for the calculation of the
gross beta activity. In practice, the beta efficiency for gross beta
measurements is commonly determined with a source prepared from ^90^Sr
(546 KeV) in secular equilibrium with ^90^Y (2281 KeV). This method only
provides a counter efficiency for the ^90^Sr/^90^Y pair, so any user of
the gross beta activity data needs to understand these limitations.

These gross activity measurements, along with the gamma spectrum from
the same sample preparation, can give the radiochemist a good starting
point to determine which additional analyses may be necessary. Also,
this information helps determine the aliquots needed for any required
radiochemical separations. These gross activities can also be used as a
data quality check to determine if all significant alpha and beta
emitters have been identified in the sample.

Samples for gross alpha and beta measurements frequently involve
dissolving solid materials such as a soil or the sample may simply be an
acidified ground water. In either case, there can be a significant
amount of dissolved solids in the sample solution. Also, some
radiochemical separation methods add carriers or other reagents that may
increase the dissolved solids within a sample solution. When these
sample solutions are evaporated onto a counting planchet there is a
buildup of solid residue of the surface of the plate that can reduce the
count rate due to self-absorption or mass-absorption. Self-absorption is
a source of counting error that must be minimized or accounted for with
suitable correction curves. A suggested upper limit for sample residue
weight on a counting plate or planchet is about 5 mg/cm^2^ for alpha
counting and about 10 mg/cm^2^ for beta counting. For example, an upper
limit for sample loading on a standard 2-inch (\~20 cm^2^) counting
planchet for alpha and beta counting would be 100 mg and 200 mg,
respectively. If the residue loading is above these limits the sample
should be diluted and a new planchet prepared. For sample loading below
these upper limits the data can be corrected for the reduced count rate
from the effects of self-absorption. Therefore, each gas-flow
proportional counter should have a set of self-absorption curves showing
the sample loading or residue (mg) vs. the counting efficiency (cpm/dpm)
prepared with both standard alpha and beta solutions. These
self-absorption curves can be used to extrapolate the correct counting
efficiency up to the recommended upper limit for sample loading.

SOURCE PREPARATION

There are many techniques for preparing a counting source suitable for
alpha and beta measurements and the analyst needs to understand the
advantages, disadvantages, and limitations of these different
approaches. These techniques range from very simple evaporation methods
to more involved procedures using electroplating or vacuum sublimation.
An ideal source for counting alpha or beta should have the following
characteristics:

-   Low solids content to minimize mass absorption problems, the
    material to be counted should be evenly distributed over the source
    backing material (i.e., thin stainless steel disk) in a very thin
    layer of low mass.

-   The alpha or beta activity needs to be firmly fixed to the counting
    source to avoid contamination of the counting system and to ensure
    the general safety of the analyst. Some laboratories cover or seal
    the source with a thin material such as Mylar^™^ to provide an
    additional layer of safety, especially when working with alpha
    emitters.

-   For beta counting the source backing material should be a thin low Z
    material to reduce the effect of backscatter.

-   For some counting systems (especially windowless gas-flow
    proportional counters) the source backing material needs to be
    conductive to avoid polarizing the source and creating a static
    charge within the counting gas volume.

LIQUID SCINTILLATION COUNTERS

Liquid scintillation counting (LSC) has been around for over fifty years
and is probably the most widely used technique for the measurement of
radioactivity. Traditional LSC applications in radiochemistry
laboratories are minor compared to the wide use LSC techniques in the
life sciences (biological and medical sciences) for the measurement of
labeled proteins, genetic materials (RNA and DNA), and other types of
biological tracer work. Liquid scintillation analysis (LSA) techniques
are also very useful for the analysis of smear samples collected for
health physics or radiation protection applications. Although liquid
scintillation counting can be used for the measurement of radioisotopes
in any liquid sample, LSC is the preferred method for both low energy
beta emitters (i.e., ^3^H, E~max~= 18.6 KeV) and volatile radionuclides
(e.g., ^99^Tc as TcO~4~^2-^) that could be easily lost preparing a
planchet for a gas-flow proportional counter. Some of the radionuclides
routinely measured by liquid scintillation counting are listed in the
following table.

+-----------------+-----------------+-----------------+-----------------+
| *               | **E~max~**      | **T~1/2~**      | **Volatile**    |
| *Radionuclide** |                 |                 |                 |
|                 | **(KeV)**       |                 | **Forms**       |
+-----------------+-----------------+-----------------+-----------------+
| ^3^H            | 18.6            | 12.3 y          | H~2~O, H~2~     |
+-----------------+-----------------+-----------------+-----------------+
| ^14^C           | 156.5           | 5730 y          | CO~2~           |
+-----------------+-----------------+-----------------+-----------------+
| ^32^P           | 1709.0          | 14.3 d          |                 |
+-----------------+-----------------+-----------------+-----------------+
| ^35^S           | 167.4           | 87.2 d          | SO~3~, H~2~S    |
+-----------------+-----------------+-----------------+-----------------+
| ^36^Cl          | 709.0           | 301000 y        | Cl~2~, HCl      |
+-----------------+-----------------+-----------------+-----------------+
| ^45^Ca          | 258.0           | 163 d           |                 |
+-----------------+-----------------+-----------------+-----------------+
| ^63^Ni          | 66.9            | 100 y           | NiCl~2~,        |
|                 |                 |                 | Ni(NO~3~)^2^    |
+-----------------+-----------------+-----------------+-----------------+
| ^99^Tc          | 292.0           | 213000 y        | TcO~4~^2-^      |
+-----------------+-----------------+-----------------+-----------------+

The counting efficiency for charged particles with a liquid
scintillation counter can approach 4 geometry. The LSC efficiency for
alpha particles is typically \> 95%, whereas the counting efficiency for
beta particles is generally \> 90% for betas with an E~max~ \> 250 KeV.

Sample preparation for Liquid Scintillation Counting

To achieve good analytical results on a Apure@ beta emitter it is
usually necessary to chemically separate the radionuclide of interest
from all other radioactive species prior to analysis.

Distillation, wet oxidation, combustion (dry oxidation)

erenkov counting

The emission of erenkov radiation occurs when a charged particle enters
a media (i.e., water) at a speed that exceeds the velocity of light in
the same media. The energy lost as the velocity of the charged particle
slows to the speed of light is emitted as erenkov radiation, which at
high enough activity levels can be visually observed as a bluish-white
light. This blueish-white light can be used to measure high energy beta
particles with a liquid scintillation counter. The production of erenkov
radiation is a function of the refractive index of the medium. For
water, which has a refractive index of 1.33, the threshold beta energy
to produce erenkov radiation is \~263 KeV. Therefore, the beta emitting
radionuclides needs to have an E~max~ well above the threshold of 263
KeV for erenkov counting to be practical.

ALPHA SPECTROMETRY

GAMMA SPECTROMETRY

It was discovered early in the history of radiation measurements that
certain materials would give off a flash of light when exposed to
radiation. It was also recognized early on that some materials were more
sensitive to different forms of radiation. As technology improved these
scintillation materials were coupled to photomultiplier tubes to measure
the flashes of light. Inorganic scintillation detectors based on alkali
halides were used in most of the early gamma spectrometry systems. The
most common scintillation crystals used in these systems were sodium
iodide doped with thallium and were referred to as NaI(Tl) detectors.

Lithium drifted semiconductor detectors were the next generation of
detectors for gamma spectrometry. These detectors were based on both
germanium and silicon and were designated as Ge(Li) and Si(Li),
respectively. The lithium drifted detectors provided higher resolution
but at a much higher cost for an efficiency similar to the NaI(Tl)
detectors. Historically the Ge(Li) detectors were much more useful for
gamma spectrometry and the Si(Li) detectors were better for low energy
gamma-rays and x-rays. The Si(Li) detectors continue to be the best
choice for x-ray spectrometry in modern counting laboratories.

In addition to cost, the three important performance characteristics of
a semiconductor detector for gamma spectrometry include the efficiency,
resolution, and peak-to-Compton ratio. These detector characteristics
should be optimized for each particular application. For example, a
NaI(Tl) detectors have excellent efficiency but poor resolution and may
be ideal for simple spectra or an online monitor system but are not very
useful for applications involving complex spectra.

The formal definition for resolution is E/E, or the peak width divided
by the peak centroid energy. The resolution is typically reported as a
percentage for NaI(Tl) detectors and as the full-width-half-maximum
(FWHM) for semiconductor detectors. The FWHM is the width of the peak in
units of energy measured at ½ of the peak height. The resolution is very
important for the interpretation of complex spectra and radionuclide
identification. The FWHM for a full energy peak is directly proportional
to the peak energy.

A standard method as described in the IEEE Standard Test Procedures
(ANSI/IEEE 325-1996) was developed to define the relative detection
efficiencies for coaxial detectors which is suppose to permit the
comparison of detectors for different sources. The relative efficiency
for a coaxial detector is defined as follows:

$$\varepsilon_{R} = \frac{\varepsilon_{\text{HPGe}}}{\varepsilon_{\text{NaI}(\text{Tl})}}$$

where,

> ~HPGe~ = absolute efficiency at 1332 KeV for a coaxial detector
> measured 25 cm from the source.
>
> ~NaI(Tl)~ = absolute efficiency at 1332 KeV for a 3\" x 3\" NaI(Tl)
> scintillation detector measured 25 cm from the source.

The absolute efficiency is the ratio of the number of counts measured in
the 1332 KeV photopeak divided by the number of gamma rays at 1332 KeV
emitted from the ^60^Co source during the same time interval.

A comparison of resolution and efficiency

+-----------+-----------+----------+----------+-----------+-----------+
| Energy    |           | NaI(Tl)  |          | HPGe      |           |
|           |           |          |          |           |           |
| (KeV)     |           | (30mm x  |          | (\~30%    |           |
|           |           | 30mm)    |          | Relative  |           |
|           |           |          |          | Eff.)     |           |
+-----------+-----------+----------+----------+-----------+-----------+
|           |           | ~abs~    | FWHM     | ~abs~     | FWHM      |
|           |           |          |          |           |           |
|           |           | (%)      | (KeV)    | (%)       | (KeV)     |
+-----------+-----------+----------+----------+-----------+-----------+
| 60        | (^241^Am) | 50       | \~8      | 22.4      | 1.2       |
|           |           |          | (14%)    |           |           |
+-----------+-----------+----------+----------+-----------+-----------+
| 662       | (^137^Cs) | 32       | \~50     | 5.0       | 1.5       |
|           |           |          | (7.5%)   |           |           |
+-----------+-----------+----------+----------+-----------+-----------+
| 1332      | (^60^Co)  | 25       | \~95     | 2.5       | 1.9       |
|           |           |          | (7.1%)   |           |           |
+-----------+-----------+----------+----------+-----------+-----------+

The efficiencies listed in this table represent nominal values that
would be expected for a point source at contact with the detector, which
is basically a 2 geometry.

The third performance characteristic that needs to be considered before
selecting a semiconductor detector for gamma spectrometry is the
Peak-to-Compton ratio. The Peak-to-Compton ratio is an important
indication of the detectors performance at lower energies, this is
especially important in the presence of higher energy gamma radiation
such as ^60^Co (1332 KeV) where the Compton scattering become more
predominate within the detector volume. The resulting Compton continuum
starts at \~256 KeV below the photopeak and increases the spectral
background across the entire low energy region below the Compton edge.
For ^60^Co the Compton edge is observed at \~1076 KeV (e.g., 1332-256).
Most detector manufactures report the Peak-to-Compton ratio using the
^60^Co photopeak at 1332 KeV, which would be the count rate measured at
1332 KeV divided by the average count rate at 1076 KeV. This ratio
typically ranges from 40 to 70 for coaxial HPGe detectors currently
produced. In general, for detectors with a similar relative efficiency,
the Peak-to Compton ratio is higher for detectors with better
resolution.

The most common type of detector currently used for gamma spectrometry
is the coaxial high purity germanium (HPGe) detector. The HPGe detectors
are usually available with either a p-type or n-type crystal. The
efficiency of a p-type detector typically drops off quickly below \~100
KeV, whereas, the n-type detectors can actually be used down to about 3
KeV with a beryllium window. The best choice for these types of
detectors depends on the application.

CHARACTERISTICS OF A GAMMA SPECTRUM

There are three ways a gamma photon can interact with matter and each
type of interaction must be understood to properly interpret gamma
spectral data with either a NaI(Tl) or HPGe detector. The three modes of
interaction are photoelectric absorption, Compton scattering, and pair
production. The probability or cross-section for each of these
interactions is a function of the energy for the incident gamma photon.
For gamma spectrometry the photoelectric absorption is the preferred
mode of interaction since it results in a full-energy photopeak in the
spectrum that is used to quantify and identify the radionuclide. If the
photon interacts with the detector material by Compton scattering there
is a loss of energy by the incident photon that is transferred to a
recoil electron. Since this energy loss is a function of the scatter
angle, and all angles are possible, this scattering interaction results
in the Compton continuum that is observed in the gamma spectrum as an
increase in the spectral background. The third mode of interaction is
pair production which can only occur with incident gamma photons that
have energy greater than 1022 KeV. The process of pair production
absorbs all the incident gamma photon energy and results in the creation
of an electron-positron pair within the active volume of the detector.
Then as the positron loses energy it will combine with an electron which
results in the annihilation of the charged particles and the release of
two annihilation photons that have an energy of 511 KeV each. Depending
upon the detector size it is possible for either one or both of the 511
KeV annihilation photons to escape from the active volume of the
detector. If both of the annihilation photons escape the active detector
volume a "double escape peak" would be observed in the gamma spectrum
that would be located 1022 KeV below the full-energy peak. It is also
possible to have a "single escape peak" located 511 KeV below the full
energy peak if only one of the annihilation photons escape the detector
volume.

To ensure a low background detectors used for gamma spectrometry are
typically surrounded with at least lead shielding. Also, the crystals
used in both NaI(Tl) and HPGe detectors are encased in an aluminum
shell. These various materials around the detector can interact with the
radiation from a sample source and cause additional background structure
in the gamma spectrum. The same interactions that occur within the
detector are also experienced with these materials surrounding the
detector; however, these interactions external to the detector have a
different impact on the spectral background. For example, Compton
scattering within the lead shielding will result in a broad backscatter
peak that is located in the lower energy region (\< \~250 KeV) of the
spectrum. The location of the peak maximum (*E~BS~*) for this
backscatter peak, which is observed at a backscatter angle of , can be
estimated from the following equation:

$$E_{\text{BS}} = \frac{E_{\gamma}}{1 + (2\frac{E_{\gamma}}{\text{511}}\text{KeV})}$$

Where, *E* is the energy in KeV for the incident gamma photon. For
instance, the backscatter peak for ^137^Cs (662 KeV) would be \~184 KeV.

For a gamma photons with an energy greater than 1022 KeV, the photon
interaction with the lead shielding can result in pair production. The
annihilation of the positron produced during this pair production
process can result in a 511 KeV gamma that escapes from the lead
shielding and ends up being superimposed on the gamma spectrum. Another
source of pair production within the lead shielding is cosmic radiation.
The sample source counted for the gamma spectrum can also contribute to
the 511 KeV gamma peak if there are any positron emitters present in the
sample. This source of 511 KeV gamma-rays results for the absorption of
the positron, and subsequent annihilation, by the aluminum casing
surrounding the detector crystal.

The photoelectric absorption of radiation by the lead shielding and
other material surrounding the detector can result in characteristic
x-rays in the lower energy region of the gamma spectrum. For example,
the lead K and K x-rays are almost always present as part of the
background in a gamma spectrum. There are commercially available gamma
detector shields that are lined with cadmium and copper to reduce the
level of lead x-rays in the background. In addition to the lead x-rays,
characteristic x-ray from any material located near the source and
detector can end up in the background spectrum.

Environmental and other low level samples may require very long counting
times to satisfy the low detection limits needed for regulatory
applications and other special projects. The background spectra
associated with these long counting periods will show additional
structure from the natural background radiation surrounding the gamma
spectrometer system. Naturally radioactive trace elements present as
impurities (i.e., uranium decay daughters, ^40^K, etc.) in the shielding
material may contribute to the background and can become significant
with long counting periods. One final source of background radiation
that must be considered with long counting periods are prompt -rays from
the thermal neutron capture of cosmic ray neutrons within the detector
and shielding materials. A excellent indicator for this type of
background problem is cadmium which is frequently present in graded
shields (Cd-Cu lined lead) for gamma spectrometry systems. One of the
naturally occurring isotopes of cadmium (^113^Cd) has an extremely high
cross section (\~20,000 barns) for thermal neutron capture and will
yield a 558 KeV prompt -ray. There can also be prompt -rays from the
lead and even the germanium crystal, but well below the level
experienced with cadmium. In general, any background structure due to
prompt -rays should be significantly lower than the 511 KeV annihilation
peak and the 1460 KeV from ^40^K.

X-RAY SPECTROMETRY

solid state detectors (energy dispersive)

crystal spectrometers (wavelength dispersive)

MEASUREMENT OF NEUTRONS

Thermal neutrons

^10^B(n,a)^7^Li, ^3^He(n,p)^3^H, ^6^Li(n,a)^3^H

> Fast Neutrons

VARIOUS FACTORS THAT MAY IMPACT RESULTS

Background, Scatter

Mass Absorption

Interference

NUCLIDE IDENTIFICATION

> Gamma emitters
>
> Alpha emitters
>
> APure@ Beta emitters

REFERENCES

Evans, R. D., *The Atomic Nucleus*, New York: McGraw-Hill, 1958.

Horrocks, D. L., *Applications of Liquid Scintillation Counting*, New
York: Academic, 1974.

Friedlander, G., Kennedy, J. W., Macias, E. S., Miller, J. M., Nuclear
and Radiochemistry, 3^rd^ ed. New York: Wiley-Interscience, 1981.

Tsoulfanidis, N., *Measurement and Detection of Radiation*, New York:
McGraw-Hill, 1983.

Knoll, G. F. *Radiation Detection and Measurement*, 2^nd^ ed. New York:
Wiley, 1989.

Ehmann, W. D. and Vance, D. E., *Radiochemistry and Nuclear Methods of
Analysis*, New York: Wiley-Interscience, 1991.