7030

7030 COUNTING INSTRUMENTS*
7030 A. Introduction
Radiochemical analytical instruments operate on the principle
that the energy expended by a radiation event is detected and
recorded by an instrument suitable for the type of radiation
emitted. The counting instruments described below are com-
monly found in radioanalytical laboratories. Other less common
instruments may also be used for analysis. Instrument back-
ground and fractional counting efficiency must be measured and
integrated into the sample calculations. These characteristics can
be compared with historical data and used to evaluate instrument
stability.
7030 B. Description and Operation of Instruments
1. Gas-Flow Proportional Counters
Gas-flow proportional counters (GPCs) detect and quantify
alpha- and beta-emitting radionuclides. If the sample contains
multiple radionuclides, the instrument will detect and count all
emissions regardless of their radionuclide source or energy. The
analysis may be made radionuclide-specific by combining this
detection method with a radiochemical separation to segregate
the desired radionuclide. The radionuclide (
226
Ra, Th, U, etc.) is
usually precipitated and mounted as a thin layer (generally less
than 5 mg/cm
2
) on planchets.
a. Types of GPCs commonly used for environmental analyses:
GPCs operate on the principle that radioactive decay particles,
such as alpha and beta particles, cause ionization in a gas-filled
detector. The liberated electrons are accelerated in an electrical
field placed across the detector cell, thereby amplifying the
signal. The signal is collected at the detector’s anode, creating a
pulse of electricity that then can be processed. The combined
magnitude of all pulses collected from one decay particle is
proportional to the energy deposited in the detector by that
particle—hence the term proportional count.
Select the detector fill gas to optimize amplification. Although
other gases have been used successfully, the most frequently
used mixture in low-background GPCs is called P-10; it consists
of 90% argon and 10% methane.
Alpha particles may be determined by operating the detector at
the alpha plateau, where the voltage is low enough that the
detector does not respond to beta particles. Detectors are fre-
quently operated at the higher-voltage beta plateau, where the
detector responds to both alpha and beta particles. Because each
pulse produced is proportional to the energy deposited in the
active volume of the detector by the decay particle, pulse-height
analysis (PHA) can be used to discriminate between the larger
pulses associated with alpha particles and the smaller pulses
associated with beta particles.
b. Internal and thin-windowed GPCs: Internal and thin-win-
dowed GPCs are commonly used for low-level environmental
measurements. With internal GPCs, the test source is introduced
directly into the counting chamber, which is then filled with the
counting gas. With thin-window GPCs, the test source is posi-
tioned outside the active volume of the detector, next to an
entrance window generally constructed of a very thin Mylar
TM
film. The entrance window allows decay particles emitted from
the source to enter the active volume of the detector.
Thin-windowed GPCs are marginally less sensitive to low-
energy decay particles than internal GPCs due to the less favor-
able counting geometry associated with positioning the source
outside the active volume of the detector. Additionally, there are
losses due to the decay particles’ interaction with air and the
window between the test source and the active volume. In
contrast, windowed GPCs are much less subject to counting-
chamber contamination and damage due to loose residues, losses
due to residual moisture, and corrosion due to vapors commonly
associated with test sources. Prepare all GPC test sources in a
manner that minimizes friable particles and vapors from mois-
ture or solvents because these can contaminate or damage the
detector chamber (internal GPCs) or detector entrance window
(thin-windowed GPCs).
c. Detection system components: The instrument consists of a
counting chamber, pre-amplifier, amplifier, scaler, high-voltage
power supply, timer, pulse-shape discriminator, and register. Use
the specified counting gas and accessories, make adjustments for
sensitivity, and operate in accordance with both the method and
the manufacturer’s instructions.
d. Background-reduction measures: Low backgrounds are im-
portant for environmental analysis. One technique for minimiz-
ing background activity is constructing the detection system with
low-background materials. Two other techniques used for back-
ground reduction—passive and active shielding—are designed
to minimize the effect of external radiation (e.g., cosmic radia-
tion as well as gamma and X-ray radiation from terrestrial
sources, building materials, and radioactive sources at the labo-
ratory). A passive shield typically consists of a 5- to 10-cm-thick
layer of lead surrounding the detector. An active shield involves
a “guard” detector placed next to the primary detector so any
radiation that impinges on the primary detector must pass
through the guard. The primary detector is operated in anti-
coincidence mode (i.e., any counts registered concurrently in the
* Approved by Standard Methods Committee, 2011.
Joint Task Group: Robert T. Shannon.
1

primary and guard detectors are not registered as valid sample
counts). With these measures, modern low-background GPCs
can routinely achieve background count rates on the order of
0.05 to 0.2 alpha counts per minute (cpm) and 0.5 to 2 beta cpm.
e. Availability: GPC detection systems are commercially
available in a number of assembled configurations. Detectors
may be configured alone or in arrays (e.g., of 4, 8, and 10
detectors), and may incorporate automatic sample changers. The
most commonly encountered detector diameter is approximately
5 cm. Ten-centimeter detectors are also available; they are most
frequently used to count larger sources (e.g., air filters).
f. Instrument setup:
1) Radioactive sources—See Section 7020A.3h.
2) Voltage plateau (alpha or beta)—A voltage plateau is a
curve used to determine an operating voltage that will provide
reproducible count rates. It is obtained by placing an appropriate
alpha or beta source in each detector and varying the detector
voltage until the change in the count rate reaches a minimum.
Further increases in voltage will produce little change in overall
detector response until the plateau region is exceeded. CAUTION:
Continuous discharge at too high a voltage will damage the
instrument.
The alpha plateau is generally a region in the 300 to 800 V
range (depending on the detector) where the instrument responds
to an alpha source but not a beta source. The beta plateau is
generally a region in the 1000 to 1700 V range where the
instrument responds to both alpha and beta radiation.
The relative counting rate (ordinate) is plotted against the
voltage (abscissa). The operating point is generally established
on the plateau curve at a point marginally beyond the knee of the
curve (see Figure 7030:1), where instrument response to a point
source varies by less than 5% per 100-V change in voltage.
Distributed planar test sources are the most common test source
counted in most laboratories. When plateaus are obtained using
distributed sources, the slope of the curve at the operating
voltage may nominally exceed a target of 5%/100 V. Check the
plateau to ensure that the optimal operating voltage has not
changed by performing instrument response and background
checks after each change of counting gas.
Most modern GPC software contains utilities for performing
plateau curve acquisition and establishing operating voltage set-
points. Consult the instrument manual for setup details.
3) Pulse-height discriminator setup—GPCs use pulse-height
analysis (PHA) to discriminate between pulses originating from
alpha or beta particles based on the amplitude of the pulses they
produce. Due to the physics of the measurement, alpha pulses are
generally much larger than beta pulses. Crosstalk refers to the
unavoidable misclassification of pulses during simultaneous
counting for alpha and beta. For example, pulses resulting from
alpha decays may be misidentified as beta pulses (alpha-to-beta
crosstalk), or pulses resulting from beta decays may be misiden-
tified as alpha pulses (beta-to-alpha crosstalk). Alpha particles
are much more strongly attenuated than beta particles, so they
often result in smaller pulses that may be misclassified as beta
pulses. In other words, alpha-to-beta crosstalk will be much
larger than beta-to-alpha crosstalk.
Setting the pulse-height discriminator establishes the instru-
ment’s crosstalk response. Some instruments require that dis-
criminators be set up after the operating voltage is established,
while others are configured at the factory. Consult the instrument
manual for setup details. As an overall measurement strategy,
setting the discriminators to minimize beta-to-alpha crosstalk
Figure 7030:1. Shape of counting rate-anode voltage curves. Key: (a) and (b) are for internal proportional counter with P-10 gas; (c) is for end-window
Geiger-Mueller counter with Geiger gas (NOTE: Beta losses depend on radiation energy and thickness of window and air path.)
COUNTING INSTRUMENTS (7030)/Description and Operation of Instruments
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COUNTING INSTRUMENTS (7030)/Description and Operation of Instruments

(e.g., to ⬃0.1%) will decrease the magnitude of crosstalk cor-
rections and improve the quality of simultaneous alpha and beta
measurements.
Once the operating voltage and discriminator settings have
been established, they must be documented and locked down.
Changing either parameter invalidates all previously established
performance check data and instrument calibrations, and the
instrument will have to be recalibrated.
4) Background (alpha or beta)—Most detectors have a back-
ground count rate that is due to several factors: cosmic radiation;
radionuclides contained in instrument materials; the counting
room’s construction material; electrical/electronic noise; and/or
the proximity of radioactive sources. The background is roughly
proportional to the size or mass of the counting chamber or
detector but can be reduced by shielding or anti-coincidence
guard circuitry (see 7030B.1d).
a) Determination of background count rate—The background
count rate is subtracted from each measurement to determine the
“net” activity of the sample test source. Determine the back-
ground count rate by counting an empty planchet in the counting
chamber in the same configuration as the sample test source. The
background count duration should be as long as or longer than
the counts from which it will be subtracted.
b) Control of backgrounds—Verify the continuing stability of
the background count rate during the count by tracking back-
ground performance via control charts. Also, evaluate method
blank control charts for persistent high or low bias in results (i.e.,
absolute bias), then determine the cause and correct it. If the
cause is intrinsic to the method, a correction factor (independent
of QC samples) may be developed and applied to eliminate
absolute bias as long as the additional correction’s uncertainty is
reflected in the combined standard uncertainty reported for the
sample result.
5) Initial calibration (alpha, beta, or radionuclide specific)—
The purpose of initial calibration is to empirically derive count-
ing efficiency factors for a geometry that matches those of the
test samples, thus ensuring that the correction for efficiency is
accurate. Parameters that must match include radionuclide, ma-
trix composition, and density/thickness (which causes self-ab-
sorption of alpha or beta particles in the sample matrix). Each
detector’s detection efficiency must be determined separately.
Initial calibration is procedure-dependent; see individual proce-
dures for specific instructions. Absorption curve values do not
need to be re-established after initial calibration as long as
continuing calibration (source check) response is monitored reg-
ularly and indicates that the instrument has remained stable
between calibration and sample measurements.
6) Continuing calibration (source check)—Verify instrument
stability at the operating voltage by counting separate check
sources for alpha and beta (where applicable) on each detector.
Monitor the major and minor channel count rates (i.e., for an
alpha source, alpha is the major channel and beta is the minor
channel) for each source counted and plot the results on a
control/tolerance chart to demonstrate the continuing stability of
the detector efficiency and crosstalk (see Section 7020A). The
tests and acceptance criteria used should be addressed in the
laboratory quality manual. If the checks do not provide evidence
of continuing instrument stability, the detector should be taken
offline until the problem is fixed and the instrument is recali-
brated, or until continuing checks demonstrate that maintenance
has not changed instrument response.
7) Sample counting—Place the prepared sample test source in
the detector in a geometry consistent with initial calibration. For
internal GPCs, ensure that there is electrical contact between
planchet and chamber, and flush the chamber with counting gas.
Count for a preset duration or a preset count to give the desired
counting uncertainty (see Section 7020D).
2. Alpha Scintillation Counter
Alpha scintillation counters detect and quantify alpha-emitting
radionuclides. If the sample contains multiple radionuclides, the
instrument will detect and count all alpha emissions regardless of
their radionuclide source or energy. The analysis may be made
more radionuclide-specific by using a radiochemical separation
to isolate the desired radionuclide. This radionuclide (
226
Ra, Th,
U, etc.) is usually precipitated and mounted as a thin layer
(ⱕ5 mg/cm
2
) on planchets. Radon-222 (radium-226 by radon
emanation) also can be counted in alpha scintillation cells in a
modified sample chamber (see Section 7500-Ra.C).
a. Principle and uses: An alpha particle interacts with zinc
sulfide phosphor (which contains a silver activator), exciting the
scintillator’s atoms. When the atoms return to the ground state,
they emit the energy as visible light. This process is called
scintillation. The light is further transformed into an electrical
current via an attached PMT, which amplifies the electrical
current into a measurable pulse. The pulses trigger a scaler,
which registers each pulse as a “count.” Depending on the
amount of radionuclide present and statistics required, count a
sample long enough to obtain required sensitivity. The counter is
calibrated with a thin-layered precipitate of a radionuclide or an
electrodeposited radionuclide.
b. Components: The alpha scintillation counter consists of a
light-tight sample chamber with a phosphor detector coupled to
a photomultiplier tube (PMT) and sample holder, a high-voltage
supply, an amplifier-discriminator, a scaler, and readout capabil-
ity. Generally, the PMT’s window has a larger diameter than that
of the samples. Put the phosphor between the sample and the
PMT. The distance between the sample surface and PMT face is
usually about 3 to 5 mm. Arrange the phosphor so the PMT
window is in optical contact with the scintillator. Under these
conditions, the counting efficiency can be 35 to 40%. For details
on operating and calibrating an alpha scintillation counter, see
manufacturer’s instructions.
c. Performance verification:
1) Radioactive source—See Section 7020A.3h.
2) Plateau—In accordance with manufacturer recommenda-
tions, use a radioactive source to find the operating voltage
where the count rate is consistent over some specified voltage
range.
3) Background—Two backgrounds are evaluated with alpha
scintillation counters: electronic instrument background and
chamber background. The electronic instrument background
characteristically measures the PMT’s electronic noise. It is
determined periodically (e.g., annually) by counting with an
empty chamber and should vary from 0 to 1 cpm.
The chamber background is counted with the zinc sulfide
phosphor or scintillation cell in place. This background is caused
by contaminated instrument parts, counting-room construction
COUNTING INSTRUMENTS (7030)/Description and Operation of Instruments
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COUNTING INSTRUMENTS (7030)/Description and Operation of Instruments

materials, and/or nearby sources of radioactivity. When measur-
ing chamber background, use a duration as long or longer than
the longest sample counting duration; this background is sub-
tracted from gross sample results.
4) Initial calibration—The purpose of initial calibration is to
determine the scintillation counting efficiency based on sample
thickness (which causes absorption of some alpha or beta parti-
cles in the sample matrix) or cell geometry (in the case of
scintillation cells). Initial calibration is method-dependent; see
individual methods for specific instructions.
Absorption curve values do not need to be re-established after
initial calibration as long as the check source response is mon-
itored regularly and control charts indicate instrument stability.
Recalibrate scintillation cells periodically.
5) Continuing calibration (source check)—See 7030B.1f6).
6) Sample counting—Place prepared sample in the counting
chamber according to manufacturer’s instructions. Take the fol-
lowing precautions:
• ensure that counting chamber is light-tight;
• ensure that PMT is not exposed to direct light while high
voltage is applied;
• let sample chamber adapt to the dark before starting count;
• count for a preset duration or count, to give the desired
counting precision;
• ensure that sample is in contact with the phosphor; and
• count samples in a geometry consistent with initial calibra-
tion.
3. Liquid Scintillation Counters
A liquid scintillation spectrometer system uses one or more
PMTs to count the number of scintillations (photons of light)
emitted from a sample vial. The number of photons produced in
the scintillator is proportional to the particle’s initial energy. This
information helps determine the specific radionuclide present.
a. Principle and uses: A sample containing beta- (or alpha-)
emitting radionuclides is mixed in a liquid scintillator. Each
beta (or alpha) particle transfers kinetic energy to solvent and
phosphor molecules in the liquid scintillation cocktail via a
large number of collisions. As the excited molecules return to
ground state, they emit photons of light that are detected by
PMTs in the instrument. Although the number of photons
produced for any given decay event is proportional to the
energy of the decay particle, it is notable that alpha particles
transfer energy at roughly one-tenth the rate of beta particles
and thus result in lower energy pulses. The PMTs amplify the
incoming light into an electronic pulse. In most instruments
currently in use at radiochemistry laboratories, background is
significantly reduced by operating two PMTs in coincidence.
This means that signal must be received from both PMTs
within the same time interval (e.g., within 20 ns of each other)
to yield a valid pulse. Coincident signals are summed, digi-
tized according to their pulse-height and then stored in a
multichannel analyzer in a channel corresponding to the en-
ergy of the original radiation.
Although the instrument outputs spectrometric data, the
continuous nature of beta spectra combined with the instru-
ment’s limited resolution limits the instrument’s use for spec-
trometric determinations. Most frequently, LS counting is
combined with chemical separations to perform sensitive,
radionuclide-specific determinations of alpha and beta emit-
ters. Liquid scintillation is commonly used to determine the
activities of some alpha (e.g.,
222
Rn and daughters) and most
low-energy beta emitters (e.g.,
3
H and
14
C). Counting effi-
ciencies approaching 100% can be obtained from alpha- and
higher-energy beta-emitting radionuclides.
b. Components: The system consists of a liquid scintillator
[organic scintillator(s) diluted with an appropriate solvent], a
polyethylene or glass vial, a liquid scintillation counter (with one
or more PMTs coupled to a single- or multichannel analyzer),
and a readout device.
c. Performance verification:
1) Radioactive sources—See Section 7020A.3h.
2) Background—Consider three backgrounds when dealing
with liquid scintillation spectrometers: electronic or instrument
background, chamber background, and background for subtrac-
tion.
a) Electronic or instrument background—Periodically deter-
mine background using an empty counting chamber and a dark
vial. Some manufacturers supply a dark or “black” vial [a count-
ing vial filled with black material (e.g., graphite)] for this pur-
pose.
b) Chamber background—The ”unquenched background” is
determined using a vial supplied by the manufacturer. This
background is used only to assess daily instrument performance
and should never be subtracted from sample results.
c) Background for subtraction—A method-specific back-
ground is determined and subtracted from each sample test
source result using vials identical to those used for samples,
except that they are free of activity. The background subtraction
sample is not a QC sample; it is part of the analytical process and
should be prepared independently of QC reagent or method
blanks. When analyzing tritium, for example, mix the same type
of vial with cocktail from the same production lot, using the
same sample-to-cocktail ratio with an aliquant of purified “dead”
water (e.g., a fossil source of water from which tritium has
already decayed).
3) Initial calibration—Before sample analysis, the counting
conditions may be optimized for measurements with constant
quench. For variable-quench measurements, counting regions
may be optimized if they take into account variation in the region
of interest as a function of quench. Some analysts maximize the
figure of merit (FOM):
E
2
/B
where:
E⫽efficiency and
B⫽background count rate.
Optimum counting conditions should not be re-established
after initial calibration because changing the analysis window
will affect the efficiency that is determined. Once optimum
conditions have been selected (via a pure source), determine
sample counting efficiency by one of several methods (see
Sections 7500-
3
H and 7500-Rn): standard additions, quench
curve, or prepared laboratory standard.
4) Continuing calibration (source check)—See Section
7500-Rn for specific calibration procedures for radon-222 by
liquid scintillation counting.
COUNTING INSTRUMENTS (7030)/Description and Operation of Instruments
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COUNTING INSTRUMENTS (7030)/Description and Operation of Instruments

5) Sample counting—Place prepared samples in the counting
chamber, let samples adapt to the dark, and count for the preset
count or duration needed to obtain the desired sensitivity.
As with other counting techniques, deal with any interferences
to the method. Such interferences include quenching (chemical,
color, or particle quenching), chemiluminescence/photolumines-
cence, static electricity, scintillation volume variations, sample
homogeneity, background, multiple radionuclides, and phase
separation.
4. Alpha Spectrometers
a. Principle and uses: An alpha spectrometer detects, identi-
fies, and quantifies specific alpha-emitting radionuclides. The
radionuclides should be chemically separated from samples and
deposited as a thin layer on filter papers or electrodeposited on
metal disks. When a sample’s alpha emissions cause ionization
in a solid-state detector, a pulse of current is produced that has
an amplitude proportional to the alpha particle energy.
The current is collected, amplified, sorted according to depos-
ited alpha energy, and displayed on a multichannel analyzer.
Chemical yield tracers should be added to each sample to ensure
accurate quantitation.
b. Components: An alpha spectrometer consists of a sample
chamber (with detector, detector/sample holder, and vacuum
chamber); a mechanical vacuum pump; detector bias voltage
supply; preamplifier; amplifier; multichannel analyzer (with
ADC and memory storage); and data readout capability. For
details on calibration and operations, see the operations manual
provided by the manufacturer.
c. Performance verification:
1) Radioactive source—See Section 7020A.3h.
2) Detector voltage—Set detector operating voltage in accor-
dance with the manufacturer’s recommendations.
3) Background—Determine the chamber background count
using a clean filter or metal disk identical to those on which
samples are mounted. The counting duration should be at least as
long as the longest sample counting duration. Monitor the back-
ground count rate (which will be subtracted from the sample
count rate) via a control chart to demonstrate its stability over the
most recent 3 to 10 counts and to identify potential long-term
trends that could prevent the detector from providing results that
meet detection limit requirements.
4) Initial calibration—The purpose of initial calibration is to
establish a detector’s energy calibration and counting efficiency.
Use a standard source, as described in Section 7020A.3h. Before
beginning calibration, ensure that the counting vacuum chamber
is well sealed and has an adequate, consistent vacuum source.
Inadequate vacuum will result in a distorted alpha spectrum.
Inconsistent vacuum can cause significant drift of the alpha
spectra.
5) Continuing calibration (source check)—Verify detector ef-
ficiency and energy calibration stability. A mixed alpha-emitting
isotopic source containing at least two distinct peaks (see Section
7020A.3h) allows the energy calibration to be confirmed by
permitting identification of specific alpha-emitting radionu-
clides. For criteria, see 7030B.1f6).
6) Sample counting—Place prepared sample in the counting
chamber according to manufacturer’s instructions. Take the fol-
lowing precautions:
• ensure that the air in the counting chamber is slowly evac-
uated to the level of vacuum defined in the laboratory’s SOPs;
• count for a preset duration or preset count, to give the
desired counting precision;
• ensure that the sample is properly positioned on the sample
holder; and
• after counting, release vacuum slowly to minimize the risk
of contaminating chamber and detector.
5. Gamma Spectrometers
Gamma spectrometry identifies and quantifies specific energy
photons (gamma rays), thereby quantitating specific radionu-
clides.
a. Principle and uses: Gamma rays from a sample enter the
sensitive volume of the detector and cause ionization. The lib-
erated charge is converted into a voltage pulse whose amplitude
is proportional to the photon energy. Pulses are stored in se-
quence in finite energy-equivalent increments over the desired
spectrum. After sample counting, the accumulated pulses over a
certain area result in photopeaks that can be identified and
attributed to specific radionuclides based on their characteristic
energies.
b. Components: A gamma spectrometer consists of a detector,
pre-amplifier and detector bias supply, pulse-height analyzer
system, data readout capability, and shielded sample enclosure.
The pulse-height analyzer system consists of a linear amplifier,
an analog-to-digital converter (ADC), memory storage, and a
logic control mechanism. The logic-control capabilities allow
data storage in various modes and data display or recall. For
details on operating and calibrating a gamma spectrum analyzer,
see manufacturer’s instructions.
Commonly used gamma detectors consist of intrinsic or high-
purity germanium (HPGe). Lithium drifted germanium [Ge(Li)]
detectors are less robust, and many have been replaced by HPGe
detectors. Sodium iodide [NaI(Tl)] and silicon [Si(Li)] detectors
are sometimes encountered, but have limited application (pri-
marily to screen samples or analyze single gamma-emitting
radionuclides). Germanium detectors offer excellent resolution
and reasonable to good efficiency (depending on the application
and cost). NaI(Tl) detectors have poor resolution and reasonable
to good efficiency at reasonable cost. Si(Li) detectors have good
efficiency and reasonable resolution for low-energy X-rays (10
to 200 KeV). Other detectors are available. See Table 7030:1 for
typical resolution for several detector types.
c. Performance verification:
1) Radioactive sources—See Section 7020A.3h.
2) Detector voltage—Set detector operating voltage according
to manufacturer’s recommendations.
3) Background—Background counts are the result of a con-
taminated sample chamber and cosmic, natural, and worldwide
fallout in the detector shielding. Determine the detector back-
ground count using an appropriate volume of reagent-grade
water in the desired geometry. The background counting dura-
tion should be at least as long as the longest sample counting
duration.
4) Initial calibration—Establish a detector’s energy calibra-
tion, shape, and counting efficiency per sample geometry. Use a
standard source, as described in Section 7020A.3h.
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