ASTM D3648-23

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D3648 − 23
Standard Practices for the
Measurement of Radioactivity
This standard is issued under the fixed designation D3648; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope D2459 Test Method for Gamma Spectrometry of Industrial
Water and Industrial Waste Water (Withdrawn 1986)
1.1 These practices cover a review of the accepted counting
D3084 Practice for Alpha-Particle Spectrometry of Water
practices currently used in radiochemical analyses. The prac-
D3085 Practice for Measurement of Low-Level Activity in
tices are divided into four sections:
Water (Withdrawn 1987)
Section
D3370 Practices for Sampling Water from Flowing Process
General Information 6 – 11
Alpha Counting 12 – 22 Streams
Beta Counting 23 – 33
D3649 Practice for High-Resolution Gamma-Ray Spec-
Gamma Counting 34 – 41
trometry of Water
1.2 The general information sections contain information
D7902 Terminology for Radiochemical Analyses
applicable to all types of radioactive measurements, while each
D8293 Guide for Evaluating and Expressing the Uncertainty
of the other sections is specific for a particular type of
of Radiochemical Measurements
radiation.
IEEE/ASTM SI 10 American National Standard for Metric
Practice
1.3 The values stated in SI units are to be regarded as
standard. No other units of measurement are included in this
2.2 ANSI Standards:
standard.
ANSI N42.14 Calibration and Use of Germanium Spectrom-
eters for the Measurement of Gamma-Ray Emission Rates
1.4 This standard does not purport to address all of the
of Radionuclides
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- 2.3 Other Documents:
JCGM 100:2008 Evaluation of measurement data—Guide to
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use. the expression of uncertainty in measurement
1.5 This international standard was developed in accor-
3. Terminology
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
3.1 Definitions:
Development of International Standards, Guides and Recom-
3.1.1 For definitions of terms used in these practices, refer
mendations issued by the World Trade Organization Technical
to Terminology D1129 and Terminology D7902. For an expla-
Barriers to Trade (TBT) Committee.
nation of the metric system, including units, symbols, and
conversion factors, see IEEE/ASTM SI 10.
2. Referenced Documents
4. Summary of Practices
2.1 ASTM Standards:
D1066 Practice for Sampling Steam 4.1 The practices are a compilation of the various counting
techniques employed in the measurement of radioactivity. The
D1129 Terminology Relating to Water
D1943 Test Method for Alpha Particle Radioactivity of important variables that affect the accuracy or precision of
counting data are presented. Because a wide variety of instru-
Water
ments and techniques are available for radiochemical
laboratories, the types of instruments and techniques to be
selected will be determined by the information desired. In a
These practices are under the jurisdiction of ASTM Committee D19 on Water
and are the direct responsibility of D19.04 on Methods of Radiochemical Analysis.
Current edition approved June 1, 2023. Published July 2023. Originally approved
in 1978. Last previous edition approved in 2014 as D3648 – 14 which was The last approved version of this historical standard is referenced on
withdrawn January 2023 and reinstated in June 2023. DOI: 10.1520/D3648-23. www.astm.org.
2 4
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM 4th Floor, New York, NY 10036, http://www.ansi.org.
Standards volume information, refer to the standard’s Document Summary page on Available from www.bipm.org/utils/common/documents/jcgm/JCGM_100_
the ASTM website. 2008_E.pdf.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D3648 − 23
simple tracer application using a single radioactive isotope processing. If low levels of radiation are to be determined, very
having favorable properties of high purity, energy, and ample large samples and complex counting equipment may be nec-
activity, a simple detector will probably be sufficient, and essary.
techniques may offer no problems other than those related to 6.3.1 More detailed discussions of the problems and inter-
reproducibility. The other extreme would be a laboratory
ferences are included in the sections for each particular type of
requiring quantitative identification of a variety of radiation to be measured.
radionuclides, preparation of standards, or studies of the
characteristic radiation from radionuclides. For the latter, a
7. Apparatus
variety of specialized instruments are required. Most radio-
7.1 Location Requirements:
chemical laboratories require a level of information between
7.1.1 The apparatus required for the measurement of radio-
these two extremes.
activity consists, in general, of the detector and associated
4.2 A basic requirement for accurate measurements is the
electronic equipment. The latter usually includes a stable
use of accurate standards for instrument calibration. With the
power supply, preamplifiers, a device to store or display the
present availability of good standards, only the highly diverse
electrical pulses generated by the detector, or both, and one or
radiochemistry laboratories require instrumentation suitable
more devices to record information.
for producing their own radioactive standards. However, it is
7.1.2 Some detectors and high-gain amplifiers are tempera-
advisable to compare each new standard received against the
ture sensitive; therefore, changes in pulse amplitude can occur
previous standard.
as room temperature varies. For this reason, it is necessary to
provide temperature-controlled air conditioning in the counting
4.3 Thus, the typical laboratory may be equipped with
room.
proportional or Geiger-Mueller counters for beta counting,
7.1.3 Instrumentation should never be located in a chemical
sodium iodide or germanium detectors, or both, in conjunction
laboratory where corrosive vapors will cause rapid deteriora-
with multichannel analyzers for gamma spectrometry, and
tion and failure.
scintillation counters suitable for alpha- or beta-emitting radio-
nuclides.
7.2 Instrument Electrical Power Supply—Detector and elec-
tronic responses are a function of the applied voltage;
5. Significance and Use
therefore, it is essential that only a very stable, low-noise
electrical supply be used or that suitable stabilization be
5.1 This practice was developed for the purpose of summa-
included in the system.
rizing the various generic radiometric techniques, equipment,
and practices that are used for the measurement of radioactiv-
7.3 Shielding:
ity.
7.3.1 The purpose of shielding is to reduce the background
count rate of a measurement system. Shielding reduces back-
GENERAL INFORMATION
ground by absorbing some of the components of cosmic
radiation and some of the radiations emitted from material in
6. Experimental Design
the surroundings. Ideally, the material used for shielding
6.1 In order to properly design valid experimental should itself be free of any radioactive material that might
procedures, careful consideration must be given to the follow- contribute to the background. In practice, this is difficult to
ing; achieve as most construction materials contain at least some
6.1.1 Radionuclide to be determined, naturally radioactive isotopes (such as K, members of the
uranium and thorium series, and so forth). The thickness of the
6.1.2 Relative activity levels of interferences,
shielding material should be such that it will absorb most of the
6.1.3 Type and energy of the radiation,
soft components of cosmic radiation. This will reduce cosmic-
6.1.4 Original sample matrix, and
ray background by approximately 25 %. Shielding of beta- or
6.1.5 Required accuracy.
gamma-ray detectors with anticoincidence systems can further
6.2 Having considered 6.1.1 – 6.1.5, it is now possible to
reduce the cosmic-ray or Compton scattering background for
make the following decisions:
very low-level counting.
6.2.1 Chemical or physical form that the sample must be in
7.3.2 Detectors have a certain background counting rate
for radioassay,
from naturally occurring radionuclides and cosmic radiation
6.2.2 Chemical purification steps,
from the surroundings; and from the radioactivity in the
6.2.3 Type of detector required,
detector itself. The background counting rate will depend on
6.2.4 Energy spectrometry, if required,
the amounts of these types of radiation and on the sensitivity of
6.2.5 Length of time the sample must be counted in order to
the detector to the radiations.
obtain statistically valid data,
7.3.3 In alpha counting, low backgrounds are readily
6.2.6 Isotopic composition, if it must be determined, and
achieved since the short range of alpha particles in most
6.2.7 Size of sample required. materials makes effective shielding easy. Furthermore, alpha
detectors are quite insensitive to the electromagnetic compo-
6.3 For example, gamma-ray measurements can usually be
nents of cosmic and other environmental radiation.
performed with little or no sample preparation, whereas both
alpha and beta counting will almost always require chemical 7.4 Care of Instruments:
D3648 − 23
7.4.1 The requirements for and advantages of operating all anode in each chamber. The source is mounted on a thin
counting equipment under conditions as constant and repro- supporting film between the two halves, and the counts
ducible as possible have been pointed out earlier in this section.
recorded in each half are summed. A 10 % methane-90 %
The same philosophy suggests the desirability of leaving all
argon gas mixture can be used; however, pure methane gives
counting equipment constantly powered. This implies leaving
flatter and longer plateaus and is preferred for the most
the line voltage on the electrical components at all times. The
accurate work. The disadvantage is that considerably higher
advantage to be gained by this practice is the elimination of the
voltages, about 3000 V, rather than the 2000 V suitable for
start-up surge voltage, which causes rapid aging, and the
methane-argon, are necessary. As with all gas-filled propor-
instability that occurs during the time the instrument is coming
tional counters, very pure gas is necessary for very high
up to normal temperature.
detector efficiency. The absence of electronegative gases that
7.4.2 A regularly scheduled and implemented program of
attach electrons is particularly important since the negative
maintenance is helpful in obtaining satisfactory results. The
pulse due to electrons is counted in this detector. Commercial
maintenance program should include not only checking the
chemically pure (cp) gases are ordinarily satisfactory, but they
necessary operating conditions and characteristics of the
should be dried for best results. A high-voltage power supply
components, but also regular cleaning of the equipment.
for the detector, an amplifier, discriminator, and a scaler
7.5 Sample and Detector Holders—In order to quantify
complete the system.
counting data, it is necessary that all samples be presented to
7.6.1.2 To convert counting rate to disintegration rate, the
the detector in the same “geometry.” This means that the
principal corrections required are for self-absorption in the
samples and standards should be prepared for counting in the
source and for absorption in the support film. The support film
same way so that the distance between the source and the
should be as thin as practicable to minimize absorption of beta
detector remains as constant as possible. In practice, this
particles emitted in the downward direction. Polyester film
usually means that the detector and the sample are in a fixed
with a thickness of about 0.9 mg/cm is readily available and
position. Another configuration often used is to have the
easily handled. However, it is too thick for accurate work with
detector in a fixed position within the shield, and beneath it a
the lower energy beta emitters. For this purpose, thin films
shelf-like arrangement for the reproducible positioning of the
2 2
(.5 μg ⁄cm to 10 μg ⁄cm ) are prepared by spreading a solu-
sample at several distances from the detector.
tion of a polymer in an organic solvent on water. VYNS (1),
7.6 Special Instrumentation—This section covers some ra-
Formvar (2), and Tygon (3) plastics have been used for this
diation detection instruments and auxiliary equipment that may
purpose.
be required for special application in the measurement of
7.6.1.3 The films must be made electrically conducting
radioactivity in water.
(since they are a part of the chamber cathode) by covering them
7.6.1 4π Counter:
2 2
with a thin layer (2 μg ⁄cm to 5 μg ⁄cm ) of gold or palladium
7.6.1.1 The 4π counter is a detector designed for the
by vacuum evaporation. The absorption loss of beta particles in
measurement of the absolute disintegration rate of a radioactive
source by counting the source under conditions that approach the film must be known. Published values can be used, if
a geometry of 4π steradians. Its most prevalent use is for the
necessary, but for accurate work an absorption curve using
absolute measurement of beta emitters (1, 2). For this purpose,
very thin absorbers should be taken (1). The “sandwich”
a gas-flow proportional counter similar to that in Fig. 1 is
method, in which the film absorption is calculated from the
common. It consists of two hemispherical or cylindrical
decrease in counting rate that occurs when the source surface
chambers whose walls form the cathode, and a looped wire
is covered with a film of the same thickness as the backing
film, is suitable for the higher beta energies.
The boldface numbers in parentheses refer to a list of references at the end of 7.6.1.4 The source itself must be very thin and deposited
this standard.
uniformly on the support to obtain negligible self-absorption.
Various techniques have been used for spreading the source;
for example, the evaporation of Ni-dimethylglyoxime onto
the support film (1), the addition of a TFE-fluorocarbon
suspension (3), colloidal silica, or insulin to the film as
spreading agents, and hydrolysis (2). Self-absorption in the
source or mount can be measured by 4π beta-gamma coinci-
dence counting (4, 5). The 4π beta counter is placed next to a
NaI(Tl) detector, or a portion of the chamber wall is replaced
by a NaI(Tl) detector, and the absolute disintegration rate is
evaluated by coincidence counting (6, 7). By adding a suitable
beta-gamma tracer, the method has been used for pure beta as
well as beta-gamma emitters (8). Accurate standardization of
pure low-energy beta emitters (for example, Ni) is difficult,
and the original literature should be consulted by those
FIG. 1 The 4π-Counting Chamber inexperienced with this technique.
D3648 − 23
7.6.1.5 Photon (gamma and strong X-ray) scintillation 7.6.3.2 In a modification of the internal gas counter, scin-
counters with geometries approaching 4π steradians can be tillation counting has been used in place of gas-ionization
constructed from NaI(Tl) crystals in either of two ways. A well counting. The inner walls of the chamber are coated with a
crystal (that is, a cylindrical crystal with a small axial hole scintillation material and the radioactive gas introduced. An
covered with a second crystal) will provide nearly 4π geometry optical window is made a part of the chamber, and the counting
for small sources, as will two solid crystals placed very close is done by placing this window on a multiplier phototube to
together with a small source between them. The counts from detect the scintillations. This system is particularly useful for
both crystals are summed as in the gas-flow counter. The counting radon gas with zinc sulfide as the scintillator. Addi-
deviation for 4π geometry can be calculated from the physical tional details on internal gas counting may be found in Watt
dimensions. For absolute gamma-ray counting, the efficiency and Ramsden (10).
of the crystal for the gamma energy being measured and the
7.6.4 Spectrometers and Energy-Dependent Detectors:
absorption in the crystal cover must be taken into account.
7.6.4.1 The availability of energy-dependent detectors (de-
Additional information on scintillation counting is given in
tectors whose output signal is proportional to the energy of the
7.6.4. The liquid scintillation counter is also essentially a 4π
radiation detected) that are easy to operate and maintain and
counter for beta particles, since nearly all the radiations are
have good resolution makes it possible to measure not only the
emitted into and interact with the detecting medium.
total activity of a radioactive sample but the energy spectrum
7.6.2 Low-Geometry Counters—This type of instrument is
of the nuclear radiations emitted. Nuclear spectrometry is most
particularly useful for the absolute counting of alpha particles.
useful for alpha particles, electromagnetic radiation (gamma
The alpha emitter, in the form of a very thin solid source, is
and X-rays), and conversion electrons, since these radiations
placed at a distance from the detector such that only a small
are emitted with discrete energies. Beta spectra have more
fraction (<1 %) of the alpha particles are emitted in a direction
limited use since beta particles are emitted from a nucleus with
to enter the counter. This solid angle is obtained from the
a continuous energy distribution up to a characteristic maxi-
physical measurements of the instrument. The space between
mum (E-max), making a spectrum containing several different
the source and the detector is evacuated to eliminate the loss of
beta emitters difficult to resolve into its components. The
alpha particles by absorption in air. The detector can be any
advantages of spectrometric over total activity measurements
counter that is 100 % efficient for all alpha particles that enter
of radioactive sources are increased selectivity, sensitivity, and
the sensitive volume—a gas-flow proportional counter with a
accuracy because nuclide identification is more certain, inter-
window that is thin (less than approximately 1 mg/cm )
ference from other radioactive nuclides in the sample is
compared to the range of the alpha particles or the semicon-
diminished or eliminated, and counter backgrounds are re-
ductor alpha detector with a 1 mg ⁄cm covering. The advan-
duced since only a small portion of the total energy region is
tages of this instrument for absolute alpha counting are that: (1)
used for each radiation. The detectors for alpha spectra are
the effect of absorption of alpha particles in the source itself is
gridded ion-chambers and silicon semiconductor detectors.
kept to a minimum since only particles that travel the minimum
These are described in Practice D3084. A variety of semicon-
distance in the source enter the detector (particles that have
ductors can be purchased, and these detectors have essentially
longer paths in the source are emitted at the wrong angle, and
replaced ion-chambers for alpha spectrometry, although the
(2) backscattered alpha particles (those that are emitted into the
chambers have the advantages of high efficiency (nearly 50 %)
source backing and are reflected back up through the source)
for large-area sources.
lose sufficient energy so that they cannot enter the detector.
7.6.4.2 The principal detectors used for gamma-ray spec-
One such instrument is described in Curtis et al. (9).
trometry are high purity germanium semiconductors (HPGe)
7.6.3 Internal Gas Counters:
and thallium-activated sodium iodide scintillation crystals
7.6.3.1 The internal gas counter is so named because the (NaI(Tl)). For X-rays and very low energy gamma rays, HPGe
radioactive material, in the gaseous state, is placed inside a and gas-filled thin (approximately 1 mg/cm ) window propor-
counting chamber and thus becomes part of the counting gas tional counters are used. Sodium iodide is hygroscopic, so the
itself. It is useful for high-efficiency counting of weak beta- crystal must be hermetically sealed, and the entire crystal-
and X-ray emitters. The radiations do not have to penetrate a phototube package must be light-tight. The complete spectrom-
counter window or solid source before entering the sensitive eter also requires a high-voltage power supply for the photo-
volume of a detector. The counter may be an ionization tube (usually operated at 800 V to 1000 V), a preamplifier,
chamber, or it may be operated in the Geiger-Mueller or linear amplifier, pulse-height analyzer, and output recorder.
proportional mode. Most present-day instruments are of the The crystal is packaged in aluminum or stainless steel. The
latter type, and they generally take the form of a metal or portion of the cover through which gamma rays enter is
metal-coated glass cylinder as a cathode with a thin anode wire normally thinner than the rest of the package in order to reduce
running coaxially through it and insulated from the cylinder low-energy photon attenuation. Sodium iodide crystals are
ends. A wire through the wall makes electrical contact to the available in a large range of sizes and shapes, from 25 mm by
cathode. The counter has a tube opening through which it may 25 mm cylinders to hemispheres and cylinders at least 305 mm
be connected to a gas-handling system for filling. The purity of in diameter. Information on the types of crystal packages and
the gas is important for efficient and reproducible counting, mountings that can be used is available from the manufactur-
particularly in the proportional mode. ers.
D3648 − 23
7.6.4.3 Germanium detectors are junction-type semiconduc- maximum” (FWHM)—or as the ratio of the FWHM (in units
tor devices in which a large sensitive region has been produced of energy) to the energy of the peak centroid. The later quantity
is typically expressed as a percentage. This is shown graphi-
by the refinement of germanium to extremely low impurity
levels. The crystal functions as a “solid ion chamber” when a cally in the gamma-ray spectrum in Fig. 2. While the FWHM
increases with increasing energy, the percent resolution im-
high voltage is applied. To provide the semiconductor function,
and in order to obtain optimum resolution, the detector must be proves. The standard for comparison is usually percent reso-
lution of the 0.662 MeV gamma ray emitted in the decay
operated at low temperatures to reduce thermal noise. At room
temperature, sufficient free electrons will be present in the of Cs. Good NaI(Tl) detectors have resolutions in the range
of 6.5 % to 7 % for Cs. Detection efficiency for the same
crystal to obscure the measurement of gamma and X-rays (but
geometry and window thickness is a function of several
not of alpha particles). Consequently, the HPGe detectors are
parameters and much published information on efficiencies for
operated and kept in the range of 80 K to 100 K by a cryostat
various energies, detector sizes, source-to-detector distances,
consisting of a metallic cold-finger immersed in a dewar
and other variables is available (11). The efficiency for gamma-
containing liquid nitrogen or by thermoelectric cooling. The
ray detection may be expressed in various ways. Of primary
detector is kept hermetically sealed in a vacuum to prevent
interest in spectrometry is the full peak efficiency—the fraction
impurities from condensing on the surface and lowering its
of incident gamma rays that give a full-energy peak for a
resistance and is thermally and electrically isolated, usually by
particular source-detector configuration. For a 102 mm thick
vacuum, to reduce heat transfer from the room to the crystal,
NaI(Tl) detector, with the source on the surface (zero distance),
and to maintain the high voltage required for the crystal to be
this fraction is approximately 0.24 for the 0.662 MeV gamma-
a diode. A low atomic number material, such as aluminum, is
ray of Cs and approximately 0.14 for the 1.33 MeV gamma-
the usual covering, and a molecular sieve pump is incorporated
ray of Co. The “peak-to-valley” or “peak-to-Compton” ratio
into the system to maintain the vacuum. The electronic
is the ratio of counts at the maximum height of the full-energy
components required to obtain spectra are similar to those for
peak to the counts at the minimum of the Compton continuum
sodium iodide crystals, except that because smaller pulses must
(Fig. 2). A high ratio indicates narrow peaks, that is, good
be measured, high-quality electronics are needed. The com-
resolution, for that particular efficiency. The Compton spec-
plete system includes a high-voltage bias supply for the
trum does not give useful information in gamma-ray spectrom-
detector (up to 5000 V for large depletion volumes), a
etry and can be considered as “noise.” The ratio varies with
preamplifier (usually charge-sensitive), amplifier, biased am-
energy and is frequently given for the 1.33 MeV peak of Co.
plifier (if needed), pulse height analyzer, and recording device.
It increases as the crystal size increases, and, after passing
Current technology provides for digital signal processing for
through a minimum, increases as the source-to-detector dis-
the signal immediately after the signal leaves the preamplifier.
tance increases, since a larger fraction of the gamma rays pass
The digital signal processing technology provides improved
through the full depth of the crystal. A peak-to-valley ratio of
resolution and peak energy stability across a large range of
12:1 for a crystal is very good. This ratio can be increased by
ambient temperatures and signal processing rates. In more
anti-coincidence shielding to cancel Compton events as de-
advanced products, the signal processing, bias supply, and
scribed in 7.6.5. The efficiency of silicon for gamma-rays is
multichannel pulse height analyzer are provided as a single
considerably less than sodium iodide because of its lower
component.
atomic number (the efficiency for photoelectric absorption of
7.6.4.4 A gamma ray entering either a NaI(Tl) crystal or a
semiconductor detector may lose all or part of its energy in the
detector. When all of the energy is absorbed in the detector,
through multiple Compton interactions or the photoelectric
effect, a full energy peak is obtained. Otherwise, only part of
the energy will be observed, and a Compton continuum
spectrum is seen. An alternative process for high-energy
gamma rays (>1.02 MeV) is pair production, in which an
electron-position pair is produced, and gamma-ray peaks are
observed at 0.511 MeV intervals below the full energy peak.
The two most important operating characteristics of gamma
detectors are efficiency and resolution. The “peak-to-Compton”
or “peak-to-valley” ratio is frequently given in the literature
and is related to both efficiency and resolution. These param-
eters should be specified by the manufacturer and the condi-
tions under which they were measured should be given,
normally by reference to an international standard (for
example, ANSI N42.14) describing the measurement terms and
conditions.
7.6.4.5 The resolution of a gamma-ray detector may be
specified in terms of the width of the full-energy gamma-ray
peak at half its maximum height—the “full width at half FIG. 2 Pulse Height or Energy Spectrum of Cesium-137
D3648 − 23
gamma-rays is proportional to Z ) and lower density (the efficiencies and backgrounds of 76 mm by 76 mm NaI(Tl)
3 3 3
density of NaI is 3.7 g/cm and of silicon 2.4 g/cm ). detector and a 35 cm (5.5 % efficiency) HPGe detector are
7.6.4.6 For a 1 MeV gamma-ray, the total absorption coef- compared.
-1 -1
ficient is about 2 mm for sodium iodide, 1.5 mm for silicon, 7.6.4.8 Spectra of beta particles and conversion electrons
-1
and 3 mm for germanium. As defined in IEEE and IEC can be obtained with sodium iodide and semiconductor detec-
standards, the efficiency of an HPGe detector is generally tors sufficiently thick (a few centimetres) to absorb the particles
expressed by comparison with that of a 76 mm by 76 mm completely. One disadvantage of NaI(Tl) and cooled semicon-
cylindrical NaI(Tl) detector. Comparison is made between the ductors is their relatively thick entrance windows. Other
full-energy peak efficiencies for the 1.33 MeV gamma ray semiconductor detectors, particularly the silicon surface barrier
of Co when the source is 250 mm from the detector. A type, have thin entrance windows and can be used for beta
germanium detector with a volume of 35 cm has an efficiency particles at room temperature. The resolution of silicon surface
approximately 5 % that of a 76 mm by 76 mm NaI(Tl) crystal. barrier detectors is 5 keV to 10 keV for 600 keV electrons and
Larger HPGe detectors are now available with relative effi- 12 keV to 30 keV resolution for 5 MeV alpha particles.
ciencies of greater than 200 %. The very large HPGe detectors 7.6.4.9 Good spectra of low-energy beta particles, conver-
provide signal to noise ratios and resolution far superior to that sion electrons, and X-rays can be obtained with a gas-flow
offered by NaI(Tl). proportional counter provided that a linear preamplifier is used.
7.6.4.7 There are limitations in the efficiency of the light The resolution is intermediate between NaI(Tl) and HPGe. To
production and collection processes in the NaI(Tl) detector reduce backscattering, the chamber should be made of low Z
system that make its resolution inferior to that of semiconduc- material. A counter constructed of a cylinder of graphite-
tor detectors. One important factor is that about 500 eV are impregnated plastic, poly(methyl methacrylate) ends, and a
required to produce an electron at the photocathode in a thin coaxial center wire gives good spectra for such radiations
NaI(Tl) detector system, while the average energy to produce (11). A hole is cut into the outer wall and covered with
the analogous electron-hole pair in silicon is only 3.5 eV and in aluminized polyester film to provide a thin entrance window.
HPGe 2.8 eV. The resolution of semiconductor detectors does Methane (10 %)-argon (90 %) is a suitable counting gas.
not change greatly with energy. Presently available HPGe 7.6.4.10 Organic scintillators, such as anthracene and poly-
detectors have resolutions of 1.5 keV to 2.8 keV at 1.33 MeV styrene polymerized with scintillating compounds, are also
and are from 10 % to 200 % efficient as compared to a 76 mm useful for beta spectrometry. They are packaged with a
by 76 mm NaI(Tl) detector. The greater resolution makes this phototube in a manner similar to a sodium iodide crystal.
detector the one of choice for gamma-ray spectrometry; the Liquid scintillation mixtures also give beta spectra, and the
ability to produce very large high purity HPGe crystals also output of a commercial liquid scintillation counter can be fed
cancels the effect of the higher efficiency previously available into a multichannel pulse-height analyzer to obtain a spectrum
210 210 210
only from NaI(Tl). Since the pulses from a single photopeak (2). A spectrum of Pb Bi Po in Fig. 3 shows the
are spread over a much smaller energy range in HPGe than in resolution obtainable by liquid scintillation counting of aque-
NaI(Tl), the background under the peak is much less. This ous samples in a dioxane-based solution. The Bi curve is
means that for small sources of moderately energetic gamma- from a beta particle, and the Po peak is from an alpha
rays, HPGe is more sensitive (that is, better detection capabil- particle. Organic scintillators are preferable to NaI(Tl) for beta
ity) than NaI(Tl). This is indicated in Table 1, where the spectrometry because less backscattering occurs.
TABLE 1 Comparative Performance of NaI(Tl) and HPGe Gamma-Ray Detectors
Note:
NaI = 76 mm by 76 mm cylindrical detector. Detection = the number of photons emitted from the source whose net
Ge(Li) = 35 cm active volume, 5.5 % efficiency. limit count equals twice the counting error, or
A = small source placed on detector.
1/2
B = 57 mm by 57 mm by 57-mm thick source place on N + N + 2(N − N )
B B
detector.
Counting = percent of photons emitted from the source that give a
efficiency full-energy peak. where N is the total number of counts recorded when the
Shielding = 152 mm of iron, 3.2 mm of lead. source is measured and N is the total number of counts
B
Counting = one 30 000 s count for both source and background. recorded when the background is measured.
time
Photon Energy Background (cps) Counting Efficiency,% Detection Limit, photons/s
Detector Under Peak
A B A B
0.14 MeV
NaI 24 26 18 2.4 3.5
Ge 0.7 12 4 0.92 2.8
0.66 MeV
NaI 20 14 9 4.1 6.3
Ge 0.11 1.3 0.68 3.6 6.8
1.33 MeV
NaI 8 5.8 3.8 6.2 9.5
Ge 0.055 0.75 0.38 4.5 8.9
D3648 − 23
210 210 210
FIG. 3 Spectrum of Pb Bi Po
7.6.4.11 The output pulses of any energy-dependent and ADC speed. The minimum number of channels useful for
detector, after linear amplification, must be sorted out accord- NaI(Tl) gamma-spectrometry is 128; HPGe detectors should be
ing to energy to obtain the spectrum of incident radiation. The used with at least a 1000-channel analyzer and alpha and beta
high resolution available in detectors requires analyzers with spectra can profitably use 100 to 400 channels, depending on
hundreds of channels to realize their full resolving power. The the energy range to be covered. Analyzers with 4096 channels
amplified pulse is digitized by an analog-to-digital converter are fairly common, and larger analyzers are available for
(ADC), and the resulting number for a particular pulse is special purposes.
recorded in a pulse counter whose location is determined by 7.6.4.13 Semiconductor detectors require low-noise,
digital circuitry. This makes it possible to use a digital charge-sensitive amplifiers. Because of their excellent
computer to count and store in its memory the number of resolution, semiconductor detectors are often used with a
pulses in each channel. This conversion and storage is rela- biased amplifier following the main amplifier to isolate a
tively slow, and the analyzer is blocked from processing a portion of the spectrum for analysis. This makes it possible to
second pulse until the previous processing is completed. The use smaller analyzers than would otherwise be necessary.
time required to process a pulse increases with channel 7.6.5 Anti-Coincidence Counters:
number. The instruments now available are sufficiently fast for 7.6.5.1 Substantial background reduction can be achieved in
almost all environmental measurement purposes. Some loss of beta and gamma counters by surrounding or covering the
pulse information is acceptable, as the analyzers measure and sample detector with another detector also sensitive to beta or
record “live time” fairly accurately. Thus, the counting time gamma radiation, and connecting them electronically so that
recorded by the analyzer will be the actual time it was in a any pulse appearing in both detectors is cancelled and not
condition to receive detector pulses, and not the elapsed time. recorded as a count. This is usually referred to as anti-
To maintain good accuracy, the activity of the sample should be coincidence shielding, and is recommended for obtaining very
adjusted to give live times of 90 % or more. A computer is low backgrounds. This type of counter was used for many
typically combined with the ADC and has a program to control years in directional studies of cosmic rays, and was first
detector operations and perform data reduction and analysis. applied to reducing the background of beta counters by Libby
The program may provide an output of the spectrum and results in his study of natural C. The thick metal shielding (lead,
to a monitor, a printer or a digital storage device. iron, or other high-density metal) ordinarily used to reduce
7.6.4.12 All multichannel pulse-height analyzers currently cosmic-ray and gamma-ray background must also be present,
available are digital, and are fairly reliable instruments and and is placed outside the anti-coincidence shielding. Gas-filled
relatively easy to operate. Their maintenance and repair is, beta detectors are generally shielded by gas-filled detectors,
however, a specialized skill similar to other computer repair. In and such anti-coincidence shielding is effective primarily
comparing analyzers, some of the important specifications to against the particulate component of cosmic rays. The anti-
consider are the number of channels, count capacity, stability, coincidence shielding for beta counters may consist of a
live-time accuracy, linearity, type of pulse input acceptable, number of long Geiger-Mueller tubes (“cosmic-ray counters”)
D3648 − 23
surrounding the sample detector or a large (approximately complete spectrum from each detector, singly and in
152 mm square) gas-flow detector, with several anode wires so coincidence, then the complete coincident gamma-ray spec-
the entire area of the counter is sensitive, placed just above the trum can be obtained with one measurement. The efficiency for
sample detector. For counting solid beta sources, the sample coincidence counting is low since it is the product of the
individual efficiencies in each detector, but the detection
detector has a diameter of 25 mm to 51 mm. Surrounding these
counters on all six sides there is frequently a layer of capability is generally improved because of the large back-
ground reduction (16). This technique is often referred to as
high-purity copper to absorb gamma rays emitted from the
outermost shielding, and 102 mm to 152 mm of lead or iron on two-parameter or multidimensional gamma-ray spectrometry.
all six sides. This is the form usually taken by the commercially 7.6.6.3 Additional background improvement is obtained if
available anti-coincidence shielded beta counters. Plastic or the two detectors are surrounded by a large annular NaI(Tl) or
inorganic scintillators could also be used as the anti- plastic scintillation detector connected in anti-coincidence with
coincidence shielding. the two inner detectors. In this case a gamma ray that gives a
pulse, but is not completely absorbed in one of the two inner
7.6.5.2 Anti-coincidence shielding of gamma-ray detectors
detectors, and also gives a pulse in the surrounding detector, is
operates in a similar way, and is particularly useful in reducing
cancelled electronically (13, 16). This provides additional
the Compton continuum background of gamma rays (12).
reduction in the Compton scattering background. HPGe detec-
Gamma rays that undergo Compton scattering and produce a
tors may be used in place of the inner NaI(Tl) detectors for
pulse in both the detector and the anti-coincidence shield are
improved resolution and sensitivities (14).
cancelled electronically. Ideally, only those gamma rays that
are completely absorbed in the sample detector itself produce
7.7 All of the equipment described in Section 7 is available
a count that is recorded with the total energy of the gamma ray
commercially.
(full-energy peak). There are second-order effects that prevent
complete elimination of Compton scattering, but the improve-
8. Sampling
ment is substantial. The anti-coincidence shield can be a large
8.1 Collect the sample in accordance with Practice D1066
NaI(Tl) or plastic scintillator suitably attached to phototubes.
or Section 14.3 of Practices D3370.
They usually have a large annular hole into which the sample
detector, a smaller NaI(Tl) detector, or HPGe detector is placed
8.2 Sample an appropriate volume depending on the ex-
(13, 14).
pected concentration of radioactivity in the water. For precise
7.6.6 Coincidence Counters: measurements without long counting times, it is advisable to
count an aliquot that contains at least 40 Bq of radioactivity.
7.6.6.1 In coincidence counting, two or more radiation
detectors are used together to measure the same sample, and
8.3 Chemical treatment of samples to prevent biological or
only those nuclear events or counts that occur simultaneously
algal growth is not recommended and should be avoided unless
in all detectors are recorded. The coincidence counting tech-
essential. When necessary, select the reagents used to avoid
nique finds considerable application in studying radioactive
chemical interaction with the radioactive species in the sample.
decay schemes; but in the measurement of radioactivity, the
Analyze samples promptly.
principal uses are for the standardization of radioactive sources
8.4 Chemical treatment of samples to retain radioactive
and for counter background reduction.
species in solution may be used but carefully select the specific
7.6.6.2 Coincidence counting is a very powerful method for
treatment. The use of oxidizing acids such as HNO is not
absolute disintegration rate measurement (6, 15). Both alpha
recommended when iodide is present since it may be oxidized
and beta emitters can be standardized if their decay schemes
to iodine and lost or be absorbed into the plastic containers if
are such that β-γ, γ-γ, β-β, α-β, or α-X-ray coincidence occur in
they are used. In some cases, extreme chemical treatment may
their decay. Gamma-gamma coincidence counting with two
be used to keep a particular chemical species in solution;
NaI(Tl) detectors, and the source placed between them, is an
examples are strongly alkaline conditions to hold molybdenum
excellent method of reducing the background from Compton
and ruthenium in solution, or acid conditions with fluoride ion
scattered events. Its use is limited, of course, to counting
to keep zirconium in solution. The addition of an acid such as
nuclides that emit two photons in cascade (which are essen-
60 hydrochloric is generally desirable to reduce hydrolysis and the
tially simultaneous), either directly as in Co, by annihilation
65 loss of activity on container walls. Frequently, samples will
of positrons as in Zn, or by immediate emission of a gamma
contain insoluble material. In such cases, treat the sample by
ray following electron capture decay. If the detectors are
one of the following methods:
operated with single-channel pulse-height analyzers to limit the
8.4.1 Filter the insoluble material and analyze both the
events recorded from each detector to one of the full-energy
filtrate and insoluble matter on the filter. During filtration, some
peaks of the photons being emitted, then essentially only those
material may be sorbed onto the filter and assumed to be
photons will be counted. Non-coincident pulses of any energy
insoluble when in fact it is soluble.
in either one of the detectors will be cancelled, including
8.4.2 Centrifuge the sample and analyze both phases. Wash
cosmic-ray photons in the background and degraded or Comp-
the insoluble phase with distilled water to remove all soluble
ton scattered photons from higher energy gamma rays in the
material without dissolving the insoluble fraction.
sample. Thus, the method reduces interference from other
gamma emitters in the sample. If, instead of single-channel 8.4.3 In either of the above separations when the total
analyzers, two multichannel analyzers are used to record the activity is required, the insoluble matter may be dissolved and
D3648 − 23
recombined with the soluble fraction. When radioactivity is left the average of at least ten measurements over a period of days.
on the walls of the sample container, desorb it and add it to the For a statistical control chart, enter the control limit bands of
sample.
¯
=
62s and 63s (s 5 C) and draw lines on the chart that allow
c c c
8.5 Co
...