ASTM C1189-11(2018)

Standard Guide to Procedures for Calibrating Automatic Pedestrian SNM Monitors

SIGNIFICANCE AND USE
5.1 SNM monitors are an effective means to search pedestrians for concealed SNM. Maintaining monitor effectiveness rests on appropriate calibration and adjustment being part of a continuing maintenance program.  
5.2 The significance of this guide for monitor users who must detect SNM is to describe calibration and adjustment procedures for the purpose.  
5.3 The significance of this guide for monitor manufacturers is to describe calibration procedures, particularly for detecting forms of SNM that may not be readily available to them.
SCOPE
1.1 This guide covers calibrating the energy response of the radiation detectors and setting the discriminator and alarm thresholds used in automatic pedestrian special nuclear material (SNM) monitors.  
1.2 Automatic pedestrian SNM Monitors and their application are described in Guide C1112, which suggests that the monitors be calibrated and tested when installed and that, thereafter, the calibration should be checked and the monitor tested with SNM at three-month intervals.  
1.3 Dependable operation of SNM monitors rests, in part, on an effective program to test, calibrate, and maintain them. The procedures and methods described in this guide may help both to achieve dependable operation and obtain timely warning of misoperation.  
1.4 This guide can be used in conjunction with other ASTM standards. Fig. 1 illustrates the relationship between calibration and other procedures described in standard guides, and it also shows how the guides relate to an SNM monitor user. The guides below the user in the figure deal with routine procedures for operational monitors. Note that Guide C993 is an in-plant performance evaluation that is used to verify acceptable detection of SNM after a monitor is calibrated. The guides shown above the user in Fig. 1 give information on applying SNM monitors (C1112) and on evaluating SNM monitors (C1169) to provide comparative information on monitor performance.  
FIG. 1 The Relationship of Calibration to Other Procedures Described in Standard Guides for SNM Monitors  
1.5 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.

General Information

• Status: Published
• Publication Date: 31-Oct-2018
• ICS: 17.240 - Radiation measurements
• Technical Committee: C26 - Nuclear Fuel Cycle
• Drafting Committee: C26.10 - Non Destructive Assay

Relations

• Refers: ASTM C859-24 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Jan-2024
• Refers: ASTM C859-14a - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Jun-2014
• Refers: ASTM C859-14 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Jan-2014
• Refers: ASTM C859-13a - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Jun-2013
• Refers: ASTM C859-13 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-May-2013
• Refers: ASTM C859-10b - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Nov-2010
• Refers: ASTM C859-10a - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Aug-2010
• Refers: ASTM C859-10 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 01-Feb-2010
• Refers: ASTM C859-09 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Feb-2009
• Refers: ASTM C859-08 - Standard Terminology Relating to Nuclear Materials
• Effective Date: 15-Sep-2008
• Refers: ASTM C1112-99(2005) - Standard Guide for Application of Radiation Monitors to the Control and Physical Security of Special Nuclear Material (Withdrawn 2014)
• Effective Date: 01-Jun-2005
• Refers: ASTM C1112-99 - Standard Guide for Application of Radiation Monitors to the Control and Physical Security of Special Nuclear Material
• Effective Date: 10-Jun-1999
• Refers: ASTM C1169-97 - Standard Guide for Laboratory Evaluation of Automatic Pedestrian SNM Monitor Performance
• Effective Date: 10-Jun-1997
• Refers: ASTM C1169-97(2003) - Standard Guide for Laboratory Evaluation of Automatic Pedestrian SNM Monitor Performance
• Effective Date: 10-Jun-1997
• Refers: ASTM C993-97 - Standard Guide for In-Plant Performance Evaluation of Automatic Pedestrian SNM Monitors
• Effective Date: 10-May-1997

Overview

ASTM C1189-11(2018), Standard Guide to Procedures for Calibrating Automatic Pedestrian SNM Monitors, developed by ASTM International, provides practical guidance for the calibration of automatic pedestrian Special Nuclear Material (SNM) monitors. These monitors are critical for reliably detecting the unauthorized movement of SNM, supporting facility security protocols and regulatory compliance. The guide outlines key calibration steps, adjustment of detection thresholds, and recommended maintenance and evaluation practices, ensuring both users and manufacturers consistently achieve dependable monitor performance.

Key Topics

• Calibration Procedures: The guide details steps for calibrating the energy response of radiation detectors, including the adjustment of pulse height discriminators and alarm thresholds specific to SNM monitors.
• Types of Detectors: Instructions cover calibration for various radiation detectors, such as inorganic scintillation detectors (e.g., NaI(Tl)), neutron proportional counters, and organic scintillators, emphasizing their role in detecting both gamma rays and neutrons associated with SNM.
• Monitor Maintenance: Effective calibration is presented as part of a comprehensive maintenance program, with procedures designed to maintain long-term reliability and early misoperation warning.
• Test and Evaluation: The standard encourages testing monitors upon installation and at regular intervals, recommending three-month periodic calibration checks using SNM.
• Threshold Adjustments: Methods are provided to set background alarm thresholds and discriminate voltage levels, crucial for timely identification of detector failure or loss of sensitivity.
• Safe Calibration Practices: Emphasis is placed on electrical and radiation safety during calibration, recommending adherence to radiation safety protocols and responsible oversight by trained safety personnel.

Applications

Automatic pedestrian SNM monitors-such as portal monitors and monitoring booths-are deployed in high-security environments where control and safeguarding of nuclear materials are paramount. Typical applications include:

• Nuclear Facilities: Entry and exit monitoring for staff to prevent unauthorized removal of SNM.
• Fuel Cycle Operations: Ongoing verification of compliance with nuclear material security regulations.
• Government and Defense Sites: Enhanced detection of concealed nuclear material to support non-proliferation efforts.
• Customs and Border Protection: Screening of individuals in sensitive areas to thwart illicit trafficking of nuclear substances.

Manufacturers benefit from ASTM C1189-11(2018) by following its calibration guidelines to ensure their SNM monitors meet performance requirements-even for forms of SNM not readily available during development. Users apply the guide’s procedures to sustain reliable and accurate detection in operational environments, maintaining security and supporting regulatory audits.

Related Standards

The calibration procedures in ASTM C1189-11(2018) are complemented by several related ASTM standards, forming a comprehensive framework for SNM monitor operation and evaluation:

• ASTM C1112 - Guide for Application of Radiation Monitors to the Control and Physical Security of Special Nuclear Material
• ASTM C1169 - Guide for Laboratory Evaluation of Automatic Pedestrian SNM Monitor Performance
• ASTM C993 - In-plant Performance Evaluation for post-calibration verification
• ASTM C859 - Terminology Relating to Nuclear Materials
• ASTM E876 - Practice for Use of Statistics in the Evaluation of Spectrometric Data

These interconnected standards reinforce consistent calibration, application, and performance verification of radiation monitoring systems for SNM detection, supporting compliance with both domestic and international security requirements.

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Keywords: ASTM C1189-11(2018), SNM monitor calibration, automatic pedestrian monitor, radiation detector calibration, special nuclear material detection, threshold adjustment, nuclear security standard, radiation detection maintenance, compliance, performance evaluation.

Standards Content (Sample)

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: C1189 − 11 (Reapproved 2018)
Standard Guide to
Procedures for Calibrating Automatic Pedestrian SNM
Monitors
This standard is issued under the fixed designation C1189; 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 2. Referenced Documents
1.1 This guide covers calibrating the energy response of the 2.1 ASTM Standards:
radiation detectors and setting the discriminator and alarm C859 Terminology Relating to Nuclear Materials
thresholds used in automatic pedestrian special nuclear mate- C993 Specification for Welded Wire Lath (Withdrawn
rial (SNM) monitors.
2021)
C1112 Guide for Application of Radiation Monitors to the
1.2 Automatic pedestrian SNM Monitors and their applica-
Control and Physical Security of Special Nuclear Material
tion are described in Guide C1112, which suggests that the
(Withdrawn 2014)
monitors be calibrated and tested when installed and that,
C1169 Guide for Laboratory Evaluation of Automatic Pe-
thereafter, the calibration should be checked and the monitor
destrian SNM Monitor Performance (Withdrawn 2021)
tested with SNM at three-month intervals.
E876 Practice for Use of Statistics in the Evaluation of
1.3 Dependable operation of SNM monitors rests, in part, Spectrometric Data (Withdrawn 2003)
on an effective program to test, calibrate, and maintain them.
The procedures and methods described in this guide may help
3. Terminology
both to achieve dependable operation and obtain timely warn-
3.1 Definitions of Terms Specific to This Standard:
ing of misoperation.
3.1.1 calibration—a multistep procedure that uniformly ad-
1.4 This guide can be used in conjunction with other ASTM
justs the energy response of a monitor’s detector array and sets
standards. Fig. 1 illustrates the relationship between calibration
the operating parameters of its detection circuits for optimum
and other procedures described in standard guides, and it also
performance. In a few monitors, an additional analog adjust-
shows how the guides relate to an SNM monitor user. The
ment of a signal detection circuit is required.
guides below the user in the figure deal with routine procedures
3.1.2 SNM—special nuclear material: plutonium of any
for operational monitors. Note that Guide C993 is an in-plant
isotopic composition, U, or enriched uranium as defined in
performance evaluation that is used to verify acceptable
Terminology C859.
detection of SNM after a monitor is calibrated. The guides
3.1.2.1 Discussion—This term is used here to describe both
shown above the user in Fig. 1 give information on applying
SNM and strategic SNM, which is plutonium, uranium-233,
SNM monitors (C1112) and on evaluating SNM monitors
and uranium enriched to 20 % or more in the U isotope.
(C1169) to provide comparative information on monitor per-
formance.
3.1.3 SNM Monitor—a radiation detection system that mea-
sures ambient radiation intensity, determines an alarm thresh-
1.5 This international standard was developed in accor-
old from the result, and then when it monitors, sounds an alarm
dance with internationally recognized principles on standard-
if its measured radiation intensity exceeds the threshold.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom- 3.1.3.1 Discussion—The automatic pedestrian SNM moni-
mendations issued by the World Trade Organization Technical tor discussed here is a walk-through or wait-in portal or
Barriers to Trade (TBT) Committee. monitoring booth.
1 2
This guide is under the jurisdiction of ASTM Committee C26 on Nuclear Fuel For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Cycle and is the direct responsibility of Subcommittee C26.10 on Non Destructive contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Assay. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Nov. 1, 2018. Published December 2018. Originally the ASTM website.
approved in 1991. Last previous edition approved in 2011 as C1189 – 11. DOI: The last approved version of this historical standard is referenced on
10.1520/C1189-11R18. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1189 − 11 (2018)
5.3 The significance of this guide for monitor manufacturers
is to describe calibration procedures, particularly for detecting
forms of SNM that may not be readily available to them.
6. Interferences
6.1 The monitor should be in proper operating condition
when calibrated. Any indication that the monitor does not stay
in calibration or that it drifts substantially during the interval
between calibration checks is cause for repair or renovation
and then recalibration.
7. Apparatus
7.1 SNM Automatic Pedestrian Monitors, having arrays of
radiation detectors that form a portal through which pedestrians
pass or that surround a pedestrian as he waits in a booth for
clearance to pass.
7.2 Radiation Detectors, used in SNM monitors may detect
gamma rays, neutrons, or both. One of three types of detector
FIG. 1 The Relationship of Calibration to Other Procedures De-
scribed in Standard Guides for SNM Monitors listed is usually used. All of types of detector operate in a
pulse-counting mode to obtain good sensitivity for detecting
small changes in radiation intensity.
7.2.1 Inorganic Scintillation Detectors, such as sodium
4. Summary of Guide
iodide [NaI(T1)], detect gamma rays but have little response to
neutrons from SNM. This detector is useful for detecting
4.1 This guide covers various instructions for calibrating
unshielded SNM.
SNM pedestrian monitors for optimum performance in normal
7.2.2 Neutron Proportional Counters, containing BF
operation. The order of procedures is as follows.
or He as a converter gas, detect thermal neutrons and are used
4.1.1 The energy response of inorganic or organic scintilla-
with a moderator to thermalize fast neutrons from SNM. This
tion detectors or of neutron proportional counters, is calibrated
detector is useful for detecting unshielded or shielded pluto-
to produce appropriate signal pulse heights for SNM radiation
nium.
(see Section 10).
7.2.3 Organic Scintillators, detect both gamma rays and fast
4.1.2 The monitor’s pulse height discriminators are cali-
neutrons from SNM. This detector is useful for detecting
brated to form a region of interest containing SNM radiation
unshielded SNM and shielded plutonium.
from highly enriched uranium or low-burnup plutonium (see
Sections 9 and 11), or for detecting neutrons in proportional
7.3 Oscilloscope or Multi-Channel Analyzer, for viewing
counters (see Section 9).
reference detector pulses produced by a specific radiation
4.1.3 The monitor’s transient signal detection logic is ad-
source during energy calibration.
justed for appropriate response to walk-through or wait-in
7.3.1 Gamma-Ray Detectors, reference pulses from 662-
monitoring (see Section 12).
keV gamma rays emitted by a Cs source with a nominal
8-microCurie (0.3-kBq) activity are used for calibration.
4.2 This guide covers adjusting various thresholds used in
7.3.2 Neutron Proportional Counters, reference pulses from
SNM monitors.
neutrons emitted by a Cf neutron source with less than
4.2.1 This guide describes setting background alarm thresh-
2 × 10 neutron/s (0.009-μg) source strength can be used for
olds that may be used to announce loss of detection sensitivity
calibration.
or detector failure (see Section 13).
4.2.2 This guide discusses setting the lowest practical dis-
NOTE 1—Acquisition, storage, and use of sources should be under the
criminator levels for the radiation detectors (see Section 11). guidance of a responsible radiation safety officer (see Section 8 on
hazards).
4.3 When calibration is complete, the monitor should be
7.4 Manufacturer’s or Designer’s Operation and Mainte-
tested using in-plant evaluation procedures described in Guide
nance Manual, essential for quick and efficient monitor cali-
C993.
bration. The manufacturer’s suggested calibration scheme is a
good starting place, if not the best approach to calibration.
5. Significance and Use
Calibration requires knowledge of test point and adjustment
5.1 SNM monitors are an effective means to search pedes-
locations that should be described in the manuals.
trians for concealed SNM. Maintaining monitor effectiveness
rests on appropriate calibration and adjustment being part of a 8. Hazards
continuing maintenance program.
8.1 Make sure that the use of radioactive materials is under
5.2 The significance of this guide for monitor users who the guidance of a responsible radiation safety officer who can
must detect SNM is to describe calibration and adjustment provide any needed radiation safety training, personnel
procedures for the purpose. dosimetry, and handling procedures for radiation sources.
C1189 − 11 (2018)
8.2 The radiation detectors in SNM monitors all operate at an organic scintillator is 11.4 to 102 keV. The resulting
high voltages that may be hazardous. Although a person is not discriminator levels for calibrations using 2 and 3.3 V for Cs
usually exposed to high voltage during calibration, make sure pulse height are as follows:
that the work is performed with the approval of a responsible (a) Calibration using 2 V in a NaI(Tl) detector: 0.18 to 0.66
safety officer with proper attention given to electrical safety V,
training and reading any warnings of high voltage exposure in (b) Calibration using 3.3 V in a NaI(Tl) detector: 0.3 to
manuals or posted on equipment. 1.10 V,
(c) Calibration using 2 V in a plastic detector: 0.05 to 0.43
9. Pulse-Height Analysis Calibration
V, and
(d) Calibration using 3.3 V in a plastic detector: 0.08 to
9.1 Once a monitor’s detector array is adjusted to uniform
0.70 V.
pulse height, the pulse-height analysis circuitry can be ad-
9.3.2 Low-Burnup Plutonium—The optimum region of in-
justed. The point is to set a lower-level discriminator to exclude
terest for low-burnup plutonium extends from 0 to 450 keV (1).
electronic noise and pulses from radiation below the SNM
The value 0 means the lowest practical value achieved by one
energy range. Most often a second-level discriminator or
of the means discussed in Section 11. The corresponding
window is also set to discriminate energy above the SNM
deposited energy range in an organic scintillator is 0 to 287
radiation, thus forming an SNM energy region of interest.
keV. The resulting discriminator levels for calibrations using 2
9.2 Discriminator Settings for SNM—The lower-level dis-
and 3.3 V for Cs pulse height are as follows:
criminator setting and the window or upper-level discriminator
(a) Calibration using 2 V in a NaI(Tl) detector: 0 to 1.36 V,
setting, if used, may depend on the type of SNM to be detected
(b) Calibration using 3.3 V in a NaI(Tl) detector: 0 to 2.24
and the type of detector used for the following reasons.
V,
9.2.1 The two types of SNM, highly enriched uranium
(c) Calibration using 2 V in a plastic detector: 0 to 1.20 V,
(HEU) and low-burnup plutonium, differ in their intrinsic
and
gamma-ray spectra.
(d) Calibration using 3.3 V in a plastic detector: 0 to 1.97
9.2.2 Inorganic and organic scintillators respond differently
V.
to gamma rays. Inorganic scintillators produce pulse heights
9.3.3 In case of other gamma-ray pulse-height calibrations
that are proportional to the detected gamma-ray energy.
for Cs gamma rays than are given here, use values directly
However, organic scintillators do not, as Fig. 2 illustrates. At
scaled from the listed values for the same type of detector.
low gamma-ray energies, a smaller fraction of the incident
9.4 Optimum Neutron Analysis Windows, for proportional
gamma-ray energy is deposited in an organic scintillator, and it
counters are given here.
produces a proportionately smaller pulse height. Hence, inor-
ganic and organic scintillators calibrated to the same reference
NOTE 2—For organic scintillators, adequate fast neutron response for
pulse height will have different upper and lower discriminator
present-day SNM monitoring applications is usually achieved using the
plastic detector discriminator levels for gamma rays given in 9.3.2.
voltage levels for an SNM region of interest. The examples
following illustrate the differences.
9.4.1 Neutron proportional counters detect moderated neu-
trons from plutonium and each type of proportional counter has
9.3 Gamma-Ray Regions of Interest for SNM:
its own pulse-height spectrum for detected neutrons.
9.3.1 HEU—The HEU gamma-ray region extends from 60
9.4.2 The upper level is unimportant in this case because
to 220 keV (1). The corresponding deposited energy range in
there is no high level background. Only a lower-level discrimi-
nator may be available in some monitors. Suggested operating
ranges are as follows:
The boldface numbers in parentheses refer to the list of references at the end of
this guide.
(a) For BF calibrated to 2 V, from 0.3 to 10 V;
(b) For He calibrated to 2 V, from 0.4 to 10 V;
(c) For BF calibrated to 8 V, from 1.2 to 10 V; and
(d) For He calibrated to 8 V, from 1.6 to 10 V.
9.4.3 In case another neutron pulse height than given here is
used, the values can be directly scaled from the listed values
for the same type of detector.
9.5 Setting the Discriminators:
9.5.1 Set the appropriate values in the monitor’s discrimi-
nators or single-channel analyzers (SCA) noting the following
special cases:
9.5.1.1 Interpreting Window Discriminator Voltage
Levels—Monitors having both a level discriminator and a
window discriminator float the window voltage level on top of
the level-discriminator voltage level. Hence, the upper dis-
criminator value, which is the upper limit of the operating
ranges just tabulated, is the sum of the monitor’s level
FIG. 2 The Relationship Between Incident Gamma-Ray Energy
and Energy Deposited in NaI(Tl) and Plastic Scintillators discriminator and window values.
C1189 − 11 (2018)
9.5.1.2 Zero Discriminator Values—The value 0 means the
lowest practical value. It will be determined later using a
procedure described in Section 11.
9.5.1.3 Backlash in Potentiometer Adjustments—When set-
ting multiturn potentiometers, adopt a convention for the
direction of rotation so that settings can be made reproducibly.
9.5.1.4 Uncalibrated Adjustments—If a calibrated multiturn
potentiometer dial is not provided, the designer or manufac-
turer will have to indicate how to make these adjustments with
the aid of a voltmeter or oscilloscope.
10. Procedures
FIG. 3 The Gamma-Ray Spectrum of Cs Detected by a NaI(Tl)
10.1 Detector Energy Calibration:
Scintillator and Viewed with an Oscilloscope
10.1.1 Detector energy calibration sets the SNM detector
response to a particular reference pulse height for gamma rays
or neutrons from a calibration source. The reference pulse
height recommendations of designers and manufacturers for
different detectors range from 2 to 8 V. Particular values for
each detector type are provided, and the corresponding energy
regions for different types of SNM are listed in the following
procedures.
10.1.2 Put the monitor into operation using the manufactur-
er’s instructions. Pay particular attention to checking or setting
the detector high voltage to the recommended value using
proper electrical safety practice (see 8.2).
10.1.3 With the detectors operating at an appropriate high
voltage, proceed with energy calibration by varying amplifier
gain or individual detector voltage dividers or both to balance
the response of each detector. This is done using the pulse-
FIG. 4 The Gamma-Ray Spectrum of Cs Detected by a NaI(Tl)
height spectrum of a reference source as viewed on an
Scintillator and Viewed with a Multi-Channel Analyzer
oscilloscope or multichannel analyzer coupled to the monitor’s
amplifier analog output. Procedures for each type of detector
follow. potentiometers on detector voltage dividers, if provided, to
obtain the same pulse height.
10.2 Inorganic Scintillators: (See 10.3 for organic (plastic)
10.2.4 In case of difficulty do as follows.
scintillators and 10.4 for neutron detectors.)
10.2.4.1 If the limit of adjustment is reached on a trimmer,
10.2.1 Inorganic scintillators, such as NaI(T1), absorb
the amplifier gain will have to be readjusted, as will all trimmer
gamma-ray energy both by photoelectric absorption and
adjustments, until the detectors have uniform pulse height, and
Compton scattering. Photoelectric absorption leads to peaks in
10.2.4.2 If uniform pulse height cannot be achieved, main-
the pulse-height spectrum that are characteristic of the incident
tenance is needed to replace faulty resistors or
gamma-ray energy, and one gamma-ray peak is used as a
photomultipliers, or to change component values so that all
reference pulse height for calibration.
detectors can be set to the same pulse height.
10.2.2 Before bringing the reference source up to the
10.2.5 The detector array is now adjusted for uniform pulse
detector, look at the background pulse-height spectrum on the
height and is ready for the next calibration step. Proceed with
oscilloscope or multichannel analyzer so that you are familiar
Section 11.
with it and will recognize the peak in the reference source
10.3 Organic Scintillators:
spectrum.
10.2.3 Adjusting the pulse height. 10.3.1 Organic scintillators, such as plastic scintillators or
liquid scintillators, absorb incident gamma-ray or neutron
10.2.3.1 Safely hold or attach the cesium ( Cs) reference
source to one of the monitor’s detectors at a reference point energy by Compton scattering from electrons or protons
present in the scintillator. Only the gamma-ray energy response
that can be used for each detector, for example, at its center or
at a manufacturer’s specified location. is calibrated in this procedure. Compton scattering does not
lead to peaks but to a distribution of pulse heights that is
10.2.3.2 Observe the pulse height spectrum and verify that it
looks like Fig. 3 or Fig. 4. characteristic of the incident gamma-ray energy at its end
point, a knee shape in the spectrum. The half-height of the
10.2.3.3 Adjust the amplifier gain to place the peak in the
slope of the knee is used for calibration.
cesium spectrum at the reference pulse height (usually 2 V, 3.3
V, or other pulse height).
NOTE 3—Adequate fast neutron response in organic scintillators for
10.2.3.4 Now attach the source to each remaining detector
present-day SNM monitoring applications is achieved by the gamma-ray
and adjust individual amplifiers, if provided, or trimmer energy calibration procedure.
C1189 − 11 (2018)
10.3.2 Before bringing the calibration source up to the
detector, carefully look at the background pulse-height spec-
trum on the oscilloscope or multichannel analyzer so that you
are familiar with it and will recognize the difference when the
calibration source is added.
10.3.3 Adjusting the pulse height:
10.3.3.1 Safely hold or attach the cesium ( Cs) calibration
source to one of the monitor’s detectors at a reference point
that can be used for each detector, for example, at its center or
the manufacturer’s specified location.
10.3.3.2 Observe the pulse-height spectrum on the oscillo-
scope or multichannel analyzer and verify that it looks like Fig.
5 or Fig. 6. If you want to verify the position of the knee on an
oscillosocope, move the source away and then back repeatedly
FIG. 6 The Gamma-Ray Spectrum of Cs Detected by a Plastic
to emphasize the contrast.
Scintillator and Viewed with a Multi-Channel Analyzer
10.3.3.3 Next, adjust the amplifier gain to place the mid-
point of the knee in the cesium spectrum at the reference pulse
height (usually 2 V, 3.3 V, or other pulse height).
10.4.1 Neutron proportional counters absorb incident neu-
10.3.3.4 Now attach the source to each remaining detector
tron energy by means of a conversion reaction that produces
and adjust individual amplifiers if provided, or trimmer
charged particles, which in turn cause ionization and detectable
potentiometer, if provided, until the recommended pulse height
current pulses. The pulse-height spectrum has a peak or knee
is obtained.
that characterizes the reaction and is used for calibration.
10.3.4 In case of difficulty do as follows:
10.4.2 Before bringing the calibration source up to the
10.3.4.1 Diffıculty Seeing the Spectrum—If the background
137 detector, you should see a low-intensity background spectrum
intensity is intense enough that it is difficult to see the Cs
because neutron backgrounds are usually very low.
spectrum, it may help to isolate the individual detectors. Do
10.4.3 Adjusting the pulse height.
this using proper electrical safety practice (see 8.2), turning off
10.4.3.1 Safely hold or attach the californium ( Cf) or
the high voltage, and disconnecting the high voltage from all
other suitable neutron calibration source to one of the moni-
but the detector being calibrated. Leave all signal cables in
tor’s detectors at a reference point that can be used for each of
place to avoid changing the total signal cable capacitance.
the detectors.
10.3.
...