Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 1 of 11 Procedure13v550

Radioactive Neutron Sources Emission Rates

Purpose

The purpose of this procedure is to describe the setup, measurement, and reporting 

procedures for radioisotope neutron source calibrations with neutron emission rates 

ranging from 5 x 105/s to 1 x 1010 /s.

Scope

This procedure covers the calibration of radioisotope sources of neutrons via the 

manganese sulfate (MnSO4) bath method, in which the emission rate of the source to be 

calibrated is compared to the emission rate of NBS-1, the national standard Ra-Be 

photoneutron source. Additional details and references can be found in Refs. [1, 2, 3].

Definitions

NBS-1 The national standard Ra-Be photoneutron (,n) source. Its emission rate has been 

absolutely determined with an estimated uncertainty of ± 0.85 % (see Ref. [1]). 

Its neutron emission rate in October 2003 was 1.234 x 106/s, and it has a half-life 

of 1600 years. The half-life uncertainty is 7 years which contributes less than 

0.02 % to the total relative uncertainty since the last calibration.

BIPM In addition to NBS-1, we have three calibrated neutron sources from the Bureau 

International des Poids et Mesures (BIPM). Their accuracy is about 1 %. They 

are an AmBe source (2.285 x 106/s in 12/1986, half life 432 years), a Ra-Be (α,n) 

source (3.360 x 106/s in 12/1986, half life 1600 years), and a Ra-Be photoneutron 

source (6.404 x 105/s in 12/1969, half life 1600 years).

Equipment

Figures 1 and 2 show the manganese sulfate bath and control rooms, respectively. The 

equipment located in the manganese sulfate bath room (building 245, room B143) is:

• 1.27 m diameter bath containing MnSO4 solution with a density of 1.25 kg/L 

• Emerson Model C63CXJBH-5252 pump for circulating the manganese sulfate 

solution; SN 587010

• Teflon source holder

• stepping motor, several pulleys, and string

• D4521 Barnstead B-pure water filter system; SN 583990330636

• Anton Paar DMA 35N density meter; SN 795678
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 2 of 11 Procedure13v550

The equipment located in the control room (building 245, room B141) is: 

• Central Research Laboratories remote manipulator G; SN 9362

• StepPak MCU-2 stepping motor controller; SN 0217980 

• two Bicron detectors ("main" and "remote") utilizing sodium iodide crystals and 

photomultipliers 

• stainless steel Marinelli beaker housing the "main" detector 

• two Ortec 556 high voltage (HV) power supplies for the photomultiplier tubes; 

SN 14125056 rev#R (remote channel), 443 rev#16 (main channel) 

• two Ortec preamps for the photomultiplier outputs; Ortec 113 SN 4013 rev# 23 

(remote channel), Ortec 113 SN 3061 rev#12 (main channel) 

• two Ortec linear amplifiers; Ortec 485 SN 2991S rev#14 (remote channel), Ortec 

575A SN IR0565 rev#03 (main channel) 

Figure 1: The manganese sulfate bath room. 
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 3 of 11 Procedure13v550

Figure 2: The manganese sulfate bath control room. 

• two NIST-built constant deadtime discriminators; SN 2385 (main channel), 2386 

(remote channel) 

• three Ortec 772 counters; SN (left to right) 907 rev#11, 1750 rev#36, 912 rev#11 

• Ortec 776 counter timer; SN 836 rev#30 

• Tennelec TC 566A time of day, year clock; SN 210 

• Ortec 779 interface controller; SN 248 rev#09 

• computer running IGOR and in-house analysis software 

• generic precision pulse generator 

• Tracor Northern TN-7200 multichannel analyzer (MCA) 

Because this measurement is a relative measurement (vendor’s source versus NBS-1), it 

is sensitive only to the stability of the employed electronics over the measurement period 

(a few days). All other effects, such as the specific choice of electronic modules, are 

common mode and do not enter into the final result. 

Health & Safety 

The calibration of neutron sources involves several different safety aspects. 

1. The radiation safety aspect emphasizes minimization of personnel exposure. The 

magnitude of neutron source strengths that can be safely handled at NIST is 

limited to 1010 /s, or less, so several hours of close proximity exposure would be 

necessary to produce a lethal dose of radiation. Our emphasis is upon obtaining 
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 4 of 11 Procedure13v550

as small an exposure as practical in consideration of As Low As Reasonably 

Achievable (ALARA) practices.

2. The other aspects are safe handling of neutron sources to prevent any damage and 

industrial safety practices when handling heavy shielding materials, shipping 

casks, and the removable parts of shipping containers.

With regard to radiation safety, Nuclear Regulatory Commission (NRC) regulations and 

NIST policy require that all personnel handling neutron sources be given a radiation 

safety and awareness training course, with a refresher course to be given every two years. 

Radiation safety and training services are provided by the NIST Gaithersburg Radiation 

Safety Division (GRSD). Safe handling of neutron sources and the industrial safety 

aspects are taught through an apprentice-type relationship with each new handler.

All neutron-source transfer operations at NIST are accomplished by trained professionals 

with the cognizance of, and frequently with the help of, GRSD personnel. Temporary, 

in-transit, source storage is accomplished with movable water tanks and shielding barrels. 

Procedures are reviewed prior to each handling operation. Whole body dosimetry (TLD) 

and extremity dosimetry (finger rings) are worn during all source transfers and a 

minimum of two authorized source users are present for a source transfer. When sources

with neutron emission rates in the range 109/s to 1010 /s are calibrated, supplemental 

gamma/neutron sensitive dosimeters are worn in order to document the radiation 

exposure received during the calibration; these transfers are carried out in close 

consultation with and in the presence of GRSD.

Procedures

Summary

Based upon email/telephone discussions with the customer, a calibration schedule is fixed 

and a NIST 364 (“Radioactive Material Request”) form is filled out. When each source 

arrives, it is carefully inspected by GSRD for damage or leaks. As these sources tend to 

be securely encapsulated, passing examination by GRSD personnel means they are very 

likely to work in the MnSO4 bath without a problem. The sources are stored in a secure 

area (245/B143), which is monitored carefully by GRSD. Any given source is likely to 

be present in storage for no more than three months.

Four separate measurements are needed to calibrate a source and establish the reliability 

of the results. They are:

1. measurement of the background

2. measurement of NBS-1

3. measurement of the customer's source

4. remeasurement of NBS-1
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 5 of 11 Procedure13v550

Ideally, the background measurement should be done when the bath has been empty for 

at least a month. These four measurements are subsequently analyzed to obtain customer 

source strength and uncertainty. Occasionally this order is changed if, for example, there 

is a backlog of sources to calibrate. In this case, several sources may be measured 

between NBS-1 measurements.

Procedure for each measurement

1. If the source strength is greater than 108 /s, turn off the HV on the "main" detector 

and tally results from the "remote" channel; otherwise, tally results from the 

"main" channel. For the purposes of these instructions, the tallied channel will be 

called the active channel.

2. Remove the neutron source from its container following the customer's 

instructions.

3. Quickly transport the source to the loading area adjacent to the bath using an 

appropriate grappling pole. Return to the control room.

4. Using the remote manipulator, load the Teflon source container with the source, 

screw the top onto the container rendering it watertight, put the source container 

in the bottom part of the Teflon carriage, position the carriage on the stand sitting 

atop the bath, grab and attach the dangling carriage top to the carriage bottom. 

Finally, release the carriage from the stand allowing it to dangle freely over the 

bath.

5. Using the motor controller's up and down controls, lower the source until the red 

mark on the string attached to the carriage is centered on the black arrow located 

on the far wall. At this point the source is centered in the bath.

6. Once the decay activity has built up sufficiently (~30 hours), use an oscilloscope 

to verify that the pulse width coming out of the active discriminator is 3.4 s (5.6 

s if using the “remote” channel). This measurement does not have to be done 

every time. It suffices to do this a couple of times per year as this quantity has 

been observed to be very stable.

7. Verify that the "time of day, year clock" unit is reading the proper time. If it is 

not, adjust it. The IGOR code that controls data acquisition inserts a time value 

into the data stream; therefore, this step is optional.

8. Over the course of the next few days, periodically adjust the gain of the active

amplifier until the 846 keV 56Fe peak occurs in channel 316 on the MCA (the 

MCA should be set for 512 channels full scale).

9. Name and activate (open) a log file in IGOR on the acquisition PC. Accumulate 

data for a couple of days. Every 50 minutes, the accumulated counts are logged.

10. Periodically type the peak channel number observed on the MCA into IGOR. 

This information will be added to the log file. If the peak has drifted away from 

channel 316, bring it back by adjusting the gain.

11. Once sufficient data have been obtained (typically after 2 days), the source can be 

removed from the bath and returned to its proper storage container. Reverse the 

steps followed above for transporting the source.
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 6 of 11 Procedure13v550

For a background measurement, only steps 7 and 9 are followed. Because a background 

measurement provides results relevant for both detectors, both amplifier gains should be 

set near their normal operating points during this measurement.

In terms of maintenance, the water level in the bath should be checked periodically. The 

level will drop due to evaporation. The level should be kept near the lower rim of the 

entrance tube on top of the bath by the addition of deionized water. It is also possible to 

add manganese sulfate if the specific gravity of the bath liquid becomes too low. In 

addition, water filters are changed periodically per manufacturer’s specifications.

This measurement is not significantly sensitive to the room or bath temperature over the 

range within which they normally vary.

Procedure for analysis

1. From each of the four file sets that have been written, select usable rows. For the 

background run this should be everything; for the other runs, it will be those rows 

where the peak was in channel 316.

2. Run mnbath.exe specifying the background file to be analyzed. Answer the 

prompts. Now background rates will be available for both channels.

3. Run mnbath.exe on the other three files. In each case, it will be necessary to 

specify the background for both channels.

4. From each of the three non-background output files, gather the number of counts 

per second (Rcustomer and RNBS-1) and their uncertainties.

5. Extract source strengths for the customer's source. The source strength S is 

derived from R through the equation 

1 1

1

− −

= −

NBS NBS

NBS

c

c

R

R

S S F , <-changed “NBS-i” to “NBS-1” in first fraction

where R is Rcustomer, SNBS-1 is the known source strength of NBS-1 on the 

calibration date, 

F = 1

if the active channel was "main", 

F = 72.25

if the active 

channel was “remote” (the remote detector is 72 times less efficient than the main 

detector), and both c's are obtained by summing the source-appropriate column in 

Table 1.

6. An additional code has been developed in the programming language R to analyze 

the data. This code carries out the same calculations as does the program 

mnbath.exe, however it has the advantage that it auto-archives all data.

Acceptance Criteria

Agreement between new values of RNBS-1 and previous values at the 1 % or better level 

provides a complete validation of the entire measurement system including electronics, 

efficiency of the detectors, and the behavior of the bath. In particular, there is always a 

long history of RNBS-1 values available. New values should be within 2 % of the trend 
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 7 of 11 Procedure13v550

suggested by previous values unless the reason for the difference is understood (for 

example additional MnSO4 has been added to the bath). When this is not the case, it 

usually means that some part of the apparatus is not functioning properly (for example,

neutron absorbing material may have been inadvertently introduced into the bath). In this 

case, calibrations must stop and the cause of the deviation must be determined and 

corrected. A thorough investigation shall accompany any deviation that cannot be 

corrected to within the expected value range. These actions shall be documented and 

submitted for technical review.

In addition, it is important to look at all the individual counts to see whether there are any 

trends or outliers. Finally, when a previously calibrated source is recalibrated, the new 

number should agree with the previous number. When the calibration is finished, the 

source is returned to the customer.

Source of 

correction

252Cf 239PuBe

AmBeRa-Be

(,n)

AmB AmF AmLiRa-Be

NBS-1

SbBe

Fast leakage 0.030 0.333 0.227 0.193 0.013 0 0 0 0

Thermal 

leakage

0.015 0.050 0.030 0.026 0.007 0.003 0 0 0

Oxygen 

capture

0.344 2.092 2.031 1.528 0.254 0 0 0 0

Sulfur 

capture

0.280 0.913 0.848 0.627 0.274 0.06 0 0 0

Teflon 

capture

0.170 0.700 0.680 0.530 0.160 0.095 0.095 0.002 0

Cavity 

flux/Q

0.186 0.141 0.143 0.190 0.156 0.197 0.350 0.43 0.52

Table 1: The magnitude of correction or effect in % for the indicated types of neutron 

source. Calculations performed by E. J. Axton, NPL, for a MnSO4 bath with dimension 

and manganese concentration same as that at NIST. We multiply the Cavity flux/Q 
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 8 of 11 Procedure13v550

correction by a factor that ranges from 1—3 depending on the size of the source 

encapsulation. Larger Sources correspond to larger factors.

Determination of uncertainties

The basis for the determination of uncertainties associated with these measurements is the 

Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results

[4]. The expanded uncertainty consists of components evaluated by statistical means (the 

so-called “Type A” uncertainties) and components determined on the basis of alternative 

techniques (the so-called “Type B” uncertainties). Type A uncertainties are the 

uncertainties on the R's, typically less than 0.5 %. Type B uncertainties include the 

following:

• NBS-1 emission-rate (± 0.85 %)

• Detector calibration (± 1 %); this is present only when the "remote" detector is 

used (customer source strength in the range 108/s to 1010 /s)

• Uncertainty associated with the applied corrections for both NBS-1 and the 

calibrated source (± 0.3 %, and typically 1 to 2 %, respectively)

The final report includes the expanded uncertainty. The expanded uncertainty 

corresponds to the quadrature-sum of the stated uncertainty components multiplied by a 

coverage factor equal to two (2). The expanded uncertainty, therefore, represents an 

approximate level of confidence of 95 %.

Documentation

All the data and analysis files are stored in customer-specific folders on the computer in 

room 245/B141 and backed up on C:\Users\msd\OneDrive - National Institute of 

Standards and Technology (NIST)\transferelwood682\My Documents\Mnbath. In 

addition, a global logbook of bath-related activity including beyond-normal maintenance 

activities is maintained in room 245/B141.

For customer calibration, prepare calibration report and obtained required signatures. 

Make copy for customer file and send original. 

References

[1] E. Dale McGarry and Edward W. Boswell. Neutron source strength calibrations. 

Technical report, National Institute of Standards and Technology, 1988. NBS Special 

Publication 250-18.
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 9 of 11 Procedure13v550

[2] J. M. Adams. Present and future trends for neutron source calibrations at the 

National Institute of Standards and Technology. Nucl. Instrum. Meth. B, 213:218—

222, 2004.

[3] Craig R. Heimbach. The neutron spectrum of NBS-1. J. ASTM International, 

Volume 3, Issue 4 (available online at www.astm.org).

[4] Barry N. Taylor and Chris Kuyatt. Guidelines for Evaluating and Expressing the 

Uncertainty of NIST Measurement Results, NIST Technical Note 1297, September 

1994.
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 10 of 11 Procedure13v550

Appendix A Calibration Report 

Report of Calibration 

Neutron Source Strength Calibration Report

NIST Report Folder Number: 682.03/O-0000000534-19

Service ID: 44020C

Report Date: August 13, 2003 

Calibration Performed for: Si Grande Corp 

Quelque part, USA 

Neutron Source Description: Source Type: 252Cf

Serial Number XXX-XX-XXXX 

Calibration Results for Source Identified Above: 

Calibrated neutron emission-rate: 1.68  108

s

-1

Relative expanded uncertainty: ± 4.30 % (95 % level of confidence) 

Calibration date: August 11, 2003 

NBS-1 emission-rate on date of calibration: 1.234  106 s-1

This calibration was performed by: This calibration was reviewed by: 

_______________________________ _______________________________ 

Maynard S. Dewey, Physicist H. Pieter Mumm, Physicist 

Neutron Physics Group Neutron Physics Group 

______________________________ 

Jeffrey S. Nico, Leader 

Neutron Physics Group 

For the Director of the National Institute of 

Standards and Technology: 

Information on technical aspects of this report _________________________________ 

may be obtained from Maynard Dewey, James M. Adams, Chief 

Radiation Physics Division National Institute of Standards and Technology, 

100 Bureau Drive Stop 8463, Gaithersburg, MD 

20899, m.dewey@nist.gov, or (301) 975-4843. 

Page 1 of 2
Radiation Physics Division 44010C, 44020C RPD-P-13

RADIOACTIVE NEUTRON SOURCES EMISSION RATES

Version Date Author Approval Pages Filename

5.50 4/6/2023 MSD AKT 11 of 11 Procedure13v550

Appendix for 252Cf Neutron Source Strength Calibration Reports

Calibration Method 

Neutron source strength measurements performed in building 245, room B142 at NIST are accomplished 

by making a traceable comparison between the emission rate of the source being calibrated and the national 

primary standard source, NBS-1, whose emission rate has been determined absolutely. The measurements 

of source emission rate are made by activating a circulating, aqueous solution of manganese sulfate, and 

continuously counting the induced 56Mn activity with a scintillation counter. During calibration, the 

neutron source is placed within a small Teflon cavity that is positioned at the center of the 1.3 m diameter 

spherical bath; activity measurements are taken once the bath has reached saturation. The purpose of the 

cavity is to reduce the absorption of thermal neutrons by the source. Corrections to the measured source 

strength have been applied in order to account for the following effects: capture of fast neutrons by oxygen 

and sulfur in the bath, capture of fast and thermal neutrons by fluorine in the Teflon source holder, neutron 

escape from the bath, and thermal neutron absorption in the source. Typical values for these corrections 

are: 

Fast neutron capture by oxygen and sulfur: 0.624 % 

Fast and thermal neutron capture by fluorine: 0.170 % 

Neutron escape from the bath: 0.045 % 

Thermal neutron absorption in the source: 0.186 % 

Because the measurement is relative, it is not sensitive to environmental conditions in the lab (temperature, 

humidity, pressure). The reported emission rate tallies neutrons of all energies emerging in all directions 

from the source. The reported number contains no information about the neutron spectrum nor the angular 

distribution of the emitted neutrons. When the emission rate is combined with a Monte Carlo model of 

neutron interactions incorporating the source neutron spectrum, source geometry, and environment, 

absolute neutron fields in the environment can be calculated. 

Uncertainties 

The expanded uncertainty consists of components evaluated by statistical means (the so-called “Type A”

uncertainties) and components determined on the basis of alternative techniques (the so-called “Type B”

uncertainties). The “Type A” and “Type B” uncertainty components relevant to this calibration are 

identified below. 

Type A uncertainties: Count-rate associated with NBS-1 (typically < 0.5 %) 

Count-rate associated with the calibrated source (typically < 0.5 %) 

Type B uncertainties: Uncertainty associated with the applied corrections for both NBS-1 and the 

calibrated source (± 0.3 %, and typically 1 to 2 %, respectively) 

NBS-1 emission-rate (± 0.85 %) 

Detector calibration (± 1.0 %) 

The expanded uncertainty reported corresponds to the quadrature-sum of the stated uncertainty components 

multiplied by a coverage factor, k, equal to two (2). The expanded uncertainty, therefore, represents an 

approximate level of confidence of 95 %.

 Page 2 of 2