NIST Technical Note 1788

Neutron Test Results of Preliminary 

Measurements and Calculations 

Using a Handheld Instrument

L. Pibida 

C. Heimbach 

B. Norman

A. Thompson

S. Dewey

http://dx.doi.org/10.6028/NIST.TN.1788
NIST Technical Note 1788

Neutron Test Results of Preliminary 

Measurements and Calculations 

Using a Handheld Instrument

L. Pibida 

C. Heimbach 

B. Norman

A. Thompson

S. Dewey

Radiation and Biomolecular Physics Division

Physical Measurement Laboratory

http://dx.doi.org/10.6028/NIST.TN.1788

March 2013

U.S. Department of Commerce

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director
Certain commercial entities, equipment, or materials may be identified in this 

document in order to describe an experimental procedure or concept adequately. 

Such identification is not intended to imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor is it intended to imply that the 

entities, materials, or equipment are necessarily the best available for the purpose.

National Institute of Standards and Technology Technical Note 1788

Natl. Inst. Stand. Technol. Tech. Note 1788, 16 pages (March 2013)

http://dx.doi.org/10.6028/NIST.TN.1788

CODEN: NTNOEF
Neutron Test Results of Preliminary Measurements and Calculations

Using a Handheld Instrument

L. Pibida, C. Heimbach, B. Norman, A. Thompson, and S. Dewey

National Institute of Standards and Technology, Gaithersburg, MD, 20899-8462

Abstract

Initial neutron measurements and calculations were performed to investigate the response of 

handheld radionuclide identifiers (RIDs) to neutron sources with different 

moderators/scattering conditions.

Key words: neutron detection, handheld radionuclide identifiers, RIDs, 252Cf moderators/scattering 

conditions.

Introduction

The purpose of these preliminary measurements and calculations is to investigate the 

differences in response of handheld radionuclide identifiers (RIDs) to neutron sources with 

different moderators/scattering conditions. Additional neutron spectral measurements and 

calculations will be performed in order to better understand the differences in instrument 

response under different measurement conditions.

RIDs are handheld instruments designed to identify radionuclides by gamma-ray identification 

and detect neutrons. The RIDs tests are performed against two document standards, the 

American National Standard Institute/Institute of Electrical and Electronics Engineers 

(ANSI/IEEE) N42.34 standard and the Domestic Nuclear Detection Office (DNDO) Technical 

Capability Standard (TCS). The TCS does not include requirementsfor neutron detection since 

these are set in the ANSI/IEEE N42.34 standard. Currently, the standard requires testing with 

unmoderated (bare) 252Cf with a neutron emission rate of 2 × 104 n/s without the presence of a 

phantom in a low-scatter environment.

Most field measurements are being carried out by users holding the instrument at an arm’s

length from their body in an environment that has substantially more neutron scattering than a 

low scatter laboratory environment. The field measurementsettings change the neutron

1
2

spectrum, substantially increasing the probability of detection by a neutron detector such as a 

bare (or lightly moderated) 3

He-based detector.

Measurements are required to assess the suitability of the ANSI/IEEE requirements and test 

methods compared to field measurements.

Measurements setup

In order to assess the difference in response of the RIDs to field measurements and standard 

test conditions a 252Cf source was placed in different moderators/scattering conditions. The

252Cf source was initially placed inside a vehicle (1983 AMC SX4) parked on a wet asphalt

surface while a RID (Flir IndetiFinder-GN)

1 was positioned outside the vehicle at a fixed distance 

from the source. The RID is equipped with the NaI(Tl) gamma-ray detector and a 3

He neutron 

detector. The 252Cf source was placed on the floor of the trunk in 2 different locations. One was 

directly over the center (both side-to-side and front-to-back) of the gasoline tank and another 

was over the edge of the gasoline tank (approximately 1 cm from the edge of the gasoline tank 

on the driver’s side towards the back end of the trunk). For this particular vehicle the gasoline 

tank was located underneath the trunk without additional parts except for the rug and vehicle 

metal wall (there was no spare tire). The gasoline tank dimensions were approximately 42 cm 

wide, 78 cm long and 20 cm high. The gasoline tank was filled to 75 % of its 82.5 l full capacity. 

The distances from the front of the RID (located outside and in the back of the vehicle) to the 

source varied between 30 cm and 34 cm. In this case, the RID was held by a person, whose

body was located at an arm’s distance from the RID, which varied between 27 cm and 30 cm 

from the front face of the RID. The distance between the floor of the trunk and the wet asphalt 

surface was 74 cm. Figure 1 shows a diagram of the measurementssetup.

1 Mention of commercial products does not imply recommendation nor endorsement by the National Institute of 

Standards and Technology, nor does it imply that the products identified are necessarily the best available for the 

purpose.
3

Figure 1: Diagram of measurements setup with phantom

In order to compare the RID response when held by a person with that obtained in the presence

of a phantom, the RID was mounted on an aluminum stand outside the vehicle at a fixed 

distance from the source. Phantoms were placed behind the detector; the source remained

inside the vehicle. Two phantoms were used for this part of the test. One phantom was made

of polymethyl methacrylate (PMMA) (dimensions: 40 × 40 × 15 cm) while the other was a water 

phantom (dimensions: 30 × 30 × 15 cm). The 252Cf source was unmoderated (bare) and

moderated by a 4 cm thick high density polyethylene (HDPE) sphere.

The 252Cf source used for these measurements was identified by NIST source number 3828 with 

a neutron emission rate of 2.16 × 104 n/s with a reference time of 7 September 2012 12:00 PM 

EST and a combined standard relative uncertainty (k = 1) of 1.2 %. The source emission rate at 

the time of the measurements was 2.11 × 104 n/s. During the outdoor measurementsthe 

ambient temperature was (13 ± 2) °C, the relative humidity was (77 ± 4) % and the atmospheric 

pressure was (1022 ± 5) hPa.

The following configurations were measured in the vehicle:

a. Unmoderated (bare) 252Cf source placed inside vehicle with the RID placed at the same 

height as the source with a person holding the RID (the source was placed at the center 

of the gasoline tank and at the edge of the gasoline tank)

b. Unmoderated (bare) 252

Cf source placed inside the vehicle with the RID placed at the

same height as the source without the phantom present (the RID was placed on the 

aluminum stand; the source was placed at the edge of the gasoline tank)

c. Unmoderated (bare) 252Cf source placed inside the vehicle with the RID placed at the

same height as the source with the PMMA phantom present (the phantom was placed 

at 30 cm from front face of the RID, the same distance as the person holding the RID; 

the source was placed at the edge of the gasoline tank)

d. Unmoderated (bare) 252

Cf source placed inside the vehicle with the RID placed at the

same height as the source with the water phantom present (the phantom was placed at

30 cm from the front face of the RID, the same distance as a person holding the RID; the 

source was placed at the edge of the gasoline tank)

e. 252Cf source moderated by 4 cm thick high density polyethylene (HDPE) sphere placed

inside vehicle with the RID placed at the same height as the source without a phantom 

present (the source was placed at the edge of the gasoline tank)

f. 252Cf source moderated by 4 cm thick HDPE sphere placed inside the vehicle with the

RID placed at the same height as the source with the phantom present (the phantom
4

was placed at 30 cm from the front face of the RID; the source was placed at the edge 

of the gasoline tank). Measurements were performed with both the PMMA and the 

water phantoms.

To assess under which test conditions the measurementsin a low scatter laboratory 

environment match those measured in a vehicle configuration, the 252Cf source with different

moderators/scattering conditions was measured inside the laboratory. During the indoor 

measurementsthe ambient temperature was (20 ± 2) °C, the relative humidity was (56 ± 4) % 

and the atmospheric pressure was (1022 ± 5) hPa.

The following configurations were measured in the low scatter laboratory environment:

g. Unmoderated (bare) 252Cf source placed in a low scatter environment with the RID 

placed at the same height as the source; phantom is not present (the source was placed 

on the aluminum stand)

h. Unmoderated (bare) 252Cf source placed in a low scatter environment with the RID

placed at the same height as the source with phantom present (phantom placed at

30 cm from the front face of the RID). Measurements were performed with both the

PMMA and the water phantoms.

i. 252Cf source moderated by 2 cm and 4 cm thick HDPE sphere placed in a low scatter 

environment with the RID placed at the same height as the source; phantom is not 

present

j. 252Cf source moderated by 2 cm and 4 cm thick HDPE sphere placed in a low scatter

environment with the RID placed at the same height as the source with phantom 

present (phantom placed at 30 cm from the front face of the RID). Measurements were 

performed with both the PMMA and the water phantoms.

The ANSI/IEEE N42.34 standard requires testing at a distance of 25 cm, this distance was not 

attainable in the vehicle configuration so the laboratory measurements were performed at the same 

vehicle distance (30 cm) in order to obtain the RID relative response between the different tests 

configurations instead of using the distance specified in the ANSI/IEEE n42.34 standard.

Results of measurements

The RID used for the measurements displays the neutron response in counts per minute (cpm). 

A neutron survey meter (Thermo Eberline ASP 2e) was used to measure the neutron 

background at the test location; the measured value was 0.055 ± 0.008 counts per second (cps) 

(3.3 cpm).
5

Table 1 and Table 2 summarize the neutron readings obtained from the RID for the different 

test configurations listed in the measurementsetup section. The mean and standard deviation 

of 10 readings are calculated at the bottom of the tables for each test configuration. The 10 

readings were performed over a period of 2 – 3 minutes. The 10 RID background readings 

without a neutron source present were all 0 cpm. The RID alarmed when the neutron display 

was 10 cpm or higher. The source to detector distance was approximately the same for all the 

measurements.

The response of the RID to an unmoderated 252Cf source with a neutron emission rate of

2 × 104 n/s in a low-scatter environment as required by the ANSI/IEEE N42.34 standard is low 

compared to the response of the same RID when the source is placed inside a vehicle when 

parked on a wet asphalt surface. The ratio of the RID response to the unmoderated 252Cf source 

when placed inside the vehicle with a person holding the RID is 5 to 9 times higher than that 

from the same source placed in the low-scatter environment, as tabulated in Table 3.

Table 3 summarizesthe effects of choosing different low scatter environment configurations on 

expected in-vehicle count rates. For example, if the detector is calibrated with a bare 252Cf 

source, the weighted average ratio of the in-vehicle measurementsto the ANSI/IEEE N42.34 

measurement requirements (i.e., low scatter environment bare source no phantom present) is

7.67 ± 3.73. This is because the in-vehicle configurations have much more scattering than the 

calibration configuration, generating more thermal neutrons, which are easily detected by the

3

He based detector. The moderated sources better predict the response of this detector for the

in-vehicle configurations. Ideally, one would find an average of 1.0. Note that the ratio of the 

standard deviation to the weighted average is similar for all configurations. This is because the 

various in-vehicle configurations differ among themselves and no single low scatter

environment can match all the configurations measured. In Table 3 the weighted average, is the 

average of the different ratios for a given low-scatter environment test configuration weighted 

by their respective uncertainty.

For the RID used in these measurements, the response to the 252Cf source when placed inside 

the trunk of the vehicle at the edge of the gasoline tank could be simulated in the low-scatter 

environment by moderating the source with a 2 cm thick HDPE sphere and placing a PMMA 

phantom at a distance of 30 cm from the front edge of the RID.

For the RID used in these measurements,the response to the 252Cf source when placed inside 

the trunk of the vehicle at the center of the gasoline tank, within the uncertainty of the 

measurements, could be simulated in the low-scatter environment by moderating the source 

with a 4 cm thick HDPE sphere without a phantom placed behind the RID. Therefore, the use of
6

moderated neutrons for testing RIDs better describes the conditions that could be encounter in 

an operation setting. Testing in a low scatter environment with a neutron bare source with no 

phantom present is an ideal configuration but it does not represent an operational scenario.

Calculation results

Several scenarios were investigated with the Monte Carlo N-Particle (MCNP) modeling code to 

examine the effects of scattered neutrons on the detection of fission based neutron sources. 

The main diagnostic used was to compare calculated 3He capture rate vs. calculated neutron 

tissue dose. The comparison was done by calculating neutron spectra and then integrating the 

neutron spectra over the appropriate cross section. For 3He, the (n,p) cross section was used. 

For dose, the MCNP-supplied ANSI/ANS-6.1.1-1977 (rem/h)/(n/s cm2) conversion function was 

used.

Relative to each other, the 3He capture rate is more sensitive to slow or scattered neutrons and 

the neutron dose is more sensitive to fast neutrons. For both 3He and dose tallies, the absolute 

magnitude is irrelevant since no physical detector was modeled. The 3

He capture rate may be 

viewed as a proxy for 3

He-based neutron detectors. The neutron dose may be viewed as a proxy 

for a generic neutron Rem-meter. For these calculations, the 3He-based neutron detector is 

taken to be a bare 3He tube. Two sets of calculations were done. The first set investigated the 

effects on neutron spectrum of the various covers. The second set investigated the effects of 

various scattering materials (asphalt, soil, water, etc.) on the neutron spectrum.

Sources in Air

Several sources were investigated without ground scattering. These were:

dh0: 252Cf source in dry air

dh1: 252Cf source inside 4 cm thick HDPE sphere 

dh2: 252Cf source inside 8 cm thick HDPE sphere.

dh3: 252Cf source inside 8 cm thick borated HDPE sphere. 

dh4: 252Cf source inside 122 cm thick pine wood sphere.

The numbers before the colon represent the MCNP modeling code run number. All these 

calculations included a sphere of dry air of radius 200 m, with the 252Cf source at the center. 

Runs dh1-dh4 had additional material around the source. The diagnostic results are shown in 

Table 4. In Table 4, the “dist” column lists the distance between source and tally point.

The relative magnitudes of the 3He and dose rates are not important, since the units are totally 

different. What is significant is that the ratios change from about 1 × 104 for air-only to about
7

1 × 107 to 3 × 107 for the high density polyethylene sphere covers. Even the borated covers 

show an increase of about a factor of 100 over the air-only results.

No calculations were done for some of the radii of the pine sphere since the calculated point 

would be inside the pine.

Neutron spectra calculated for a source-detector (tally) distance of 200 cm are plotted in Figure

2. A distance of 200 cm was chosen because it was outside the pine. The spectra differ 

dramatically. The bare 252Cf spectrum has the highest high-energy component, but is lacking in 

slow neutrons. Below 1 × 10-4 MeV, the statistics for the bare 252Cf are poor because of the 

scarcity of neutrons in this region.

Both of the unborated HDPE spectra show a pronounced thermal neutron peak. The borated 

HDPE sphere has the thermal neutron attenuated by about a factor of 100 while having about 

the same fast fluence as the 8 cm unborated sphere.

The pine sphere is more massive than the other covers and shows a larger overall attenuation 

than the other covers, but with a spectrum similar to the HDPE.

Sources over a surface

Additional calculations were done for geometries where the 252Cf source was over a plane 

surface. These were:

dh5: 252Cf source inside 4 cm HDPE placed 50 cm over asphalt

dh6: 252Cf source inside 4 cm HDPE placed 100 cm over asphalt 

dh7: bare 252Cf source placed 50 cm over asphalt

dh8: bare 252Cf source placed 50 cm over soil

dh9: bare 252Cf source placed 100 cm over extended soil (with gasoline)

dh10: bare 252Cf source placed 50 cm over water

dh11: bare 252Cf source placed 100 cm over extended soil (no gasoline)

dh12: bare 252Cf source placed 50 cm over concrete

For these calculations, the ground was a cylinder 182.9 cm (6 foot) in radius and 2.5 cm thick

50 cm or 100 cm below the source. Tally locations were offset 100 cm horizontally from the 

source at 50 cm and 100 cm above the ground plane. Figure 3 illustrates the geometry. Results 

are given in Table 5. Statistical uncertainties in the MCNP results are all under 1.3 % (k = 1).

The asphalt composition was taken from “Concise International Chemical Assessment 

Document 59” Table 2, California asphalt with a density of 1.1 g/cm3

. The soil was dry soil from 

“Basic considerations for Monte Carlo calculations in soil”, Applied radiation and Isotopes 62 

(2005) 97-107. Table 5, Soil C, density = 2.6 g/cm3. The concrete was “ordinary concrete” from
8

pnnl-15870 (2.21 % hydrogen by mass). Gasoline was also taken from pnnl-15870. The gasoline 

was a 20.8 cm radius sphere (10 gallons) centered at 75 cm above the ground (25 cm below the 

source), offset by 50 cm horizontally from the source at a right angle to the detector.

Sources inside the HDPE gave the highest relative 3He tallies, roughly similar to the in-air tallies 

with the same covers. In this case, the thermal neutrons are due to the source cover. Spectra 

are plotted in Figure 4.

For bare sources, the presence of hydrogen moderator controlled the 3He detection. The dry 

soil used did not provide many scattered neutronsfor 3He detection. Concrete gave a much 

larger thermal neutron component, and then asphalt and water gave increasingly superior 3He 

neutron detectability because of the higher thermal neutron fluences. Gasoline was also quite 

effective in scattering neutrons.

The asphalt actually gave more 3He sensitivity than the water. The hydrogen content was about

11 % in each. Actual asphalt might also contain rock, which would make it less effective.

Generally, the calculated ANSI dose was not affected by the surroundings. The 3He tallies 

differed by orders of magnitude. A detector with an unshielded 3He counter would show a 

greater variation with surroundings than one based on ANSI dose. If a bare 252Cf source were 

used to calibrate an unshielded 3

He detector, and the detector were used in a field with 

substantial scattered neutrons, the response per neutron would be substantially higher than 

one might expect, unless the variation in spectrum were taken into account.

If unshielded 3He detectors are to be used, one might optimize the environment by measuring 

over asphalt or moist soil as opposed to concrete or dry soil.

Conclusions

From the calculations it can be observed that there are substantial differences in the neutron 

spectra when a 252Cf source is moderated by different types of materials and placed on top of 

different types of surfaces. The number of possible configurations in an operational 

environment is of course much larger than the simulated cases.

From these preliminary measurementsit was observed that the use of an unmoderated (bare)

252Cf source with a neutron emission rate of 2 × 104 n/s in a low-scatter environment as 

required in the ANSI/IEEE N42.34 standard produces a low response compared to the case when

the source is placed inside the trunk of a parked vehicle when on a wet asphalt surface. A
9

more realistic test configuration for standard performance testing would be to moderate 252Cf 

source with a 4 cm thick HDPE moderator and place a PMMA phantom behind the RID to 

simulate the presence of a person. Some RIDs have the neutron detector placed in the handle 

of the instrument. In these cases an additional hand-phantom may be needed to simulate the 

presence of a person.

The use of moderated neutrons for testing RIDs better describes the conditions that could be 

encounter in an operation setting. Testing in a low scatter environment with a neutron bare 

source with no phantom present is an ideal configuration but it does not represent an 

operational scenario. Additional measurements and calculations are required to better describe 

the test conditions to be used for the RIDs neutron tests required by the ANSI/IEEE N42.34 

standard.

Acknowledgments

The authors would like to thank the Department of Homeland Security (DHS) Domestic Nuclear

Detection Office (DNDO) for funding this work.

References

1. ANSI/IEEE N42.34, Performance Criteria for Hand-held Instruments for the Detection 

and Identification for Radionuclides.

2. “Concise International Chemical Assessment Document 59”, Ms Joann A. Wess, Dr. Larry

D. Olsen, and Dr. Marie Haring Sweeney, National Institute for Occupational Safety and

Health, Cincinnati, Ohio, USA. 2005.

3. “Basic considerations for Monte Carlo calculations in soil”. Wielopolski, Lucian; Song, 

Zhiguang; Orion, Itzhak; Hanson, Albert L.; Hendrey, George. Applied radiation and 

Isotopes Vol. 62, pp. 97-107, 2005.

4. “Compendium of Material Composition Data for Radiation Transport Modeling

Citation”. McConn R. J., Jr, C. J. Gesh, R. T. Pagh, R. A. Rucker, and R. Williams III. PNNL15870 Rev. 1, Pacific Northwest National Laboratory, Richland, WA. 2011.
10

Table 1: Results of vehicle measurements over wet pavement

Reading

Number

Bare edge 

tank person 

holding RID 

(cpm)

( a )

Bare center 

tank person 

holding RID 

(cpm)

( a )

Bare edge tank 

no phantom 

(cpm)

( b )

Bare edge tank

PMMA phantom

30 cm behind

RID (cpm) 

( c )

Bare edge tank 

water phantom

30 cm behind

RID (cpm) 

( d )

Moderated 4 cm 

HDPE edge tank 

no phantom 

(cpm)

( e )

Moderated 4 cm 

HDPE edge tank 

PMMA phantom

30 cm behind

RID (cpm) 

( f )

Moderated 4 cm 

HDPE edge tank 

water phantom

30 cm behind RID

(cpm) 

( f )

1 16 33 9 19 12 41 25 33

2 15 28 8 18 10 43 24 34

3 12 32 8 20 6 42 23 33

4 15 26 9 21 8 40 32 34

5 16 23 12 19 9 45 34 33

6 12 23 12 20 12 41 33 32

7 13 30 8 18 16 37 35 34

8 15 25 9 18 15 36 39 32

9 17 27 8 19 17 31 42 36

10 14 28 9 17 8 34 31 32

Average 14.5 27.5 9.2 18.9 11.3 39 31.8 33.3

Std dev 1.7 3.4 1.5 1.2 3.7 4.4 6.3 1.3

Table 2: Results of low scatter laboratory environment measurements

Reading

Number

Bare no 

phantom 

(cpm)

( g )

Bare PMMA 

phantom 30 cm 

behind RID 

(cpm)

( h )

Bare water 

phantom 30 cm 

behind RID 

(cpm)

( h )

Moderated 4 cm

HDPE no 

phantom (cpm)

( i )

Moderated 4 cm 

HDPE PMMA 

phantom 30 cm 

behind RID 

(cpm)

( j )

Moderated 4 cm 

HDPE water 

phantom 30 cm 

behind RID 

(cpm)

( j )

Moderated 2 cm 

HDPE no 

phantom (cpm)

( i )

Moderated 2 cm 

HDPE PMMA 

phantom 30 cm 

behind RID (cpm) 

( j )

1 2 8 4 30 31 32 16 13

2 3 6 7 28 38 28 15 13

3 4 3 6 31 33 27 15 12

4 3 5 8 32 34 31 15 12

5 2 4 4 34 42 29 17 15

6 4 3 8 35 39 30 16 13

7 3 8 6 32 40 33 15 15

8 3 10 3 34 38 29 13 17

9 2 6 6 29 41 28 12 16

10 4 4 8 30 40 31 14 17

Average 3 5.7 6 31.5 37.6 29.8 14.8 14.3

Std dev 0.8 2.4 1.8 2.3 3.7 1.9 1.5 1.9
11

Table 3: Ratios of counts for in-vehicle configurationsto the low scatter configurations

Low scatter environment Ratio of In-Vehicle to equivalent low scatter configuration

Weighted

Average

Standard

Deviation

St Dev/ 

Test Configuration Average RID 

(cpm)

Bare edge 

tank person 

holding RID

Ba re center 

ta nk pers on 

holding RID

Bare edge 

tank no 

phantom

Bare edge tank

PMMA phantom

30 cm behind

RID

Bare edge tank 

water

phantom 30 cm

behind RID

Moderated 4 cm 

HDPE edge tank 

no phantom

Modera ted 4 cm 

HDPE edge ta nk 

PMMA phantom

30 cm behind RID

Modera ted 4 cm 

HDPE edge ta nk 

water phantom

30 cm behind RID

bare source no

phantom 3.00 4.83 9.17 3.07 6.30 3.77 13.00 10.60 11.10 9.15 3.73 0.41

uncertainty (k = 1) 0.82 1.43 2.75 0.98 1.76 1.61 3.83 3.57 3.05

bare source PMMA

phantom 5.70 2.54 4.82 1.61 3.32 1.98 6.84 5.58 5.84 4.85 1.96 0.40

uncertainty (k = 1) 2.36 1.10 2.09 0.72 1.39 1.05 2.93 2.56 2.43

bare source water

phantom 6.00 2.42 4.58 1.53 3.15 1.88 6.50 5.30 5.55 4.59 1.86 0.41

uncertainty (k = 1) 1.83 0.79 1.51 0.53 0.98 0.85 2.11 1.92 1.70

4 cm HDPE no

phantom 31.50 0.46 0.87 0.29 0.60 0.36 1.24 1.01 1.06 0.84 0.36 0.42

uncertainty (k = 1) 2.32 0.06 0.13 0.05 0.06 0.12 0.17 0.21 0.09

4 cm HDPE PMMA 37.60 0.39 0.73 0.24 0.50 0.30 1.04 0.85 0.89 0.71 0.30 0.42

uncertainty (k = 1) 3.69 0.06 0.12 0.05 0.06 0.10 0.15 0.19 0.09

4 cm HDPE water 29.80 0.49 0.92 0.31 0.63 0.38 1.31 1.07 1.12 0.88 0.38 0.42

uncertainty (k = 1) 1.93 0.07 0.13 0.06 0.06 0.13 0.17 0.22 0.08

2 cm HDPE No

phantom 14.80 0.98 1.86 0.62 1.28 0.76 2.64 2.15 2.25 1.80 0.76 0.42

uncertainty (k = 1) 1.48 0.15 0.30 0.12 0.15 0.26 0.40 0.48 0.24

2 cm HDPE PMMA

phantom 14.30 1.01 1.92 0.64 1.32 0.79 2.73 2.22 2.33 1.88 0.78 0.42

uncertainty (k = 1) 1.95 0.18 0.36 0.14 0.20 0.28 0.48 0.53 0.33
12

Table 4: Dose and 3He diagnosticsfor no-ground simulations

Infinite dry air (dh0) 4cm HDPE (dh1) 8cm HDPE (dh2) 8cm borated HDPE (dh3) 122 cm pine sphere (dh4)

dist dose 3He dist dose 3He dist dose 3He dist dose 3He dist dose 3He

(cm) (cm) (cm) (cm) (cm)

20 2.36E-08 1.65E-04 20 1.35E-08 1.39E-01 20 7.13E-09 2.45E-01 20 6.90E-09 6.77E-03 20

50 3.79E-09 2.66E-05 50 2.15E-09 2.21E-02 50 1.12E-09 3.79E-02 50 1.08E-09 1.05E-03 50

100 9.52E-10 6.73E-06 100 5.41E-10 5.66E-03 100 2.81E-10 9.67E-03 100 2.72E-10 2.69E-04 100

200 2.41E-10 1.72E-06 200 1.37E-10 1.49E-03 200 7.11E-11 2.53E-03 200 6.87E-11 7.18E-05 200 2.40E-13 1.58E-05

2000 2.85E-12 3.98E-08 2000 1.64E-12 2.31E-05 2000 8.56E-13 3.81E-05 2000 8.20E-13 1.72E-06 2000 2.62E-15 2.12E-07

3He/dose Normalized 3He/dose Normalized 3He/dose Normalized 3He/dose Normalized 3He/dose Normalized

20 7.01E+03 1.000 20 1.03E+07 1.000 20 3.44E+07 1.000 20 9.82E+05 1.000 20

50 7.02E+03 1.002 50 1.03E+07 0.997 50 3.38E+07 0.983 50 9.68E+05 0.986 50

100 7.06E+03 1.008 100 1.05E+07 1.016 100 3.44E+07 0.999 100 9.90E+05 1.008 100

200 7.15E+03 1.021 200 1.09E+07 1.055 200 3.56E+07 1.035 200 1.04E+06 1.064 200 6.57E+07

2000 1.40E+04 1.994 2000 1.41E+07 1.370 2000 4.45E+07 1.293 2000 2.09E+06 2.134 2000 8.09E+07
13

Figure 2: Calculated neutron spectra for no-ground geometries
14

Figure 3: Schematic of geometry for calculations with ground
15

Table 5: Dose and 3He diagnosticsfor ground simulations

4 cm HDPE placed 50 cm over 4 cm HDPE placed 100 cm Bare Cf placed 50 cm over Bare Cf placed 50 cm over

asphalt (dh5) over asphalt (dh6) asphalt (dh7) soil (dh8)

height dose 3He height dose 3He height dose 3He height dose 3He

(cm) (cm) (cm) (cm)

50 5.83E-10 9.86E-03 50 4.58E-10 7.40E-03 50 1.02E-09 6.67E-04 50 1.10E-09 8.15E-06

100 4.59E-10 7.47E-03 100 5.56E-10 7.52E-03 100 8.08E-10 5.02E-04 100 8.57E-10 6.30E-06

3He/dose 3He/dose 3He/dose 3He/dose

50 1.69E+07 50 1.62E+07 50 6.52E+05 50 7.40E+03

100 1.63E+07 100 1.35E+07 100 6.21E+05 100 7.35E+03

Bare Cf placed 100 cm over extended

soil (no gasoline) (dh9) Bare Cf placed 50 cm over water (dh10) Bare Cf placed 100 cm over extended

soil (with gasoline) (dh11) Bare Cf 50 cm over concrete (dh12)

height dose 3He height dose 3He height dose 3He height dose 3He

(cm) (cm) (cm) (cm)

50 8.57E-10 6.30E-06 50 1.01E-09 5.20E-04 50 8.60E-10 2.94E-04 50 1.07E-09 6.89E-05

100 1.01E-09 7.31E-06 100 7.98E-10 3.83E-04 100 1.02E-09 3.49E-04 100 8.37E-10 5.00E-05

3He/dose 3He/dose 3He/dose 3He/dose

50 7.35E+03 50 5.15E+05 50 3.42E+05 50 6.45E+04

100 7.21E+03 100 4.79E+05 100 3.43E+05 100 5.97E+04
16

Figure 4: Calculated neutron spectra for ground geometries