A Measurement of the Neutron and Gamma Transmission 

of a Protective Vest 

by Samuel F. Trevino 

ARL-TR-3545 July 2005

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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5069 

ARL-TR-3545 July 2005

A Measurement of the Neutron and Gamma Transmission 

of a Protective Vest 

Samuel F. Trevino 

Weapons and Materials Research Directorate, ARL 

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22 December 2004–5 February 2005 

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4. TITLE AND SUBTITLE 

A Measurement of the Neutron and Gamma Transmission of a Protective Vest 

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AH80

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6. AUTHOR(S) 

Samuel F. Trevino 

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U.S. Army Research Laboratory 

ATTN: AMSRD-ARL-WM-MA 

Aberdeen Proving Ground, MD 21005-5069 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

The thermal (meV) and energetic (MeV) neutron transmission of a protective vest is measured. The vest is provided by the 

National Ground Intelligence Center. The energetic neutron transmission of 0.51 (0.025) was measured with the neutrons from

an AmBe source. The thermal neutron transmission of 0.0034 (0.00017) was measured with the neutron activated prompt

gamma instrument Prompt Gamma Activation Analysis. The numbers in parentheses are one standard deviation. Both of these 

instruments are located at the National Institute for Standards and Technology Center for Neutron Research. Additionally, the 

following isotopes were found, by delayed gamma emissions after neutron adsorption, to exist in the sample: 66Cu, 56Mn 24Na 65Zn, and 60Co. Quantitative amounts of these were not determined. No nuclei specific for neutron protection (boron, lithium,

and cadmium) are found in sufficient quantities for the purpose. The gamma ray transmission of the sample is also measured. 

Several natural radioactive sources producing gamma rays in the energy range from 60 to 1250 keV are used. The

transmission varies from ~0.6 to 1 for the lowest to the highest energy gamma rays. 

15. SUBJECT TERMS

monochromaton, neutron, crystal, press 

16. SECURITY CLASSIFICATION OF: 19a. NAME OF RESPONSIBLE PERSON

Sam F. Trevino a. REPORT 

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UNCLASSIFIED

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Prescribed by ANSI Std. Z39.18

ii 
Contents 

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Experimental 1

2.1 Energetic Neutrons..........................................................................................................1 

2.2 Thermal Neutrons............................................................................................................3 

2.3 Gamma Ray Transmission ..............................................................................................5 

3. Results 6

3.1 Energetic Neutrons..........................................................................................................6 

3.2 Thermal Neutrons............................................................................................................7 

3.3 Gamma Ray Transmission ..............................................................................................7 

4. Conclusion 8

Distribution List 9

iii 
List of Figures 

Figure 1. The vest under investigation............................................................................................1 

Figure 2. The various measurements of the spectra of neutrons from an AmBe source. ...............2 

Figure 3. The spatial distribution of neutrons from the energetic neutron source. Zero 

represents the nominal position of the collimated energetic neutron beam...............................3 

Figure 4. The thermal neutron spectrum from the NCNR research reactor....................................4 

Figure 5. A schematic of the PGAA instrument used to measure the thermal neutron 

transmission. ..............................................................................................................................5

Figure 6. The spatial distribution of neutrons from the energetic neutron source with the vest 

centered over the channel. Zero represents the nominal position of the collimated 

energetic neutron beam..............................................................................................................6 

Figure 7. The gamma ray transmission of the vest as a function of gamma ray energy. The 

line is a guide to the eye.............................................................................................................7 

List of Tables 

Table 1. The measured gamma ray transmission of the vest. .........................................................5 

Table 2. The parameters of the Gaussian fit to the spatial distribution of energetic neutrons. 

(The numbers in parentheses are one standard deviation.)........................................................6 

iv 
Acknowledgments 

We thank the health physics group at the National Institute for Standards and Technology Center 

for Neutron Research for providing the energetic neutron source and detector and Elizabeth 

Mackey for the use and aid in data collection with the Prompt Gamma Activation Analysis 

instrument for the thermal neutron measurement. 

v 
INTENTIONALLY LEFT BLANK. 

vi 
1. Introduction 

A vest, presumably designed for protection from nuclear radiation, is investigated for 

effectiveness. A possible use of the vest is the diminution of exposure to neutron and gamma 

radiation. For this purpose, the transmission of both energetic (MeV) and thermal (meV) 

neutrons and gamma rays are measured. 

2. Experimental 

The vest consists of several “pockets” of the shielding material enclosed by a canvas type 

material. The size of each pocket is ~50 × 50 mm2

 and 25 mm thick. A photograph of the vest is 

shown in figure 1. The composition of neither substance, the shielding or enclosure, is known. 

Two instruments are used in the transmission measurement of neutrons. These serve for the 

determination of the transmission of the material for energetic and thermal neutrons. Several 

natural radioactive sources of gamma rays are used to determine gamma transmission as a 

function of energy. 

Figure 1. The vest under investigation. 

2.1 Energetic Neutrons 

The energetic neutrons are obtained from an AmBe source of ~1.8 Curies activity. The several 

measurements1

 of the spectrum of these neutrons is shown in figure 2. The measurement of the 

spectra of energetic neutrons is not an easy task. This is reflected in the several spectra in the 

figure obtained by different methods, each of which has a different efficiency vs. energy. This 

source produces energetic neutrons by natural radioactive processes. As such, the energies of the 

neutrons reflect resonances characteristic of the nuclei. These resonances are clearly detected by 

 1 Report 13; International Commission on Radiation Units and Measurements: Bethesda, MD; pp. 20–21. 

1 
_____ proton-recoil counter

------- nuclear emulsions 

....... stilbene crystal 

-.-.-.-. proton-recoil spectrometer 

Figure 2. The various measurements of the spectra of neutrons from an AmBe source. 

all the methods used in their characterization, although not in the absolute intensities. This is the 

state of the art in these measurements. The source is contained in a cylindrical cavity coincident 

with the cylindrical axis of a cylindrical barrel of paraffin. The cavity is 35 mm in diameter and 

530 mm in depth. The barrel of paraffin is 610 mm in diameter and 870 mm in height. The top 

of the barrel is covered with a 0.5-mm-thick cadmium sheet. This precaution is taken to reduce 

the neutron background produced by the thermalization of the neutrons by the paraffin of the 

barrel. Measurement determined that 30% of the counting rate off center was due to thermal 

neutrons. The detector is a cylindrical proportional counter with a 16-mm diameter and 25 mm 

long-filled with BF3 (with 96% 10B) to a pressure of 600-mm Hg. It is housed in a 76.2-mm 

diameter plastic sphere. This is a standard configuration for the measurement of energetic 

neutrons. A measurement of the detected neutrons as a function of the detector position with 

respect to the cavity is shown in figure 3. The distance d is measured along a line perpendicular 

to the cylindrical axis. The solid line is a Gaussian function, 

( ) ( / 2 ) exp( [( )/ 2 ] ) 2 I d = Io σ π − d − do σ , (1)

representing the energetic neutron flux and a linear function (81 + 0.038d) representing the 

“background,” d in centimeters. This linear function is found to faithfully describe the 

“background” near the peak. Here, Io is the integrated intensity, σ is the standard deviation, and 

 is the peak position with respect to the nominal zero. Io is taken as the incident flux for the 

purpose of the transmission measurement. The value of σ = 2.7 cm corresponds to a full width at 

half maximum of ~5.86 cm. This width is adequate to cover a substantial portion of one of the 

pockets of the vest. 

do

2 
60

80

100

120

140

160

180

200

-20 -15 -10 -5 0 5 10 15 20

Cts/min

Dist (cm)

Figure 3. The spatial distribution of neutrons from the energetic neutron source. 

Zero represents the nominal position of the collimated energetic 

neutron beam. 

2.2 Thermal Neutrons 

The thermal neutron source is the research reactor of the National Institute for Standards and 

Technology Center for Neutron Research (NCNR). This research reactor operates at 20 MW

thermal. It serves as a source of thermal and cold neutrons for materials research. The 

instrument used in this investigation is the Prompt Gamma Activation Analysis (PGAA) facility 

located on a vertical beam line V5. The spectrum of neutrons is well described by a MaxwellBoltzmann distribution characterized by an equilibrium temperature of ~300 K (the temperature 

of the heavy water reactor moderator). This spectrum2

 is displayed in figure 4. The PGAA 

instrument is designed to measure the gamma rays emitted promptly by a material after 

absorption of a thermal neutron. Figure 5 is a schematic of the instrument. A sample whose 

prompt gamma spectrum is to be measured is placed in the “prompt gamma sample position.” 

The sample is illuminated with the vertical thermal neutron beam. The Ge detector measures the 

gamma ray spectrum emitted, after capture of the neutrons. The energy of the gamma rays is 

characteristic of the absorbing isotope. If well calibrated, their measured intensity may be used 

to determine the abundance of that isotope. In the present measurement, a sample of boron 

carbide (BC) in a graphite matrix is used as the “sample.” The natural isotopic abundance of 10B

and its adsorption cross section of ~4000 b is sufficient to produce measurable results of the 

reaction 

 2

 Williams, R. NIST Center for Neutron Research. Private communication, 2005. 

3 
Thermal Neutron Spectrum at NBSR 

0.0E+00

2.0E+11

4.0E+11

6.0E+11

8.0E+11

1.0E+12

1.2E+12

1.4E+12

1.6E+12

1.8E+12

0 20 40 60 80

Energy (meV)

Flu

x (n/c

m^2-s-meV)

100

Figure 4. The thermal neutron spectrum from the NCNR research reactor. 

 10B (n,α) 7*Li 7*Li 7

Li + γ ( 477 keV). (2) 

The 7*Li isotope is in an excited state that promptly decays by the emission of the 477-keV 

gamma ray. This detected gamma intensity is well correlated with the incident flux of neutrons 

on the sample. It is now a matter of measuring this gamma intensity with and without the sample

in the “transmission measurement sample position.” The collimation before the sample produces 

a neutron beam with an 11-mm diameter, substantially smaller than the pocket. The transmission

sample is well below the collimator defining the entrance to the detector. This prevents any 

signal emanating directly from the sample from reaching the detector. As a precaution to ensure 

that no signal from the sample contaminated the detector when the sample was in place, a 

spectrum was obtained with the vest in the sample position. 

4 
Borated polyethylene

Lithiated polyethylene

Lead

6Li polymer

BGO

HPGe

Beam Shutter

6Li2CO3 (fused)

Sapphire

Aluminum

Floor

Mg Window

Prompt

gamma

sample

position

Transmission

measurement

sample position

Figure 5. A schematic of the PGAA instrument used to measure the thermal neutron transmission. 

2.3 Gamma Ray Transmission 

Several sources of gamma rays were used in this measurement. These are listed in table 1, which 

contains the energy of the emitted gamma ray. A Victoreen model 451P pressurized ion 

chamber was used to detect the gamma rays. The source and detector were juxtaposed with 

enough distance between them to introduce the vest. Transmission measurements are 

accomplished by determining the detection rate with and without the vest between the source and 

detector. Errors in the measured intensity were estimated by noting the fluctuation of the count 

rate during the measurement. These errors were of order 5%, quite adequate for the present. 

Table 1. The measured gamma ray transmission of the vest. 

Source Gamma Energy 

(keV) 

Transmission

241Am 59.5 0.61 235U 186 0.75 137Cs 662 0.88 60Co 1252 1

5 
3. Results 

3.1 Energetic Neutrons 

Figure 6 shows the spatial distribution of the neutrons detected by the energetic neutron 

instrument as a function of position with respect to the central channel. This is with the sample 

covering that channel. Again, the background is approximated by a linear function (88 + 0.55d) 

of d and the signal with a Gaussian. The parameters that result from the fit are given in table 2. 

The ratio of integrated intensity with the sample to that without is 0.51 (0.025). The number in 

parentheses is one standard deviation. 

60

80

100

120

140

160

180

200

-20 -15 -10 -5 0 5 10 15 20

Cts/min

Dist (cm)

Figure 6. The spatial distribution of neutrons from the energetic neutron

source with the vest centered over the channel. Zero represents 

the nominal position of the collimated energetic neutron beam.

Table 2. The parameters of the Gaussian fit to the spatial distribution of 

energetic neutrons. (The numbers in parentheses are one standard

deviation.) 

Io 

(cts/s) 

do 

(cm) 

σ (cm) 

No sample 700 (15.5) 0.0 (0.057) 2.7 (0.052) 

With sample 357 (15.4) 0.0 (0.11) 2, 84 (0.11) 

6 
3.2 Thermal Neutrons 

The spectra of gamma rays was obtained with the vest in the sample position. This measurement 

is not intended to produce absolute quantities of the various isotopes in the sample. In order for 

that, a careful measure of the total mass of sample must be known. It is, however, possible to 

indicate the presence of various isotopes. This may still be of some use. Hydrogen and carbon 

were readily detected with the prompt gamma detector. Delayed gamma activity, measured 

shortly after exposure to the thermal neutron flux, indicated the presence of 66Cu, 56Mn, 24Na 

65Zn, and 60Co. These were easily detected. Their existence reflects the presence of the isotope 

of each material of atomic number one less that the radioisotope detected (these having been 

produces by neutron adsorption). The absence of other isotopes from this list does not indicate 

their absence from the sample. The ratio of the flux of the 477-keV gamma ray with and without 

the sample in the beam is 0.0034 (0.00017) and is therefore the transmission of thermal neutrons. 

3.3 Gamma Ray Transmission 

Table 1 contains the result of the gamma ray transmission measurement. The data is also plotted 

in figure 7. The resulting energy dependence is quite reasonable. 

0.6

0.7

0.8

0.9

1

1.1

0 200 400 600 800 1000 1200 1400

Gamma Ray Transmission

Energy keV

Figure 7. The gamma ray transmission of the vest as a function of gamma ray

energy. The line is a guide to the eye. 

7 
4. Conclusion 

This ratio of energetic neutron to thermal neutron transmission, ~150, is quite reasonable. It is 

not surprising that the transmission of thermal neutrons is so small considering the thickness of 

the material. However, as a protective device, this transmission is not small enough. For 

example, a flux of thermal neutrons of 109 neutrons/cm2

/s produces a dose of 1 rem. Health 

Physics considers a safe dose to be 0.5 mrem/hr. This would require a transmission of 

~1.4 × 10–5, substantially smaller than 3.4 × 10–3. There is also no detection of nuclei specific 

for neutron protection (boron, lithium, and cadmium). If this is designed for neutron protection, 

it is poorly done. The gamma ray transmission measurements also do not reflect any 

extraordinary properties for radiation protection. This material is not well suited to protect the 

human body from these radiations.

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