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F. M. Tomnovec and J. M. Ferguson 


TisiA a 

Issunr.'c* Date: September 19B.S 



This report was prepared as an account of Government sponsored work. Neither the United 
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USAEC Divition of Tsehnicol Infoimotion Extemion, Ook Ridgs, Tsnnsuss 




F. M. Tomnovec and J. M. Ferguson 

Approved by: L. J. DEAL 

Chief, Civil Effects Branch 
Division of Biology and Medicine 

Naval Radiological Defense Laboratory 
San Francisco, California 


This report is published in the interest of providing informttion which may prove of 
value to the reader in his study of effects data derived principally from nuclear weapons 
tests and from experiments designed to duplicate various characteristics of nuclear 

This document is based on information available at the time of preparation which 
may have sid>sequently been expanded and re-evaluated. Alto, in preparing this report 
for publication, tome clanified material may have been removed. Users are cautioned 
to avo'd interpretations and conclusioiu bated on unknown or iiKomplcte data. 


Operation BREN (Bare Reactor Experiment, Nevada) was an experiment that used a 1500- 
ft tower and a bare (no shielding) fast nuclear reactor patterned on "Godiva” to simulate a nu¬ 
clear weapon detonated at various heights above the ground. Certain characteristics of the 
neutron field from this unshielded reactor were measured. The activation of gold, manganese, 
sulfur, cadmium-covered gold, and cadmium-covered manganese was determined as a function 
of distance from the reactor and of depth in the ground. The data show how those parameters, 
important for calculating neutron-induced activity in the soil, vary as a function of the height 
of the source and the slant range from the source. 


The authors gratefully acknowledge the following persons who gave assistance in Opera¬ 
tion BREN and in the preparation of this report. 

J. A. Auxier, Technical Director, F. W. Sanders, Deputy Technical Director, and F. F. 
Haywood, Oak Ridge National Laboratory, who assisted the authors during the many 
months of data taking. 

W. H. Buser and E. W. Jones, Naval Radiological Defense Laboratory, preparation and ex¬ 
perimental arrangements. 

R. A. Taylor and R. F. Ehat, Naval Radiological Defense Laboratory, electronics. 

D. M. Weldon, Naval Radiological Defense Laboratory, data analyses. 

LCDR G. W. Werner and P. A. Read, Naval Radiological Defense Laboratory, determina¬ 
tions of neutron flux above the sulfur threshold. 






1.1 Background.9 

1.2 Objectives.9 

1.2.1 Measurements near Ground Zero.10 

1.2.2 Epicadmium-neutron Activation.10 

1.2.3 Effect of Source Altitude.10 


2.1 Sample Irradiation.11 

2.2 Measurement of Relative Activities.12 


3.1 Discussion of Experimental Results.23 

3.2 Summary.24 

3.3 Conclusions.25 



2.1 View from the Tower Showing the BREN Experimental Area During 

the Last Phase of the Tower Construction.13 

2.2 Type of Terrain Around the 100-yd Station.13 

2.3 Type of Terrain Around the 150-yd Station.14 

2.4 Type of Terrain Around the 200-yd Station.14 

2.5 Type of Terrain Around the 250-yd Station.15 

2.6 Type of Terrain Around the 3(K)-yd Station.15 

2.7 Soil from a Hole near the 100-yd Station.16 

2.8 Soil from a Hole near the 300-yd Station.16 

2.9 View of the ORNL Revetment Showing the Strata That Are Usually Seen 

in This Area.17 

2.10 Drill and Auger Used to Place the “Sabers” in the Ground ... .17 

2.11 NRDL Personnel Drilling Holes with the Auger and Inserting Saber 

Sheaths into the Ground.18 

2.12 Samples and Sample Holders for the Irradiation.Ig 

2.13 Pulse-height ^ctrum of **Mn. 19 



2.14 Method of Obtaining the Activity from the Pulse-height Spectrum ... 19 

2.15 Efficiency Curve for the Gamma-ray Spectrometer.20 


3.1 Relative Gold Thermal-neutron Activation as a Function of Depth ... 26 

3.2 Gold Epithermal Activations Vs. Depth.27 

3.3 Gold Thermal Activation Vs. Depth, Normalized at the Surface • . . 28 

3.4 Gold Epithermal Activation Vs. Depth, Normalized at the Surface ... 29 

3.5 Gold Thermal Activation Vs. Depth, Normalized at the Surface ... 31 

3.6 Gold Epithermal Activation Vs. Depth, Normalized at the Surface ... 32 

3.7 Gold Thermal Activation Vs. Depth for the 100-yd Station and 27-ft 

Reactor Height .34 

3.8 Soil Moisture Content Vs. Time.34 

3.9 Surface Gold Activation Vs. Slant Range.35 

3.10 Manganese Activation Vs. Depth.35 

3.11 Manganese Epithermal Activation Vs. Depth.37 

3.12 Manganese Activation Vs. Depth.38 

3.13 Manganese Epithermal Activation Vs. Depth.40 

3.14 Sulfur Neutron Activation.41 

3.15 Activation of Manganese, Copper, and Arsenic .42 



2.1 Properties of Induced Activities.12 


3.1 Density Measurements (g/cm*).21 

3.2 Precipitation Recorded at Yucca Valley . 22 

3.3 Calculated Roentgens per Hour at Each Station and Reactor Height 
for a Given Sulfur Flux and Amount of Manganese in the Soil at 

Each Station.23 

3.4 Percentage of the Total Radiation Field That Is Contributed by 
Manganese Activated by Neutrons Above the Cadmium 

Cutoff Energy.24 

Chapter 1 



In airbursts of nuclear weapons, neutron'induced activity in the soil is one of the major 
residual hazards. The U. S. Naval Radiological Defense Laboratory (NRDL) has a continuing 
program for the study of this hazard. At Operation Teapot (1955), samples of different soils 
were exposed to nuclear weapon radiations and studied for the type and intensity of Induced 
radioactivity.' For most soils, ^*A1, ^*Mn, and ^*Na are the dominating activities. Other ac¬ 
tivities may become important in unusual types of soil. 

So that the dose rates from the induced activity can be predicted, certain measurements 
must be made near the ground-air interface. First, it is necessary to know the neutron-flux 
intensity and energy spectrum as a function of distance from Ground Zero and of depth in the 
ground. Second, it is necessary to know the effective neutron-activation cross sections of the 
important soil elements. These cross sections can be computed if the field neutron-energy 
spectrum is well known and if the activation cross sections have been measured as a function 
of energy for each element. However, these quantities generally are not known, and thus field 
measurements are necessary. 

At Operation Plumbbob in 1957, NRDL undertook the measurement of some of these 
quantities.^ Samples of the important soil elements were placed in an array at various dis¬ 
tances and depths around nuclear airbursts. The specific activities of about 30 different 
elements were determined with a gamma-ray spectrometer. The neutron flux over various 
energy ranges was measured by using gold, cadmium-covered gold, and sulfur threshold de¬ 
tectors. These data and a description of the measurements are given in Ref. 2, and an analy¬ 
sis of the data is given in Ref. 3. 

In the period since 1957, these results have been supplemented by a series of laboratory 
experiments and calculations. This later work was directed toward the problem of applying 
the field results to conditions other than those at Operation Plumbbob. The information still 
had gaps. In particular, some questions remained about the activation of manganese by epi¬ 
thermal neutrons; there was little or no data concerning the neutron flux, neutron activation, 
or neutron-induced activity dose rates for the area within about 200 yd of Ground Zero; and 
there was little information about the effects of varying the burst height. 


The experimental technique us.d at Operation BREN was to simulate the neutron radia¬ 
tion field from a nuclear weapon with a reactor. The unshielded fast reactor [the Oak Ridge 
National Laboratory Health Physics Research Reactor (HPRR)] was raised in a hoist to vari¬ 
ous elevations on a 1500-ft tower. Measurements of dose rate by Program 1 were taken, in 
various configurations, inside facsimiles of Japanese houses typical of those in Hiroshima and 
Nagasaki in 1945. This study is to help evaluate the radiation doses received by persons ex¬ 
posed to nuclear weapons, especially the residents of Hiroshima and Nagasaki, Japan. Other 


selected programs that could utilize the radiation fields available on a noninterference basis 
were included as parts of the operation.* 

The general aim of Program 5 was to determine the neutron-energy spectrum and soil 
radioactivity resulting from a neutron source in the air as a function of distance from the 
source and of depth in the ground. Gold and sulfur activation data serve as measurements of 
the relative neutron flux, and the data on manganese give its relative effective activation cross 
section in the neutron field near the ground-air interface. 

The reactions involved in this experiment are as follows: 

®S(n,p)**P Effective threshold is 3.0 Mev 

**^Au(n,>')***Au Thermal-neutron activated 

®*Mn(n,y)®*Mn Thermal-neutron activated 

The specific objectives of this program are described in Secs. 1.2.1 to 1.2.3, which follow. 

1.2.1 Measurements near Ground Zero 

In a nuclear blast it is difficult to expose and recover samples near Ground Zero since the 
samples must not be shielded from neutrons, and yet must withstand the blast and thermal ra¬ 
diation. On the other hand, this is the easiest region in which to make reactor measurements 
since it is close to the source. At BREN, therefore, particular emphasis was placed on getting 
data extending from close to the base of the tower to 200 yd away, since data for these distances 
could be normalized to weapons-test results. 

1.2.2 Epicadmium-neutron Activation 

The Plumbbob measurements^ indicate that certain elements, notably manganese, are sub¬ 
stantially activated by epicadmium neutrons^ --neutrons with energies greater than about 0.3 
ev. If the field data are used, it is hard to correlate this activation with the epicadmium flux 
since the thermal-neutron activation must be subtracted out. At BREN, both gold and manga¬ 
nese were cadmium shielded. Originally it was intended that similar measurements be made 
for sodium, copper, and arsenic. However, data for these elements were obtained only at the 
30-yd station because the flux levels were too low. 

1.2.3 Effect of Source Altitude 

Most of the weapons-test data are for bursts at a 500-ft altitude. Since air absorption is 
important, a simple inverse-square correction cannot be used to extrapolate to other altitudes. 
Data un the activation at fixed positions in the soil and for a fixed type of source, but for dif¬ 
ferent altitudes, should provide the information needed for altitude corrections. 


1. R. F. Johnson, C. S. Cook, L. A. Webb, and R. L. Mather, Ncnlron-mlnced Radioactirc Iso¬ 
topes in Soil, Project 2.3a, Operation Teapot, Report WT-1117, Aug. 7, 1958. 

2. C. S. Cook, W. E. Thompson, F. M. Tomnovec, R. L. Mather, J. M. Ferguson, and P. R. 
Howland, Seutron-imiuced Activities in Soil Elements (U), Project 2.2, Operation Plumbbob, 
Report WT-1411, July 31, 1959. (Classified) 

3. R. L. Mather, New/row Energy Effects and Induced Activation (Plumbbob Observations), Re¬ 
port USNRDL-TR-465, Naval Radiological Defense Laboratory, Sept. 28, I960. (Classified) 

4. F. W. Sanders, F. F. Haywood, M. J. Lundin, L. W. Gilley, J. S. Cheka, and D. R. Ward, 
Operation Plan atui Ilaiards Report — Operation BREN, USAEC Report CEX-62.02, April 


Chapter 2 


The type of data needed is the relative activities of certain elements exposed at various 
distances from the reactor and at various depths in the ground. The discussion of experimental 
technique is divided into two parts: (1) the positioning and irradiation of the samples and (?) 
the determination of the relative activities of the irradiated samples. 


Small samples of gold, sulfur, and manganese were placed in an array at various distances 
and depths around the reactor tower. The distances varied from 30 to 400 yd; the depths ranged 
from the surface to 20 in. under ground. The activity induced in the samples at the further sta¬ 
tions often was too weak to measure. The greatest distance at which data were obtained was 
400 yd. Figure 2.1 shows a view from the tower and the relation of He station array relative 
to the BREN e.' perimental area.^ Figures 2.2 to 2.6 show views of the stations and of the type 
of terrain at each station. The terrain varied from one station to another. Figures 2.7 and 2.8 
show the difference in soil composition between the 100- and the 300-yd station, the 300-yd 
station indicating a rather large increase in rock content, especially in f.ize. Figure 2.9 shows 
another view of the stratum effect which was noticed at the BREN area and which is common 
to this area. The total effect of this variation in soil composition was not noticeable in the final 

At each distance “saber” type sample holders were put in the ground. Figure 2.10 shows 
the auger drill useu to make holes in the soil which take the holders and some of the sample 
holders used in this experiment. Figure 2.11 shows personnel placing the sample holders into 
the ground. Each hole was filled with soil after the sample holder was in place. The "sheath" 
of the saber is a light aluminum tube of cylindrical or rectangular cross section, and the saber 
itself is a light piece of aluminum which holds the samples and which slides in and out of the 
sheath. After the sheaths had been placed in the ground, the samples were positioned or re¬ 
covered by sliding the saber in or out. Figure 2.12 shows the sample holders and some of the 

The obvious question about this technique is whether the aluminum perturbs the neutron 
flux at the sample position. At Operation Plumbbob, most of the samples were suspended in¬ 
side large dirt-filled canisters buried in the ground.^ At some stations "sabers” were buried 
near the canisters for comparison. The differences between the saber results and the canister 
results were not systematic and were well within the experimental error. 

An experimental run was made at the 60-in. cyclotron of the University of California by 
using the neutron source of protons on beryllium to investigate the moderating influence of 
aluminum sample holders on fast neutrons. Gold detectors established the thermal-neutron 
flux with the presence of the aluminum holders and without the aluminum. No observable ef¬ 
fects were noticed. Calculations also show that the absorption of thermal neutrons by the 
aluminum holders is less than 1%. From Plumbbob data we have, for Nevada Test Site (NTS) 


soil with a moisture content of 6'^, a mean free path for sulfur neutrons of 5.1 in. Since the 
holders for the thermal-neutron detectors have '/\ in. of aluminum, thicknesses of about Vooth 
of a mean free path are involved. We conclude that differences between aluminum and NTS 
soil are not sufficient to perturb the flux in this situation. 

The gold and sulfur samples are disk shaped; their size and shape are determined by the 
counting technique described in Sec. 2.2, The manganese samples weigh 1 g each, and they are 
sealed in pharmaceutical gelatin pills. The gold and manganese samples were counted with a 
gamma-ray spectrometer. The thermal-neutron capture cross section of manganese is large 
enough that neutron attenuation in the sample must be corrected for; this correction factor has 
been determined in the laboratory.' 

Cadmium covers were provided for both the gold and the manganese samples. A bare 
sample and a cadmium-covered sample were placed at each position. So that flux perturbation 
by the cadmium would be avoided, the two types of samples were kept well separated. 


The properties of the induced activities are given in Table 2.1. No sampics of manganese 
were irradiated for more than one day because of its short half-life. The sat;,pies were put in 
position the day before irradiation and were recovered when the reactor was shut down for 
the day. They were then taken back to the NRDL mobile-laboratory trailer at Camp Mercury. 




Activity counted 

(from sulfur) 

14 days 

Beta rays 


2.58 hr 

0.845-Mev gamma 


2.70 days 

0.412-Mev gamma 

The trailer contained a gamnia-ray spectrometer, which was used to measure the activity 
in the manganese and gold samples. This spectrometer, which is basically the same as the one 
used at Plumbbob, consists of a 4-in.-high 4-in.-diameter Nal(Tl) crystal, a 100-channel 
analyzer, and the associated electronics. This equipment is described in Ref. 2. The sample 
to be measured is put in a holder which positions the sample 1 in. from the face of the crystal. 
Figure 2.13 shows a typical pulse-height spectrum taken with this apparatus. 

The full-energy peak of the prominent gamma ray in each spectrum is used to determine 
the activity in the sample. The area under the peak is determined as shown in Fig. 2.14. The 
activity of the sample, in counts per second, is obtained from the area by using an empirical, 
energy-dependent correction factor (Fig. 2.15). In the determination of the relative activity 
of the sample at the end of the irradiation, the sample weight and the decay of the activity with 
time must also be corrected for. Reference 2 elaborates on this technique. 

The sulfur activity was determined by beta-ray counting by using Geiger-Mueller tubes 
in a reproducible geometry. The factor for converting from counts to neutron flux is empirical 
and has been determined by intercomparison with other laboratories. The sulfur-measurement 
system was calibrated by using the 14-Mev neutron source at Los Alamos. The half-life of the 
^■P is such that there was enough time to ship the samples back to NRDL for counting. As an 
alternative methcxl when the neutron flux was low. the samples were ashed to increase the 
counting efficiency, and their activities were determined with a calibrated Geiger-Mueller 
counter mounted in a Baird-Atomic. Inc., model 750 automatic counting apparatus. Each count¬ 
ing system at any laboratory has to be calibrated in terms of the various parameters, detector 
size, thickness, geometry, etc.; therefore calibration data here are not presented since they 
are not |x*rtinent to other laboratories. 


Fin. 2.1 —V'ifw from the tower showing the BHKN experimental area during the last phase of the tower 
construetion. The trailer of F’rogram 4 is at the right background, the Japanese houses are visible in the 
background, and the induced-activit\ static ns of Program 5 are to the left of the road. Close 
scrutin\ will show the flags indicating each station, starting with the 10(i-\d station. NRDL activity is 
visible at the 150-vd station. 

Fig. 2.3 — Type of terrain around the 150-yd station 


IK. -.7 — Soil from a hok’ ni’ar lh»' 10(i-yd station. T he Kround in this area is toniposod ol small stum s 
nd soil. 


if thi' OHNI. rfvctmi'nt .showing iho strata that aro usually si-en in this area 

element sample 

Fig. 2.10 — Drill and auger used to place the 
holder are shown. 

F'ig. 2.11 —NKDL personnt‘1 drilling holos with thf auger and inserting saber sheaths into tlu' ground 

Kig. 2.12 — Sani(>les and sample holders for the irradiation, tin the left is a gold sample holder (lartially 
inserted in its sheath. The reeesses for the lop two samples are e.xposed. The next three items are a 
cadmium holder, a gold sample, and a cadmium lid. The three small items on the right are a cadmium lid, 
a sodium sample, and a cadmium holder for the sodium. On the right is a saber for positioning the sodium 
samiiles, partially inserted in its sheath. 

Fig. 2.13—Pulse-height spectrum of **Mn. This figure shows the **Mn gamma-ray pulse-height spec¬ 
trum taken with the mobile-laboratory spectrometer. Counting rate is plotted against channel number 
(pulse height). The peaks at channels 24, 55, and 64 correspond to gamma rays with energies of 0.845, 
1.8, and 2.1 Mev, respectively. The 0.845-Mev gamma-ray peak was used to determine the activity, 
as shown in Fig. 2.3. 

sa 4i 4 « 50 54 *-a 


Fig. 2.14—Method of obtaining the activity from the pulse-height spectrum. The full-energy peaks in the 
pulse-height spectrum are well represented by a Gaussian curve. The curve is fitted to the high-energy side 
of the peak, and the area of the curve is proportional to the activity in the sample. 


Fig. 2.15 — Efficiency curve for the gamma-ray spectrometer. The ordinate represents the percentage of 
photons emitted from the sample which will contribute to the full-energy peak of the pulse-height spectrum. 


1. F. W. Sanders. F. F. Haywood, M. I. Lundin, L. W. Gilley, J. S. Cheka, and D. R. Ward, 
Operation Plan and Hazards — Operation BREN, USAEC Report CEX-62.02, April 1962. 

2. C. S. Cook, W. E. Thompson, F. M. Tomnovec, R. L. Mather, J. M, Ferguson, and P. R. 
Howland, Nentron-itulneed Activities in Soil Elements (U), Project 2.2, Operation Plumbbob, 
Report WT-1411, July 31, 1959. (Classified) 


Chapter 3 

The results of the experiment are presented in Figs. 3.1 to 3.15. The relative activation 
of the samples is usually plotted vs. depth in the ground for each detector station. The curves 
labeled epithermal neutron activation represent the activities of cadmium-covered samples, 
and the data labeled thermal-neutron activation were obtained by subtracting the cadmium- 
covered activation from the activation of the bare samples. 

The relative gold thermal-neutron activities at the different stations for four reactor 
heights are shown in Fig. 3.1. The probable error of each number is about 10%. Similar plots 
for the gold epithermal activity are shown in Fig. 3.2. 

Figure 3.3 shows the same data as in Fig. 3.1, except that the curves are normalized at 
the surface for comparison of their shapes, showing the buildup of thermal-neutron flux in the 
ground. Figure 3.4 is the same type of plot for the gold epithermal activity. 

In Fig. 3.5 the gold thermal flux is again normalized at the surface, bat in this case each 
station is plotted separately, with varying reactor heights. Figure 3.6 is the corresponding 
curve for the gold epithermal flux. 

It is evident from Figs. 3.1 to 3.6 that the neutron distribution in the ground is not a simple 
function of a few parameters. For a given station the shape changes with reactor height. For 
the same reactor height, the shape changes with the distance from the tower. Also, the data 
show that the shape should be sensitive to local variations in the soil density and composition 
as well as to the number of air mean free paths from the reactor. The soil density at each of 
the stations is given in Table 3.1. 


(yards from 
tower base) 


















1.54 to 1.74 







1.68 to 1.80 







1.27 to 1.47 







1.43 to 1.59 







1.61 to 1.95 







1.54 to 1.68 







1.50 to 1.66 







1.34 to 1.42 







1.69 to 1.95 

'Experimenter’s initials. 

A particularly interesting variable is the soil moisture content.^ The moderating proper 
ties of the soil depend critically on the moisture. The effect is shown in Fig. 3.1. These two 
sets of data were taken under identical conditions except that one set was taken later in the 


year after the rain had stop|x?d and the soil moisture eontent had dropiK'd. The rainfall and 
soil-moisture variations for the data-taking ix'riod are fiiven in Table 3.2 and Fin. 3-8. The 
curve for the drier soil shows less |)eakinn and less rapid falioff with de|)th.‘ 



Bain, in. 

Snow, in. 




T race 



2 (on ground) 



4 (on ground) 


















T race 



T race 










0.01 (watt'r e(|uiv.) 


T race 





0.09 (with hail) 




T race 


T race 




0.04 (with hail) 





T race 






T race 




‘These data were reeorded at the weather station at the Yueea 
airstrip. The rainfall at the BKEN tower site would have been 
somewhat different. However, the Yueea airstrip data should give 
a good indieation of the amo .it of rain, or iaek of it, at the BHUN 
tower site. 

Figure 3.9 shows the variation of the surface neutron flux (gold thermal and epithermal) 
with slant range from the reactor. The data roughly follow a smooth curve, but it i.s evident 
that, for a given slant range, the flux is not independent of reactor height. 

The manganese data are given in Figs. 3.10 to 3.13. The manganese thermal flux follows 
the gold thermal flux well for a given set of conditions. However, mangam se and gold runs 
taken for the same geometries but at different times of tht‘ year (which means different soil 
moisture) are not the same. The manganese epithermal and gold epithermal results also show 
about the same Ix'havior, although the two elements are sensitive to somewhat different ranges 
of neutron energies. 

Some fast-neutron data obtained by using sulfur threshold detectors are shown in Fig. 3.14. 

One set of data was obtained for two other elements, copix'r and arsenic, at the 30-yd sta¬ 
tion and a reactor height of 27 ft. These elements were chosen because they are potentially 
important in the induced-activities problem and because they have high epithermal cross sec¬ 
tions. The relati.’e activities are shown in Fig. 3.15. No attempt was made to correct for the 


various disintegration rates of the gamma ray chosen for each isotope. The absolute relation 
between each element from a given neu'.ron flux is not presented here either but, rather, a 
comiwrison of the relative activity of each isotope as a function of depth in the ground; thus 
the shapes of the curves show the differences that can be attributed to the different energy 
variations of the activation cross sections. 


The manganese data were normalized to the surface-activated manganese. These 
manganese-activation profiles were then inserted in the NRDL 704 computer program. By as¬ 
suming a constant neutron field and a fixed percentage of manganese in the soil, the program 
computed the radiation field from each activation profile for all the stations and reactor 
heights for which data were obtained. A future NRDL report will discuss this program.* Table 
3.3 shows the resulting dose in roentgens per hour at 3 ft above the ground. The data taken at 
30 yd show a reduction in the radiation field as the neutron source goes higher. A qualitative 
examination of color slides taken of the reactor at each height and from each station indicates 
that structure members of the tower arc interposed between the reactor and the 30-yd station 
for reactor heights at 1125 and 1500 ft. The photographs also show that only the reactor at 
1500 ft has attenuation present for the 100-yd station. The important point we should like to 
emphasize here is that we do not have enough data to make a precise calculation on the effect 
of the tower structure on the emitted neutron spectrum. 


height, ft 


30 yd 

100 yd 

150 yd 

200 yd 

250 yd 

300 yd 





























= 34.311:11^ 

•Neglected in the average because of excessive water content in the soil at 
time of data taking. 

tNeglected in the average because of tower structure effects on the neutron 

The computed radiation fields at the reactor heights of 500, 299, and 27 ft for the 30-yd 
station were greater than the average radiation-field value at the other stations because the 
data for the 30-yd station were taken early in April whereas data for the other stations were 
taken in late May and in June. During this time there was a dramatic change in moisture con¬ 
tent of the soil, and the effect can be seen in Figs. 3.7 and 3.8. The change in moisture con¬ 
tent of the soil changes the activation profile in the soil and the resultant radiation field. 

Because of the interference and uncertainty resulting from structure effects of the tower 
and moisture content of the soil, we ignored these marked stations and looked at effects of 
height and distance from the tower on the other stations. The average dose rate over the rest 
of the stations is 34.3 r/hr i If/.t. and there is no apparent systematic effect caused by the 
height and the distance from the source. 

Figure 3.3E shows the thermal-neutron flux averaged over all stations for each reactor 
height. There is definitely a changing profile with reactor height, but in the first 6 in. of the 
soil there is very little variation. The computer program indicates that iTi of the observed 
radiation comes from the first 6 in. of soil, and this is one of the factors that limits any 
change in the radiation field when there are small changes in the activation profile in the 



f • 

Measurements were made on the activation of ^^Mn, both with and without cadmium cov¬ 
ers on the samples. With the use of our data for the activation of niant^anese by neutrons with 
energies above the cadmium cutoff energy, which were normalized at the surface in Fig. 3.11, 
the same computer program and conditions were used to determine what percentage of the 
total radiation field came from activated manganese by other than thermal neutrons. Table 3.4 
shows the percentage of the total dose that is contributed by manganese activated by neutrons 
with energies above the cadmium cutoff energy, which averages to 13'c of the dose that is due 
to the spectrum of all neutron energies. 


Height of 

reactor, ft 

Station, yd 


















An evaluation of the effect of controlled moisture was attempted by adding moisture to 
the soil at one station. The difficulty of adding moisture to a large area. es|K'cially in the 
desert, can be shown by our effort to increase the moisture content around one station. Two 
thousand gallons of water were dumped into a 12-ft-diameter circle that was ditched to help 
confine the water. The water was delivered in the night, and the ne.xt day a sample was taken 
to see how much moisture had been retained. The following table describes the conditions: 

Soil depth Before After 

0 to 3 in. 1.0'( H.O 5.2'(’ H.O 

3 to 6 in. 3.9'”c HjO 7.O'? HoO 

The difficulty of getting water to the area and the rapid evaporation lead one to feel that 
it would take at least 10,000 gal to saturate this soil with moisture. 


When tactical nuclear wea|X)ns are used so as to minimize local fallout, the most importatit 
residual hazard is neutron-induced radioactivity in the soil. The “no fallout” condition is ob¬ 
tained by detonating the weajion so that the fireball dcjes not tcjuch the ground. Therefore study 
of the neutron-induced soil activity due to a neutron source in the air was important: this was 
the objective of Program 5. 

Operation BREN was an e.xperiment that used a 1500-ft tower and an unshielded fast reac¬ 
tor similar to "Godiva” to simulate the neutron flux from a nuclear detonation. 

Samples of manganese, cadmium-covered manganese. g(dd, cadmium-covered gold, and 
sulfur were placed from 0 to 20 in. in the ground and at distances out to 400 yd from the base 
of the tower. The reactor was then oiwrated at various heights so that the effect of the height 
of a neutron source on the neutron-induced activity of a soil could Ix’ studied. 

No systematic effect due to the variation in soil com|H)Sition or to the effect of reactor 
height was present which would change the manganese activation profiles sufficiently to cause 
a corresponding change in the computed radiation field of greatir than i\0',. Manganese acti¬ 
vated by epicadmium neutrons contributed 13'f of the total radiation field. 



Through the BREN experiment three important effects of induced activity from a nuclear 
detonation were determined, the effect of the height of the weapon, the effect of distance from 
Ground Zero, and the effect of activation of manganese by neutrons with enerjjy {greater than 
that of thermal neutrons. By normalizin^i each manganese activation profile for every reactor 
height an 1 station and then using these profiles in our computer program, we were able to com¬ 
pute the dose rate due to radiation field at every station. Examination of the results shows the 
greatest deviation of dose rate from the average to be lU(’. and no systematic error was noted. 
The activation of manganese by neutrons with energies above the thermal region could make a 
contribution in the manganese radiation field of 13'( of the total dose .■'a;e for this source. 


1. R. L. Mather, Scidrini Disiribiitiuns m ar an Air—Sail lioiintlary as a Fain lion of Soil-water 
Coiilenl. Report USNRDL-TR-465, Naval Radio .ogical Defense Laboratory, Sept. 28, 1960. 

2. F. M. Tomnovec and R. L. Mather. The Inflnenee oj Soil Conifiosilion on the Thermal Neu¬ 
tron Coininnient oj Larf^e Seale Neutron Fields. Report USNRDL-TR-413, Naval Radiological 
Defense Laboratory. July 19. 1960. 

3. P. A. Read, Calculations and Nonio^ranis /or a Neutron-induced Aclirity Prediction Sys- 
tein, to be published at Naval Radiological Defense Laboratory. 

Fig. 3.2—Gold epithermal activations vs. depth. These data represent the activity in cadmium-covered 
gold samples. Parts A, B, C, and D are for reactor heights of 27, 299, 500, and 1125 ft, respectivel>. 


4 -6 8 -to -12 -t 4 -16 >8 -20 


Fig. 3.4 —Gold epithermal activation vs. depth, normalized at the surface. These are the same data as 
in Fig. 3.2 except that curves for the different stations were normalized at the surface in order to com¬ 
pare the shapes of the curves. Figure 3.4 is the average of all stations for each reactor height. 



DCPTh in Gf»OUND «NCHf Si MPTm tN CIKM»0 ti<«CMCS) 

Fig. 3.6—Gold epithermal activation vs. depth, normalized at the surface. This is a replot of the data 
in Figs. 3.2 and 3.4, but, instead of comparing different stations for the same reactor height, each sta¬ 
tion is plotted separately, with varying reactor heights. Parts A, B, C, D, E, and F refer to 30-, 100-, 
150-, 200-, 250-, and 300-yd stations, respectively. 

DtPTM IN GROUND {inches* 

Fig. 3.7—Gold thermal activation vs. depth for the 100-yd station and 27-ft reactor height. The 
two sets of data were taken under identical conditions except that during the period between runs the 
soil had dried out, changing its moderating properties. See Tabic 3.2 and Fig. 3.8 for rainfall and 
moisture in this time period. 


Fig. 3.8 —Soil moisture content vs. time. This curve shows how the soil moisture content varied during 
the course of the experiment. The data represent averages over all the stations. 


C «X) 400 600 too 1000 1?00 1400 0 ?O0 400 600 tOO '000 I?00 1400 


Fig. 3.9—Surface gold activation vs. slant range. Part A gives the relative activation produced by 
neutrons below the cadmium cutoff, obtained by subtracting the specific activity of a cadmium- 
covered gold sample from that of a bare gold sample at the same location. Part B gives the relative 
activation of gold produced by neutrons above the cadmium cutoff, obtained from the specific activities 
of cadmium-covered gold samples. Data for different reactor heights are marked by different kinds of 
points. Runs were normalized to each other. The solid line indicates the major trend of the points. 

Fig. 3.1u—(Sec following page for legend.) 


4 ♦ H , 4 A) • 4 f # ,*l 

f‘'** N •. -M s ^ HC ^ 

F*g- 3.10—Manganese activation vs. depUi. These data represent die relative activities of bare man¬ 
ganese samples. For each station the data have been normalized at the surface to compare the shapes 
of the curves. Parts A, B, C, D, E, and F are data for the 30-, 100-, 150-, 200-, 2.^0- and ,300-yd sta¬ 
tions, respectively. 


OCP^M iN G«OUNO liNCMESl Of pt« o»CuND 'NCMf S) 

Fig. 3.11—Manganese epithermal activation vs. depth. These data represent the relative activities of 
cadmium-covered manganese samples. For each station the data have been normalized at the surface 
to compare the shapes of the curves. Parts A, B, C, D, E, and F are data for the 30-, 100-, 150-, 200-, 
and 300-yd stations, respectively. Lines are drawn through the points corresponding to the same 
height in slant range to guide the eye, although in some cases there are not enough data to define 
clearly the trend of the cuive. 

Fig. ^.12—(S. e (oMc-.vuig page for legeiul.) 


DCPTm in ground ONCMfSl 

OIPTm in ground (iNCHtS) 

♦ ? 0 -2 -4 -fc -0 -’0 >? ->4 -ifc IR ?0 0 -2 -4 -6 •« -10 '«? »4 It '• /»■ 

DC PTm IN GROUND (iNCHf Si Of PTn in i^NOuNO nNCMf Si 

Fig. 3.12—Manganese activation vs. deptii. These are the same data as in Fig. 3.10, but, instead of 
comparing different reactor heights for the same station, each reactor height is compared separately, 
with var>ing stations. Parts A, B, C, D, and E are for reactor heights of 27, 299, 500, 1125, and 1500 ft, 
respectively. Part F gives curves averaged over all stations for each reactor height. 


Fig- 3.13—(See following page for legend.) 

Fig. 3 .13—Manganese epithermal activation vs. depth. These are the same data as in Fig. 3.11, but, 
instead of different reactor heights for the same station being compared, each reactor height is com¬ 
pared separately, with varying stations. Parts A, B, C, and D are for reactor heights of 27, 299, 500, 
and 1125 ft, respectively. Part E gives curves averaged over all stations for each reactor height. 



I I 

AC f ’ HtOH I 

O ABOvI Th| SO't ' 


% ^ - __ -J---*---i--- 

O ?OC'' 50C AOC SOj too 


Fig. 3.14—(See following page for legend.) 


Fig. 3.14—Sulfur neutron activation. These curves give the relative activation in sulfur due to the 
(n,p) reaction. The ordinates are proportional to the neutron flux above about 3 Mev. Part A gives the 
relative surface flux vs. distance from the base of the tower for a reactor height of 1125 ft. The marked 
flux depression at the 30-yd station is attributed to attenuation by the tower. ParU B and C give the 
sulfur activation in the ground for three different geometries. Data were taken at 2 in. above the ground 
at each station. 

DCPTh \H Oi>OU*itO «NC<S) 

Fig. 3 .15—Activation of manganese, copper, and arsenic. This curve shows the relative distributions 
with depth of the **Mn, *^Cu, and ^*As induced by (n,y) reactions in samples of the respective elements. 
The differences in shapes of the curves are attributec' to the different energy variations of the activation 
cross sections. 



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