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ORNO-P-311
DTIES
OCT 51964
(CONF-706-1
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DENSITY INTERFACE EFFECTS ON INITIAL RADIATION. EROHL NUCLEAR WEAPONS
MASTER
J
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J, A. Auxier
Health Physics Division
Oak Ridge National laboratory
Oak Ridge, Tennessee
Several Monte Carlo calculations of radiation transport have yielded
some Information on the air-ground interface effects. ",2,Because of
uncertainties due to cross sections, initial spectra, poor statistics,
etc., there has been a need for experimental data for comparison with
the computed values. Haywood measured the spatial distribution of the
neutron and gamma radiation field generated by a small bare reactor sus-
med
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-LEGAL NOTICE
pended on a tall tower."5,Myasurements were made for source heights
in the range of 27 to 1500 feet, detector heights of 0 to 100 feet, and
source-detector separation distances of 3 to 1500 yards.
Figures 1 and 2 show dose vs distance relationships for neutrons
and gamma rays, respectively. In each figure, curves are shown for three
reactor heights (Hg); the detectors were on the ground (H, : 0) for all
points except those at 10 feet which were used to compute the unperturbed
values at "unit" distance; 1.c., one foot. Values for points on the
curves for Hq - 300 feet in Figures 1 and 2 were not measured for
R < 1500 feat. Dose and dose-rate values are in "first collision" absorbed
dose units (rads) in all cases. The dosimeters included proportional
counters”,for fast neutrons and the "philm19) for gamma radiation.
For certain high dose-rate measurements, threshold detectors (10) and
metaphosphate glass detectors were used for fast neutrons and gamma
.
*
*operunat by Vintor tarbido componentamatonta
tilauslomadkankamin organ Commission.
2
*Rawanoh sponsored by the U. S. Atomic Energy Comuni mion under contract with the
Union Carbide Corporation.
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Figure 1
Neutron Dose as a function of Slant Distance
From the HPRR on BREN Tower
immuniteit work
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UNCLASSIFIED
ORNL-LR-DWG 1164A
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HPRR NEUTRON AIR DOSE
DETECTORS AT GROUND LEVEL
to He = 27
LO HA = 300
A HR = 1925
AIR DENSITY = 4.02g/liter
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NEUTRON DOSE RATE XR? (millirads ft2 nro kw"').

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1000
4000
5000
2000 3000
R, SLANT RANGE (ft)
Figure 2
Gamma-Radiation Dose as a function of Slant Distance
From the HPRR an BREN Tower

UNCLASSIFIED
ORNL-DWG. 64-2659

o HR = 27', Ho=0
A HR = 300', Ho = 0
♡ HR = 1125', H = 0
AIR DENSITY 1.02 g/1
>
DxR? (MR-hrl. kw!.ft?)
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600
1200
600
1800
3000
2400
3600
4200
1200
3600
4200
1800 2400 3000
SLANT RANGE (FEET)
4800
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rays, respectively. Ritchied calculated the perturbation due to the
interface as a function of source heights his data agres, to within 15 pero
cent, with the experimental data.
French127 has developed a simple mathematical model which can be
used for evaluating the ratio of tha dose in a finite medium of air above
ground to that in an infinite medium of air. Results obtained with the
inodel agree tu within 18 percent with Haywood's experimental data if the
normalization is done at source heights of 1125 feet or 1888. For greator
source heights, the model is less accurúte.
Figure 3 shows the neutron to gamma dose ratio as a functior. of slant
range for several values of source and detector o
ngoit'.
The source was
the Oak Ridge National Laboratory's Health Physics Research Reactor which
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had, when used in the aluminum hoist car, a n/y dose rate ratio of 10 at
10 feet. It is apparent, for the values shown, that the shapes of the
curves are the same and that the ratio is a more sensitive function of
H, than of H, in this range of values; i.e., the four sets of points define
two curves which are determined by He:
• Haywood also measured the spatial distribution of the gamma radiation
field from a Co60 source on the BREN tower. Figure 4 shows buildup factors
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based on the experimental data for Co 60 gamma rays for several source
heights. In addition, Figura 4 shows computed buildup factors for co 60
gamma rays in an infinite medium of air.
From these and similar data, it is apparent that the air-ground inter-
face enhances the radiation field (both neutrons and gamma rays) for
source-detector separation distances of less than approximately one relaxa-
tion longth and decreases the radiatia field at greater distances. The
magnitude of the effect ranges to 2.5 or greater. It is evident that
DET
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Figure 3
Neutron- to Gamma-Dose Ratio as a Function of Slant Distance from the HPRR
on the BREN Tower for Several Values of Source and Detector Height
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UNCLASSIFIED
ORNL-DWG 63-1162A

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• HR = 300 ft, Ho = 16.5 ft
A HR = 300 ft, Ho = 100 ft
HR = 1125 ft, Ho = 16.5 ft
• HR = 1125 ft, Ho = 100 ft
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1000
4000
5000
2000 3000
R, SLANT RANGE (ft)
S
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Figure 4
Computed Build-up Factors for Co60 Gamma Radiation in Water
and for l-Mev Camma Radiation in Air
UNCLASSIFIED
ORNL-DWG 63-3949

5
***necemmenopane ***er comment
A INFINITE WATER MEDIUM ,
. COGO GAMMA RAYS
O INFINITE AIR MEDIUM, 1 Mev
GAMMA RAYS
o Hg = 1125 f1
A Hg = 300 ft
v Hs = 27 ft

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EXPOSURE BUILDUP FACTOR
3
MEAN FREE PATHS
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the Interface perturbation must be taken into account in predicting the
effects of nuclear weapons. Further work is nocessary and is planned
by several groups. Haywood plans to continue his experimental program
during BKN II, a proposed extension of BREX, which would utilize a
small accelerator for producing 14-Mev neutrons,
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REFERENCES
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1. R. H. Ritchie, "Health Physics Annual Progress Report July 31, 1961",
ORNL-3189, pp. 145-148.
2. M. J. Berger, J. of Appl. Phys. 28, No. 12, 1502-1508 (December 1957).
3. M. B. Wells, Health Phys. 8, 543-549 (1962).
4. F. F. Haywood, "Spatial Dose Distribution in Air-Over-Ground Geometry",
Health Phys. (in press).
5. F. W. Sanders, F. F. Haywocd, M. I. Lundin, L. W. Gilley, J. S. Cheka,
and D. R. Ward, "Operation Plan and Hazards Report - Operation BREN",
CEX-62.02 (1962).
6. J. A. Auxier, F. F. Haywood, and L. W. Gilley, "General Correlative
Studies - Operation BREN", CEX-62.03 (1963).
7. G. S. Hurst, Brit. J. of Radiol. 27, 353-357 (1954).
8. E. B. Wagner and G. S. Hurst, Rev. Sci. Instrum. 29, 153-158 (1958).
9. E. B. Wagner and G. S. Hurst, Health Phys. 5, 20-26 (1961).
P. W. Reinhardt and F. J. Davis, Health Phys. 1, 169-175 (1958).
11. C. H. Bernard, W. T. Thornton, and J. A. Auxier, Health Phys. 4, 236-243
(1961).
12. R. L. French, "A First-Last Collision Model of the Air-Ground Interface
Effects on Fission Neutron Distributions", RRA-M32 (1963).
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DATE FILMED
12/8/64
LEGAL NOTICE
This roport was proparod as an account of Government sponsored work. Nolther the Unitod
Suatos, nor the Commission, nor any person acting on behall of the Commission:
A. Makos any warranty or roprosentation, exprossod or impliod, with respect to the accu-
racy, complotonors, or usohulnose of the information contained in this roport, or that the uso
of any information, apparatus, mothod, or procoss disclosod in this roport may not Infringo
privately ownod rights; or
B. Assumos any liabilities with rospoot to the uso of, or for damago, rosulung from tho
uso of any information, apparatus, method, or process disolosod in this roport.
As usod in the abovo, “por son acting on behall of tho Commission" includes any va-
ployoo or contractor of the Commission, or omployoo of such contractor, to the oxtont that
such omployoo or contractor of the Commission, or employoo of such contractor propares,
disseminates, or providos a0004. lo, any Information pursuant to his omployment or contract
with the Commission, or his omployment with such contractor.
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