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Pilnas toie included in proceedino in Syrr o'siuni on Protection against
Priatio:s in Space, Gatlinburg, ';::;*!!!.. , October 12-lli, 1961:
NFC 3 1964
CONF-720G
THE SECONDARY PARTICLE CONTRIBUTION TO THE DOSE FROM MONOENERGETIC PROTON BEAMS littf
----*AND"TTO VALIDITY OF CURRENT-TO-DOSE CONVERSION FACTORS*
lined
D. C. Irving, R. G. Alsmiller, Jr., W. E. Kinney, and H. S. Moran
Oak Ridge National Laboratory
Oak Ridge, Tennessee
The validity of the current-to-dosc conversion factors reported el.sewhere in
these proceedings has been investigated for the case of monoenergetic protons iso-
torpically incident on an infinite slab shield followed by a slab of tissue. The
calculations were done by the Monte Carlo method using the NTC code. We considered
as shield materials carbon, aluminum, ana copper in order to investigate any variation
in atomic mass or number of the shield. For each shield material we considered
shield thicknesses of 10 and 30 g/cm'.
Ir: all calculations a 30-cm-thick slab of
tissue followed the shield. Monoenergetic protons of energy 100 or 400 MeV were taken
to be incident on the shield with the angular distribution of a current due to an
isotropic flux.
The particle histories were trac..tc. by Monte Carlo through the shield up
to the shield-tissue interface. At this point the current into the tissue was divided
into three parts: primaries, secondary protons, and secondary neutrons.
The current-
to-dose conversion factors were applied to these currents, and the average whole-body
and 5-cm-depth doses were obtained in both rads and rems.
To test the accuracy of
these doses, the Monte Carlo calculation was continued with the tracking of the
particles through the tissue and the calculation of the actual doses in the tissue.
Any particle that crossed from the tissue back into the shield was tagged as a
"backscattered" particle, and a separate account was kept of any dose resulting from
-
-
-
such backscattered particles.
j
*Research sponsored by the National Aeronautics and Space Administration (NASA
Order R-104) under Union Carbide Corporation's Contract with the U. S. Atomic
Energy Commission.
It is essential to the validity concurrent-to-dose conversion factors that
the backscattered contribution be neglicibly small.
The calculation of the con-
version factors was carried out in a gerinctry consisting of tissue alone with no
shield present. Then the current leavirs the shield was calculated as if no tissue
were present, and the dose in the tissue was computed by means of the conversion
factors. Any appreciable interaction between the shield and the tissue, in the
form of particles passing from one to the otier several times, would invalidate such
an approach. Our calculations showed that the hackscattered contribution is de-
finitely negligible, amounting in general to less than 0.1% of the total dose in
the tissue.
There is, however, one difficulty in using the current-to-dose factors:
They have been calculated for only two types of incidence, normal and isotropic,
whereas the actual angular distribution of the currents at the shield-tissue inter-
face is, in general, neither normal nor isotropic. Consequently, we carried out
the cose calculations twice, applying both conversion factors to the currents in
the hope that this might provide upper and lower limits to the actual dose. This
did not turn out to be true in all cases; however, in almost all cases the actual
dose did not exceed the bounds provided by the two conversion factors by more
than the statistical error of the Monte Carlo calculations. Standard deviations
of 1 to 3% for the average dose and of 3 to 10 for the doses at a 5-cm depth were
obtained in these calculations.
Typical results for 400-MeV protons are shown in Table 1, which gives the
average doses and the 5-cm-depth doses for incidence on a 30 g/cm slab of aluminum.
The first column gives the dose computeci by trocking the particle histories through
the tissue, while the second and third columns give the dose computed from the
1.'
current-to-dose conversion factors.
The headings at the left identify the current :
-,
.
.
.-
TE
from which the done was derived; 1.e., lines marked "primary protons" include the
doses from primaries and from secondaries arising in the tissue from primaries,
while the lines marked "secondary protons" are the doses from secondary protons
born in the shield and entering the tissue. Similarly, Table 2 shows the results
for 100-MeV protons incident on 10 g/cm of carbon. In these cases the primaries
were stopped in the shield, and only secondary neutrons contributed to the dose.
As may be seen from the tables, the current-to-cose conversions general.ly provide
a fair estimate of the actual dose, and in most, but not all., cases the actual.
dosc is bracketed by the two estimates. In the cases considered, we found rio
significant variation with thickness of the shicid or with atomic mass of the
shield material. In no case did a current-to-dose conversion disagree with the
actual dose by more than a factor of 2.
A secondary objective of our calculations was to determine the relative
contribution of primary and secondary particles to the total dose and to estimate
the error involved in a calculation which neglected secondary particles. In
Figs. 1 and 2 respectively are shown the dose in rads and in rems as a function of
depth in tissue res:uting frota 400-MeV protons incident on 30 g/cm of alminum.
The dose has been divided into five contributions:
1. the dose from ionization of primaries,
2. the dose from secondaries produced in the tissue by the primary
protons,
the dose from secondary protons produced in the shield,
i
the dose from secondary neutrons produced in the shield,
į i
the backscattered dose from particles which crossed from the
the tissue to the shield and back again.
It is readily evident from the figures that the backscatter 18 negligible,
being a factor of 10 or more smaller than any other contribution. Ionization
of the primary protons is the most important factor, and the secondaries pro-
duced in the tissue are next in importance. This is significant since one could
account for tissue secondaries by means of current-to-dose conversion in a cal-
culation that would otherwise neglect seconda ries. The tissue secondaries
certainly cannot be neglected in the rem dose where their contribution is
approximately equal to that of the primary ionization. The dose from secondaries
..
•
produced in the shield is of less importance. In fact, from Table 1. we can see
that the "primary protons" (which here includes both ionization and tissue
secondaries) constitute about 80% of the total rad dose and 70 to 75% of the
rem dose and that a calculation which had considered only the trazsport of
primeries through the shield and used the larger of the current-to-dose con-
versions would have obtained more than 90% of the total rad dose.
References:
1. W. E. Kinney and C. D. Zerby, "Calculated Tissue Current-to-Dose Conversion
Factors for Nucleons of Energy Below 400 Mev," these proceedings.
2. W. E. Kinney, The Nucleon Transport Code, NTC, ORNL-3610 (1964).
Table 1. Doses Calculated for 400-MeV Protono Isotropically Incident
on a 30-g/cm2-Thick slab of Aluminum Followed by Mssue
Actual
Dose
Calculated Dose
with Normal
With Isotropic
Incidence Con Incidence Con-
version Factor version Factor
Average Dose (rado )
Primary protons
Secondary protons
Secondary neutrons
Total
0.300x10
0.444x100
0.164x10
0.361x10*7
0.235x10
0.416x108
0.126x1.
00
0.289x1007
0.326x10?
0.447x10
0.185x100,
0.389x10-7
Primary protons
Secondary protons
Secondary neutzons
Total
Average Dose (rems)
0.452x10 0.316x10-1
0.614x108 0.543x10,
0.793x10
0.616x10,
0.592x107 0.1132x107
0.446x10
0.594x100
0.832x100
0.588x107
5-cm-Depth Dose (rads)
0.338x10
0.214x10
0.745x100
0,562x108
0.201x100
0.433x10
0.284x107
Primary protons
Secondary protons
Secondary neutrons
Total
O
0.409x10
0.791x10
0.230x10,
0.512x107
0.140x107
Primary protons
Secondary protons
Secondary neutrons
Total
5-cm-Depth Dose (rems)
0.51'7x10
0.314x107
0.104x10
0.732x10.
0.109x104 0.746x100
0.730x10? 0.462x107
0.557x10-7
0.104x102
0.108x10°
0.768x10-7
..
.cocom ...
.-..
.
-.--. .
..
. ..
Table 2. Doses Calculated for 100-Me V Protons Isotropically Incident
on a 10-g/cm2-Thick Slab of Carbon Follo:rcà by Tissue
Actual
Dose
Dose Calculated Assuming
Normal Incidence Isotropic Incidence
Conversion
Conversion
Average Dose (rads )
Primary protons
Secondary protons
Secondary neutrons
Total
0.109x102
0.109x10-9
OOOO
0.100x10-9
0.100x10-9
0.126x10-9
0.126:10-9
Average Dose (rems)
Primary protons
Secondary protons
Secondary neutrons
Total
0.784x10-9
0.784x.20-9
0.615x10
0.615x10°9
0.752x102
0.752x10-9
5-cm-Depth Dose (rads )
Primary protons
Secondary protons
Secondary neutrons
Total
Ooo
0.152x109
0.152x10-9
0.135x103
0.135x109
0.188x10-9
0.188x10-9
5-cm-Depth Dose (rems)
Primary protons
Secondary protons
Secondary neutrons
Total
0.105x10
0.105x10
0.780x102
0.780x109
0.105x10-
0.105x10-8
.
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List of Figures
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Fig. No.
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Dwg. No.
64-9312
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-
-.
Dose (in rads ) for the Case of 400-MeV
Protons Isotropically Incident on a
30-g/cm2-thick slab of Aluminum Followed
by Tissue
64-9317
Dose (in rems ) for the Case of 400-MeV
Protons Isotropically Incident on a
30-g/cm-thick Slab of Aluminum Followed
by Tissue
UNCLASSIFIED
ORNL DWG. 64-9312
400 MEV PROTONS INCIDENT ON 30 GM./SQ. CM. OF AL+TISSUE.

DOSE, RAD/SEC./UNIT ISOTROPIC FLUX
1. PRIMARY PROTON IONIZATION DOSE
2. PRIMARY PROTONS, OTHER DOSE
3. SECONDARY PROTON DOSE
4. SECONDARY NEUTRON DOSE
5. BACKSCATTERED DOSE
2
5.0
15
20
35
.....
DEPTH IN TISSUE. CM.
..
*.11. 11.
insininen
c
,
UNCLASSIFIED
ORNL DWG. 64-9;117
400 MEV PROTONS INCIDENT ON 30 GM./50. CM. OF AL+TISSUE.
10-9

Arrhy
DOSE. REM/SEC./UNIT ISOTROPIC FLUX
1. PRIMAPY PROTON IONIZATION DOSE
2. PRIMARY PROTONS, OTHER DOSE
3. SECONDARY PROTON DOSE
4. SECONDARY NEUTRON DOSE
5. BACKSCATTERED DOSE
15
DEPTH IN TISSUE. CM.
Ls1
DATE FILMED
4 / 7 /65


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