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UNITED STATES ATOMIC ENERGY COMMISSION
NEUTRON MONITORING BY MEANS OF "SPECIAL FINE-GRAIN ALPHA-EMULSION" FILM
J. S. Cheka
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Date Declassified: April 3, 1947
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NEUTRON MONITORING BY MEANS OF "SPECIAL FINE-GRAIN ALPHA-EMULSION" FILM
By J. S. Cheka
Fine-grain alpha-emulsion film is used at Clinton Laboratories to monitor personnel for neutron
exposure. Part of the film is behind a cadmium shield, which removes thermal neutrons; part of it
is exposed without a shield, intercepting both thermal and fast neutrons.
Calibrations were made to determine the sensitivity of the emulsion for both thermal and fast
neutrons in terms of proton tracks observable under a microscope of 970X magnification. The ratio
3.01 x 105 nth/observed track was noted for the N4(n,p) C4 reaction, and the ratio 2.05 x 104 nfp ob-
served track was noted for proton recoils. Evaluating expected track densities, based on N-atom
density for nth and H-atom density for nf, the first process was found to be about 53% effective and
the latter 46% effective in producing recognizable tracks of 3 grains or more in this emulsion.
Using tolerance values of 4700 nth/(cm2 sec) and 266 nf/(cm2 sec),* two-weeks tolerance doses
are indicated by 39 tracks/50 fields for nth, and 33 tracks/50 fields for nf. However, since different
batches of the same type of film were found to differ in sensitivity, each batch must be calibrated to
establish the track density which indicates a tolerance dose.
Alpha-particle-sensitive film has been used at Clinton Laboratories since October 1944, to mon-
itor personnel for neutron exposure. This film is now in the regular film badge, in addition to the
beta- and gamma-ray-sensitive film.
This practice is possible because this film is sensitive to protons. Fast neutrons produce recoil
protons in the hydrogenous emulsion of the film. Thermal neutrons produce protons in reaction with
nitrogen, N" (n,p) C4. Part of the film in the badge is shielded with cadmium and part of it remains
uishielded. Thus it is possible to use one film for both fast and thermal neutron monitoring.
The present series of experiments included qualitative tests to determine the relative effective-
ness of thermal and fast neutrons in producing protons sufficiently energetic to generate recognizable
tracks in the emulsion. Distribution of track lengths in terms of number of silver grains per track
S was also studied. Quantitative tests were made, using various fluxes of both fast and thermal neu-
EXPOSURES AND TECHNIQUES
The source of the neutrons was the Clinton pile, which is a graphite pile with a uranium lattice.
The exposures were made in a tunnel on the top of the pile. This tunnel has thirty inches of graphite
between it and the nearest uranium. It is also shielded by four inches of lead. The graphite serves
*nf taken it 1 Mev.
MDDC 890 [
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2] MDDC 890
as a moderator and slows down most of the neutrons from the fission energies up to 2.0 Mev to ther-
mal energies of < 1 ev. The lead absorbs most of the fission gamma radiation. The resulting flux
consists mainly of thermal neutrons, and has a magnitude of 3 x 10' neutrons/cmu/sec at the power
Thermal flux calibrations were made by irradiating copper wires at the points of exposure, both
with and without cadmium shields. The thermal neutron fluxes were evaluated from the induced cop-
per activity by T. Arnette, according to the usual procedure by the use of the formulas developed by
Fast neutron fluxes were obtained in the same tunnel by using the "fast neutron cart." This
cart consists of an aluminum framework holding 42 kg of uranium slugs shielded above by four inches
of lead. The thermal neutrons in the tunnel cause fission in the uranium, liberating fast neutrons of
fission energies. The lead cuts down fission gamma radiation to a small value. The exposure com-
partment is placed over the lead and is further shielded by sheets of boron-carbide impregnated lu-
cite. The lucite contains 0.324 g of boron per c&m, which if uniformly distributed would cut down
the thermal neutron flux by a factor of 2 x 10', making thermal flux inside the compartment neg-
Fast neutron fluxes were measured in n-units by means of a Victoreen r-meter, standardized
with the r-meter at Berkeley. The n-unit is the flux which will produce the same discharge in this
meter as that produced by one r of gamma radiation.
The tracks were counted under a microscope, using a 97X objective and a 10X eyepiece. The
diameter of a field was 0.15 mm, giving an area of 1.768 x 10"cm'/field. This is equivalent to 5.66
x 103 fields per cm'. Emulsion thickness was 40M, and the depth of focus of the above-mentioned
lens system was 4#, so that the objective had to be shifted vertically to count all the tracks in a
Three or more grains in line were considered to constitute a track, although it is probable that
protons of low energy produce tracks of but two grains. These latter, however, cannot be recognized
with any degree of certainty and so were disregarded. There is also sometimes some question as to
whether a 3-grain track is a true track or only a random configuration of fog particles.
RESULTS AND DISCUSSION
Exposure of films with and without cadmium jackets to 8.66 x 109 thermal neutrons/cma (as de-
termined with copper wires) showed that cadmium cut down track density by a factor of 4. Doubling
the flux gave essentially the same ratio.
Exposure of films in the "fast cart," with and without cadmium jackets, gave 5% fewer tracks
in the shielded films. These tests were designed to determme whether thermal neutrons produced
tracks. Since cadmium has a resonance at 0.18 ev, and does not transmit-neutrons appreciably
below 0.4 ev, the differences in track density previously noted established the fact that nitrogen cap-
ture of neutrons of energies below the cadmium cutoff was a factor in track formation.
Track distribution was determined in terms of the number of silver grains per track. The re-
sults are given in Table 1.
The films exposed to thermal neutrons with a cadmium jacket had so much gamma fog from ra-
diative capture of neutrons by the cadmium that this determination could not be made.
The foregoing data show that the percentage of tracks of 5 grains or less is only slightly higher
for thermal neutrons than for fast, and that there is a slightly higher percentage of tracks of six or
more grains due to fast neutrons. Assuming that the number of silver grains developed is a function
of the energy of the proton, one might expect that there should be a higher average energy to the re-
coil protons. However, since the energy of the recoil protons is also a function of the impact angle at
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Table 1. Track distribution based on silver grains per track.
3-grain 4-grain 5-grain 6-grain > 6-grain
(%) (%) (%) (") (%)
F 50.9 25.8 11.7 6.1 5.6
F, 48.0 27.8 12.6 6.2 5.6
Th 50.2 29.2 14.8 4.6 1.2
Where F indicates films exposed to fast neutrons without a cadmium jacket, Fe indicates
films exposed to fast neutrons with a cadmium jacket, and Th indicates films exposed to
thermal neutrons without a cadmium jacket.
which collision occurs, it can be seen that the proton energy may vary from near zero, for a glancing
blow, to the full energy of the incident neutron, for a head-on collision. From these considerations
it is obvious that track length (in terms of number of silver grains) cannot be a criterion of whether
the proton-producing neutrons were thermal or fast.
Exposures were also made of a nitrogen-free film, which had been made up for J. Floyd by East-
man Kodak Co. This film proved to be so highly gamma-sensitive that cadmium-jacketed films were
fogged too much to evaluate track densities. Investigation of this film was discontinued.
Calibration for Thermal Neutrons
Films were calibrated for track frequency due to thermal neutrons by making a series of ex-
posures-in the slow neutron tunnel, using 15-second increments. A film was run in and out of the
,exposure end of the tunnel to determine the number of tracks formed by the neutron flux gathered en
route. A correction was made on each subsequent determination by subtracting this "background"
value from the observed track density.
Two films were exposed for each time interval, and 25 fields counted on each film, making a
total of 50 fields counted for each exposure. Table 2 shows the results of these counts.
Calculations were performed on these data to determine the number of neutrons causing a track
in the film. The figure used to calculate total flux is 2076 thermal neutrons/ cm2'sec. This is the
result of several determinations with copper wire as previously mentioned and agrees with numerous
similar determinations made by the Biology Group. Since the area of each field is 1.768 x 10-4 cm2,
the area of 50 fields is 8.84 x 10-s cm2, so that dividing a track value for 50 fields by this figure
Table 3 shows the results of these determinations. These figures give an average value of
(3.01 0.09)x 105 thermal neutrons, track.
Theoretical expectancy of thermal neutrons/track was calculated, using the results of an analysis
of the emulsion made by the Chemistry Division for L. B. Borst some time ago.' This analysis showed
the N:C:O:H ratio to be 1:2:2:5, or the same as that for glycine. Actually, gelatin has a more com-
plex structure. However, using the values for glycine:
sp gr = 1.6
mol wt = 15.07
ratio of nitrogen atoms to total number of atoms = 0.1
thickness of emulsion = 40p
Table 2. Tracks due to thermal neutrons.
Exposure time (sec)
Table 3. Determination of number of thermal neutrons per track.
Exposure time Tracks/50 fields Tracks/cm2 Thermal Thermal neu-
(sec) (backgrd subtracted) neutrons/cm' trons/track
15 71 8.02 x 10' 3.11 x 10" 3.88 x 10'
30 191 2.16 x 10' 6.22 x 10' 2.88 x 10'
45 268 3.03 x 10' 9.32 x 10' 3.08 x 10*
60 401 4.54 x 10 1.25 x 10'0 2.20 x 10'
75 487 5.51 x 10' 1.56 x 10"0 2.83 x 10"
90 553 6.26 x 10 1.87 x 10'0 2.94 x 10'
105 633 7.05 x 10' 2.18 x 1010 3.09 x 10"
120 688 7.78 x 10' 2.49 x 1010 3.20 x 10'
DN = sp gr x rN x Avogadro's number x thickness
where DN = density of nitrogen atoms
and rN = ratio of nitrogen atoms to total.
Substituting numerical values:
DN = 1.6 x 0.1 x 6.02 x 0.10 x 4 x 10- =
5.12 x 10.1 nitrogen atoms/cm'.
Proton flux due to N"1(n,p)C" is
PN =OaN x DN x n,
where PN is the proton flux produced in the above reaction,
a aN is the thermal capture cross-section of nitrogen,
and n is the thermal-neutron flux.
Substituting numerical values:
PN = 1.7 x 10-2 x 5.12 x 10" x 9.32 x 109* = 8.11 x 104 protons/cm2 in 45 sec.
A correction must be applied here. The chemical analysis' previously mentioned showed the
nitrogen content of the emulsion to be 14 per cent, instead of the 18.5 per cent which it would have
been if the emulsion had been pure gelatine. A part of the emulsion comprises silver halides, and it
has been determined2 that at a relative humidity of 50 per cent (which prevails in this area) gelatin
absorbs 20 per cent of its weight of water. Judging by the nitrogen percentages, it appears that 75
per cent of the emulsion is gelatin. This correction brings proton expectancy down to 6.10 x 10' pro-
Calculation of the corresponding values for each exposure, checked against the figures in column
three of Table 3, shows that 53 per cent of the protons formed recognizable tracks of 3 grains or
more. The rest probably produced 2-grain tracks, but, since these are difficult to identify, it was
considered advantageous to disregard them for purposes of personnel monitoring.
Calibration for Fast Neutrons
Calibrations for fast neutrons were made in the "fast cart.' Increments of 0.2 n-units were
used. Background correction was determined in like manner as for thermals. The results of the
exposures appear in Table 4.:
Table 4. Tracks due to fast neutrons.
Exposure-n Tracks / field
(background) Tracks/50 fields Tracks/field (Background subtracted)
0.0 68 1.36
0.2 139 2.78 1.42
0.4 203 4.06 2.70
0.6 293 5.86 4.50
0.8 387 6.94 5.58
1.0 390 7.80 6.44
1.2 520 10.40 9.04
1.4 610 12.20 10.84
1.6 675 13.50 12.14
1.8 756 14.12 13.76
2.0 842 16.84 15.48
*Value for 45-sec exposure.
Using the foregoing data, calculations were made to determine the number of fast neutrons indi-
cated by each track. Neutron exposures were calculated from n-units. As previously mentioned, an
n-unit is the fast neutron exposure which causes a discharge in the Victoreen r-meter equivalent to
that caused by one r of gamma radiation. P. C. Aebersold has estimated that tissue absorbs about
205 ergs/g when exposed to one n-unit of fast neutrons. Since one rep of radiation is, by definition,
that amount which loses 83 ergs/g of tissue, one n-unit is approximately 2.5 rep.
C. C. Gamertsfelder has calculateds that 2.84 x 10o neutrons of 2 Mev will lose 83 ergs/g of tis-
sue, or 3.83 x 10o neutrons of 1 Mev will lose the same amount. Consequently, to produce one n-unit
requires 7.10 x 10" 2-Mev neutrons/cm2, or 9.58 x 10 1-Mev neutrons/cm2. Since the mean value of
the fission energy spectrum is nearer 1 Mev than 2, the latter value was used in the calculations.
A correction is required for the fission gamma radiation which penetrates the four-inch lead
shield. This has been estimated to account for 12 per cent of the r-meter readings. With this correc-
tion applied, an n-unit, as measured on the r-meter, indicates 8.42 x 10' neutrons/cm2. The results
of the determinations follow.
Table 5. Determination of number of fast neutrons per track.
Exposure (n) (Background subtracted) Tracks/cm2 Neutrons/cm2 Neutrons/track
0.2 71 8.02 x 10S 1.69 x 10' 2.11 x 10
0.4 135 1.53 x 10' 3.37 x 108 2.20 x 10'
0.6 225 2.54 x 104 5.06 x 10' 1.99 x 10'
0.8 239 3.15 x 10' 6.75 x 10' 2.14 x 10'
1.0 321 3.63 x 10' 8.42 x 108 2.32 x 10
1.2 452 5.11 x 10' 1.01 x 10o 1.97 x 10
1.4 542 6.13 x 10' 1.18 x 10o 1.93 x 10
1.6 607 6.87 x 10' 1.35 x 10 1.96 x 10
1.8 688 7.78 x 10' 1.52 x 10' 1.97 x 10'
2.0 774 8.76 x 10 1.69 x 10 1.93 x 10'
These figures give an average value of (2.05 .03) x 10' fast neutrons/track.
A theoretical estimate was then made of the expectancy of tracks due to fast neutrons
The chemical analysis previously mentioned' was used as a basis for the estimate of prol
position. Using glycine as the representative compound, as above,
sp gr = 1.6
mol wt = 75.07
ratio of H atoms to total number = 0.5
thickness is 40jL
Then: DH = sp gr/mol wt x rH x Avogadro's number x thickness
where DH = density of hydrogen atoms
and rH = ratio of H atoms to total.
s in the film.
Substituting numerical values:
DH = 1.6/75.07 x 0.5 x 6.02 x 10" x 4 x 10-s = 2.56 x 10"' H atoms/cma.
Since only 75 per cent of the emulsion is gelatin, the corrected value of H atom density is 1.92 z
There is an additional hydrogen content of the emulsion due to the approximately 20 per cent
water content of the emulsion.2 The number of water molecules per unit weight of the emulsion is in
inverse ratio to glycine molecules as their molecular weights, and their relative weights are as their
percentages. Then, since there are 3.84 x 10is glycine molecules/cm2, the number of water mole-
3.84 x 10s x x 4.27 x 10e
and there are twice as many H atoms, or 8.54 x 10'" H atoms/cm2, due to water content.
Adding this to the H atom density due to glycine, the result is
DH = 2.77 x 10"9 H atoms/cm2 of emulsion.
Proton flux, P = uH x DH x n
where oH = the neutron cross section for hydrogen at 1 Mev
DH = the hydrogen atom density
n = fast neutron flux.
Substituting numerical values
P = 4.16 x 10-24 x 2.77 x 101" x 8.42 x 10"* = 9.70 x 10' protons/cm2.
Calculating the corresponding values for each exposure and checking against column three of
Table 4, it appears that the number of tracks is only 42.4 per cent of the expected value. A part of
this discrepancy may be accounted for by the formation of 2-grain tracks, which were not counted.
A few corrections are in order, due to the fact that neither of these flux fields was pure. Al-
though theoretically the boron screen should cut thermal flux in the fast cart to 1/2.2 x 10s of the orig-
inal, copper monitoring showed a transmission factor of 1.5 per cent. This, however, introduces an
error of only 0.22 per cent, which is negligible. More significant, however, is the epithermal contam-
ination of what is considered "thermal" exposure. The Biology Group estimates that there is an epi-
thermal flux which would amount to 8.71 x 10 fast neutrons/cm2/min at the power level at which the
thermal calibration was made, which in turn would produce 4.25 x 10s tracks/cm2, or 12.8 per cent of
the total tracks found at this exposure. Making this correction, it was found that thermal neutrons pro-
duced 46 per cent of the tracks expected.
CONCLUSION AND REMARKS
When these films are used for personnel monitoring, a badge is worn for two weeks without a
film change. It then becomes necessary to evaluate neutron tolerances in terms of tracks formed in
two-weeks exposure periods. Fast neutron to tolerance is given by Karl Z. Morgan4 as 266/cm2/sec
for fast neutrons of 1-Mev energy. Thermal neutron tolerance has recently been calculated by R. R.
Coveyou as 4700/cm2/sec.
Value for 1 n-unit.
8 ] MDDC 890
4700 n/cm'/sec gives 1.36 x 10' n/cm'/8-hour day, or 1.36 x 10' n/cm'/2 weeks of 5 days each.
This flux, at 3.01 x 10' thermal neutrons/track, gives 4.51 x .10 tracks/cm' for a two-weeks toler-
ance of thermal neutrons. 266 n/cm'/sec gives 7.66 x 10' n/cm'/8-hour day, or 7.66 x 10C n/cm/2
weeks of 5 days each. This flux, at 2.05 x 10' fast neutrons/track, produces 3.74 x 10' tracks/cm'
for a two-weeks tolerance of fast neutrons. Both of these values are of the same order of magnitude.
This may indicate that the reactions of neutrons with gelatin and tissue are quite similar and thus
that the track density on this film is a good measure of tissue ionization, it being immaterial what
the energy of the incident neutron may be.
Translated into terms of observed values, 3.74 x 103 tracks/cma are equivalent to 33 tracks/50
fields, or 1.5 fields/track. The value of 4.51 x 10' tracks/cm2 is equivalent to 39 tracks/50 fields,
or 1.3 fields/track. Using the first value of 1.5 fields/track requires counting of 25 fields of vision
to obtain an accuracy of 25 per cent.
It was found in practice that different observers vary in their determination of track density. It
was further found that different batches of film, made up to the same specifications, showed varying
track densities for the same exposure. This latter phenomenon may be due to differences in silver
grain sensitivity caused by the original processing of the emulsion.
Because of these two variables, it is necessary in using the film for personnel monitoring to
standardize each new batch of film with a standard exposure, and to have the same person who will
read the worn films evaluate the track density of the standard.
The track density for tolerance value can be calculated from the comparison of the standard
with the corresponding exposure reported herein.
2. Mees, Theory of the Photographic Process.
3. Gamertsfelder, C. C., Calculations for Use with the Fast Neutron Meter, July 21, 1944.
4. Morgan, K. Z., Table of Reference in Determining Tolerance Values, Sept. 4, 1945.
I~~~~~~~ |.i'. B.) -E"I I 'I
UNIVERSITY OF FLORIDA
3 1262 08909 7694
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