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MARTIN MARIETTA ENERGY SYSTEMS L
3 4145b nasanq 3
ORNL 1479In strumrtiwifciHon
A PROTON RECOIL TYPE
FAST-NEUTRON SPECTROMETER
OAK RIDGE NATIONAL LABORATORYOPERATED BY
CARBIDE AND CARBON CHEMICALS COMPANYA DIVISION 0^ UNION CARSIOC ANO CARBON CORPORATION
QQ3POST OFFICE BOX P
OAK RIDGE. TENNESSEE
•-N
ORNL-1^79
This document consists of 36 pages.Copy^ of 311 copies. Series A.
Contract No. W-7l»-05-eng-26
Physics Division
A PROTON RECOIL TYPE FAST-NEUTRONSPECTROMETER
R. G. Cochran
K. M. Henry
Date Issued
APR 2 19St
OAK RIDGE NATIONAL LABORATORYOperated by
CARBIDE AND CARBON CHEMICALS COMPANYA Division of Union Carbide and Carbon Corporation
Post Office Box P
Oak Ridge, Tennessee
••"A"~—v uanAwEs
3 44*b 03S3n4
- 2 -
ORNL 1V79Instrumentation
INTERNAL DISTRIBUTION
1. C. E. Center 29. K. Z. Morgan
2. Biology Library 30. J. S. Felton
3. Health Physics Library 31. A. S. Householder
k-5. Central Research Lflp&ry 32. C. S. Harrill
6. Reactor Experimental,^ 33. C. E. Winters
Engineering Library 3U. D. S. Billington
7-12. Central Files 35. D. W. Card-well
13. C. E. Larson 36. E. M. King
Ik. W. B. Humes (K-25) 37. E. 0. Wollan
15. L. B. Emlet (Y-12) 38. A. J. Miller
l6. A. M. Weinberg 39. R. B. Briggs
17. E. H. Taylor kO. L. D. Roberts
18. E. D. Shipley 1»-1. R. N. Lyon
19. A: H. Snell k2. W. C. Koehler
20. F. C. VonderLage U3. W. K. Ergen
21. R. C. Briant kk. E. P. Blizard
22. J. A. Swartout I4-5. M. E. Rose
23. S. C. Lind k6. D. D. Cowen
2k. F. L. Steahly 14-7. P. M. Reyling
25. A. Hollaender k&. G. C. Williams
26. W. M. Good 1+9. K. M. Henry
27. M. T. Kelley 50-7li-. R. G. Cochran
28. G. H. Clewett 75. M. J. Skinner
EXTERNAL DISTRIBUTION
76. R. F. Bacher, California Institute of Technology77-311. Given distribution as shown in TID-l)-500 under Instrumentation
Category. *' '
DISTRIBUTION PAGE TO BE REMOVED IF REPORT IS GIVEN PUBLIC DISTRIBUTION
- 3
Table of Contents
Page
Abstract c
Introduction g
Equipment q
Proportional Counter g
Electronic Components lg
Method of Operation 21
Calibration Data and Calculations 22
Spectral Measurements 20
Conclusion ^c
Acknowledgements og
- k -
List of Figures
Figure
I Hydrogeneous Radiator .........•••••<>•'••• •
2a Proton Recoil Neutron Spectrometer, Schematic Diagram ... 9
2b Photograph of Counter, Radiator-Absorber Wheel, andPreamplifier Assembly ..........••.••••••• 10
2c ^trirasr^iwtf^c^^ n
3 Fast-Neutron Proportional Coincidence Counter. ....... 13
k. Triple-Coincidence Proton-Recoil Proportional Counter. ... Ik
5 Integral and Differential Bias Curves for Alphas. =..„».. 15
6 Counter Voltage vs. Gas Amplification for 3-Section Counter 17
7 Counter High-Voltage Power Supply .. . . ....... - - 1°
8 Coincidence Circuit ..........•.•»•••••• 20
9 Stability Test; Alpha-Source Counts from Front Sectionof the Counter, Set on Alpha-Peak ........... 23
10 Van de Graaff Data .......... ,.......•« 27
II Polonium-Beryllium Spectrum Source PB-236 ......... 3°
12 Polonium-Beryllium Spectrum Source PB-256 .......-• 31
13 Polonium-Boron Spectrum Source PB-235 .......•••• 33
Ik Polonium-Boron Spectrum Source PB-281 3*
Page
- 5
A PROTON RECOIL TYPE FAST-NEUTRON SPECTROMETER
R. G. CochranK. M. Henry
ABSTRACT
A triple coincidence type proton recoil spectrometer has been
developed for measuring neutron spectra of interest in shielding design.
The instrument is composed of a triple-section proportional counter with
suitable radiator-absorber wheels, amplifiers, electronic pulse height
selectors, and a triple-coincidence circuit. The energy resolution is
comparable to photographic plates which are at present the best method
for neutron spectra measurements. Considerable effort has been made to
obtain maximum sensitivity by optimizing the available parameters. The
spectrometer calibration was checked by measuring monoenergetic neutrons
from the ORNL Van de Graaff generator. Excellent agreement with calcu
lations was obtained for both energy and intensity.
Several spectral measurements of polonium-beryllium and polonium-
boron sources have been included in this report as examples of the type
of measurements that can be obtained with this instrument.
- 6 -
INTRODUCTION
In order to design shields for mobile reactors optimized for minimum
weight and maximum absorption, an accurate knowledge of neutron spectraand angular distribution is needed. Therefore, the development of a
neutron spectrometer at the Bulk Shielding Facility was undertaken to
provide the shielding group with asuitable instrument for measuringneutron spectra from reactors under various conditions. Several methods
of measuring neutron spectra were investigated. It was eventually con
cluded that a "proton recoil" type instrument using amultisection
proportional counter gave the most promise of fulfilling the requirements
of the shielding program.
The basic principle of most neutron measuring devices is that of
counting recoil protons from ahydrogeneous radiator (Ft*, l).
(1) J. H. Coon and R. A. Nobles, Hydrogen RecoiTProportional Counterfor Neutron Detection, MDDC-96 tJune 20, 19^6).
(2) K. M. Case, Tb£pry of Neutron C^u^tersUs^MDDC-92 (Feb. 15, 19^5)-
Beam of
Neutrons
- 7 -
Recoil Protons
CountingDevice
Hydrogeneous Radiator
Fig. 1
Protons from a hydrogeneous radiator have an isotropic distribution in
the center of mass system and have the following simple energy relationship:
Ep =En cos2 9
In order to utilize the proton recoil method in a neutron spectrometer, a
number of design features must be incorporated. The radiator must be thin
enough to insure that self-absorption is small compared with the energy
resolution desired. The thinner the radiator the better the resolution;
however, the sensitivity of the instrument is approximately proportioned
to the number of hydrogen atoms in the radiator and the hydrogen cross
section. In order to realize maximum sensitivity for a given energy
resolution, radiator thickness, counter solid angle, and detector width
are optimized for each neutron energy to be measured.'3/ Even so, this
type of instrument is at best an insensitive device requiring at least
1000 neutrons/cnr/sec at the radiator in an energy interval to register
counts above the normal background.
(3) B. R. Gossick, General Principles of a Proton-Recoil Fast-NeutronSpectrometer, ORNL-I283 (Aug. Ik, 1952).
8 -
EQUIPMENT
The Bulk Shielding Facility fast-neutron spectrometer consists of
a three-section proportional counter with a suitable radiator-absorber
wheel between the counter and the source of neutrons (Fig. 2a, 2b, and
2c). Each section of the counter is connected to a preamplifier which
is in turn connected through 75 ft of coaxial cable to a pulse amplifier.
The output of each amplifier is connected to a pulse height discriminator
(referred to as P.H.S. in this report), the trigger tube of which is in
turn connected to a monitor scaler. The P.H.S. of each amplifier is
also connected to a section of the three-fold coincidence circuit.
Coincident counts are recorded on a scaler connected to the output of
the coincidence unit. A well regulated power supply furnishes voltage
for the proportional counter tube. The radiator-absorber wheel is
rotated and positioned by use of a selsyn system.
Proportional Counter
The proportional counter used in this instrument is probably the
most important single component. The counter must have good energy
resolution and must produce pulses with a fast rise time. In addition,
the counter must give a low background from neutrons and gamma rays
passing through it.
The three-section counter used in this instrument was developed
after many experiments with counter shapes, sizes and gas fillings. <•The first counters used in the spectrometer were cylindrical with a
(!+) B. R. Gossick and K. M. Henry, Neutron Energy Spectrometer,ORNL-711 (June 12, 1950).
Fig.2a. Proton Recoil Neutron Spectrometer, Schematic Diagram
UNCLASSIFIED
DWG 14706
10
I
Fig. 2b. Front View Showing Radiator Absorber Wheel Assembly.
Fig. 2c. Side View Showing Counter, Pre-Amplifiers and Selsyn Motor.
12
mica window on one end (see Fig. 3). The radiator-absorber wheel was
placed directly in front of the counter window with the appropriate
radiator-absorber combination or an alpha-calibration source. It was
found that these axial counters gave the best pulse rise time when
filled with an argon -2$ CO2 gas mixture at a pressure of 10 cm Hg.
Since these counters gave rather poor resolution, a three-section counter
was developed in which the protons are brought in through the side (see
Fig .!»•). A comparison of the resolution of the first counter design
and the present counter is given in Fig. 5- These curves were obtained
by placing a plutonium alpha source in front of the counter and obtaining
an integral bias curve. The integral curves were then differentiated to
obtain the differential bias curves. The point of inflection of the
integral pulse height curve corresponds to the peak of the pulse height
distribution for monochromatic alpha particles from a thin plutonium
source. This fact is used throughout the calibration of the spectrometer.
It can be seen in Fig. &i that the entire neutron beam passes
through the counter. Since this method normally causes an excessive
background, every effort was made to minimize the background effect.
(Background measurements are made by using a position of the radiator-
absorber wheel that introduces only an aluminum absorber and no radiator',)
The background is minimized most successfully by lining the counter with
10 mils of platinum and filling the counter with spectroscopically
pure krypton gas instead of the argon-C02 mixture previously used.
Further purification of the gas was accomplished by freezing out impurities
KOVAR SEAL
UNCLASSIFIED
DWG. Q-1072-IA
•COPPER FILLING TUBE
BRASS COLLAR
SILVER SOLDER
A„,„r,<>,£2£A h'mvww,,,,,,,,,,,,,^,,,,^,,,,^,^,,,,,,)
PLATINUMLINER \
15-mil NICKEL WIRE
4-mil TUNGSTEN WIRE-
= >*?;;;;/***
v«««««'"Wf«««/»;»/,,,,,,,,,,;,»,,,,,,,,,,,,;;;;,.„£„,;;;;;,,;,;;;;;„„„J
KOVAR SEAL
35-mil STAINLESS STEEL CAPILLARY TUBE
I BRASS COUNTER SHELL
Fig. 3. Fast Neutron Proportional Coincidence Counter.
I
INCHES
Fig.4. Triple-Coincidence Proton-Recoil Proportional Counter.
UNCLASSIFIED
DWG. Q-1265-1A
I
10
8
£ 7rr
& ^? 6
5
4
3
3oo
oLUN
<
oro
UNCLASSIFIED
DWG. 17710
•>
^/INTEGRAL BIAS CURVE
^^o***---^,
A THE PRESENT THREE-SECTIONTRIPLE-COINCIDENCE COUNTER
1 i i
o F RST EXPERT/1ENTAL COU NITER
7.5% \/A \ y
.DIFFERENTIAL OF BIAS (:urve
A-14.5%\
°"X16
0 10 20 30 40 50
PULSE HEIGHT SELECTOR
60 70 80 90
Fig.5. Integral and Differential Bias Curves-Alpha Curves.
i
ai
- 16
with liquid nitrogen before filling the counter. The use of krypton in
the counter lowered the background effects by about 50 percent. At least
a part of the background effects in argon-C02 is due to a known n-a
reaction in argon. Figure 6 shows the gas amplification versus counter
voltage for the three-section counter used in the present model of the
spectrometer. The counter is normally operated with a gas amplification
of about 20. The steepness of the gas amplification factor-voltage curve
requires that the counter voltage-be extremely well regulated.
Electronic Components
The high-voltage power supply found satisfactory for use in this
spectrometer has a half-wave rectifier with an LRC type filter system
(Fig. 7). Extremely good stability is obtained by first regulating the
high voltage with a string of 0A2-0b2 voltage regulator tubes, and then
feeding a string of voltage reference tubes with this regulated voltage.
Regulation of the order of 80 to 100 parts per million per day is obtained(5)
with this power supply. The prototype of this supply is NEPA Model E-109,
with such modification as coarse and fine controls, polarity reversing
switch, and voltmeter.
The output terminal of each section of the three-section proportional
counter is connected to an Atomic Instrument Company preamplifier which
has been slightly modified to reduce gamma-ray pileup. The cathode follower
(5) J. K. Bair and E. E. St. John, An Extremely Stable Power Supplyfor Scintillation Counter Use, NEPA-I265-SER-8 (Jan. 20, 1950).
O
en<
UJ>
_jUJor
100
-17-
200 300 400 500
COUNTER VOLTAGE (volts)
600
UNCLASSIFIED
DWG. 17 711
700 800
Fig.6. Counter Voltage vs. Gas Amplification for 3-Section Counter.
SIDPST
TOGGLESWITCH
115v 60
Fig. 7-Counter High-Voltage Power Supply.
UNCLASSIFIEDDWG. Q-1356-1A
S3 SWITCH3 POLE 3 POSITION CERAMIC WAFER
WESTON 741\ )o-100/j.a METER
WITH 0-1 kv SCALE
PC 11800
19
output drives a terminated 75 ft of RG-62/U cable to three ORNL A-l
amplifiers.' ' The band switch on the OML amplifiers is set on the
0.2^1-second position. The cathode follower output stage of the A-l
amplifiers was changed to improve the linearity.* It was also found
necessary to remove the differentiating network in the P.H.S. output
of the amplifier units associated with the front and middle sections
of the counter to simplify the design of the coincidence circuit.
The coincidence circuit designed for this spectrometer is relatively
simple because the counting rates involved are low, the input pulses have
a magnitude of 10 to 15 volts, and the dynamic impedance of the A-l
amplifier P.H.S. output is of the order of 100 ohms. The low impedance
of the input to the coincidence circuit is very desirable when using
crystal diodes, since this factor prevents large variations in the diode
characteristics from affecting the performance. The circuit consists of
three diodes with their anodes tied together through a common biasing
resistor to the B+ voltage (see Fig. 8). The coincident output is taken
from the anode junction and provides a signal to an amplifier stage, which
in turn drives a scaler.
(6) W. H. Jordan and P. R. Bell, Instructions for Use of A-l Amplifierand Preamplifier, MonP-323 (Feb. 15, 19^917
* A similar revision by Bell and Jordan *t. al, has been incorporated inthe latest model A-l amplifiers.
FRONTAMPLIFIER O
MIDDLEAMPLIFIERO
REAR
AMPLIFIERO
-20-
COINCIDENCE CIRCUIT
12 AT 7
ALL RESISTORS ONE HALF WATT
EXCEPT AS NOTED.
UNCLASSIFIED
DWG. 17709
B +
OUTPUT
U T ©
I MEG
MODIFICATIONS TO THE A-l AMPLIFIER PHS SECTION
V 15
6AK5
VI6
6AK5
Fig. 8
- 21 -
METHOD OF OPERATION
The spectrometer is put into operation in the following manner. The
radiator-absorber wheel is rotated to the position that places the
plutonium alpha source directly in front of the counter. (The alpha
source is spaced so that the plutonium alpha particles end their range
in the rear section of the counter, so that advantage can be taken of
the peak of the Bragg specific ionization curve). The P.H.S. dial is
then set to about mid-scale and the gain on the A-l amplifier is adjusted
until the scaler begins to register counts. Then an integral P.H.S. or
bias curve is obtained similar to those of Fig. 5-
The peak of the derivative of this counting rate curve establishes a
correspondence between pulse height and maximum stopping power of the
plutonium alphas in the counter gas. From the relation between stopping
power of protons and that of alpha particles the appropriate discriminator
pulse height selector settings are calculated for the desired window
widths. This same procedure is carried out for each of the three-counter
sections.
Various radiator-absorber wheels have been constructed to cover the
energy range from 1.3 Mev to 1^ Mev for the spectrometer. The 1-3 Mev
limit is set by the mica window thickness and the gas of the three-section
counter. Each radiator-absorber wheel has a plutonium alpha source
mounted in it for calibration purposes.
22 -
This spectrometer gives good stability considering the amount of
electronic equipment used and the rigid stability requirements. A
typical plot of the alpha peak counting rate versus time is shown in
Fig. 9. Figure 9 certainly indicates that the electronic equipment is
sufficiently stable for a 7- or 8-hr run. Since the data in Fig. 9
was taken without controlling the air temperature around the counter,
even better stability would be expected in a temperature controlled
room. During a spectral measurement, the calibration is usually checked
several times a day.
CALIBRATION DATA AND CALCULATIONS
The spectrometer has been checked for energy calibration and intensity
by measuring a monochromatic beam from the ORNL Van de Graaff generator.
Protons were used to bombard a tritium gas target. Neutron energies of
2 Mev, 3 Mev, and k Mev were measured with the spectrometer. Figure 10
shows a plot of the Van de Graaff data. It should be pointed out here that
data was not taken at energies much lower than the peaks owing to the small
amount of Van de Graaff machine time available. However, during a
previous Van de Graaff test of the spectrometer, considerable data was
taken at energies lower than the neutron peak, and it was found that the
tail of the curve went to zero very sharply. This previous data did not
have a sufficient number of points on the peaks to merit inclusion in this
report.
tz
oo
1100
1050
1000
950
900
850
-23-
UNCLASSIFIED
DWG. 17712
—o—
6 o <
c
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)
>
o
O>
o
O
9
OUR DR
o
IFT LE 3S THA
' O
°
)
7"H M O /O
Ci—
<j?
°
0 4 5 6
TIME (hr)
8
Fig.9. Stability Test: Alpha-Source Counts From Front Sectionof The Counter, Set On Alpha-Peak.
10
- 2k
For example, to show the correlation between calculations and
Van de Graaff data, consider the 4-Mev peak: the point at the peak
was taken with wheel B, position 10. Wheel B, position 10, has
6.3O mg/cm2 polystyrene radiator and 21.94 mg/cm2 of aluminum absorber.
In these calculations everything is converted to equivalent range in
mg/cm2 polystyrene.
Converting the aluminum absorber to an equivalent amount of
polystyrene:
21.9% mg/cm2 (aluminum) xr^r =14.9 mg/cm2 polystyrene•
1.06 is the stopping power for polystyrene and I.56 is the stopping
power for aluminum. Now to obtain the total proton range,
14.9 mg/cm2 +2^30 ing/cm2 +2.84 mg/cm2 =20.9 mg/cm2.
The radiator thickness is divided by 2 because it is assumed that on
on the average the proton must penetrate half of the radiator
thickness. The 2.84 mg/cm2 is the equivalent range in the counter,
considering the mica window and the gas path.
From proton range energy curves'^ it is found that 20.9 mg/cm2
range in polystyrene corresponds to an energy of 4.0 Mev. (Range in
paraffin rather than in polystyrene is actually used - this con
tributes a very small error). Now the resolution can be calculated
from Geiger's Law:
(7) J. C. Hirschfelder and J. L. Magee, Efficiencies of NeutronCounters Using Recoils of Protons in Argon and Xenon, MDDC-115(July 23,T$55).
SsasBew-aawW'-?
- 25 -
dR = g dER 2 E
From the data for 4-Mev protons, it is seen that the energy resolution
becomes:
dE = 2 x 6.30 = O.I69, or 16.9$E 3-55 x 20.9
where
R = range in mg/cm2 polystyrene, E = energy in Mev, dR = radiator
thickness in mg/cm2, and g = 3-55 (this is obtained from the energy-range
relation).
The calculated resolution can be compared to the measured value
of 18.7$. The above simple calculation represents only a very approximate
approach neglecting several rather large errors.
A more precise calculation of resolution can be made by using a
method devised by B. R. Gossick^ ' to calculate the total range error
which is denoted by:
(8) B. R. Gossick, ORNL-I283 op. cit., p. 28.
c 25 P tot
where 6 p' 2 /RexS jf H,
Rr
6 p%, = 29.1 ft)t2l + t22
5 P t =12 (R)2
For 4 Mev
- 26
= 6 pc8:, + 5 p2a + 5 p2t
2 (9)"straggling error''
"angular straggling error"
'error due to radiator and detector width"
5p\ot =(2'5 x10~^ +^6.35 xlO"3) +(3.28 x10^) . 82.! xio^
Now (82.1 x10-lf) ("Y"J (2.36) =12.040 half-level width.Since this calculation is based on a perfect instrument, the:
discrepancy between 120 and 18.70 half-level width indicates the deviation
of this instrument from an ideal spectrometer.
Consider the intensity N(E) in Ffg. 10 (Van de Graaff data). This is
calculated from the expression:^10'
2 2
bl ^PN(E) = I „ fr
2n S a^-fiLS.
tj %2 E, x C neutrons/sec/Mev/unitn 1 solid angle
(9) H. A. Bethe, The Properties of Atomic Nuclei, II Range-Energy CurvesBNL-T-7. '
(10) B. R. Gossick, ORNL-I283 op. cit., p. 19.
27-
2.0 3.0 4.0 5.0
NEUTRON ENERGY (Mev)
UNCLASSIFIED
DWG. 16821
Fig. 10. Van De Graaft Data for Neutron Spectrometer, 9/26/52.
O
c
o
•o
o
c
3
>CD
O<D<n
\
(/)
c
oi_
-*—
0)c
28 -
where
C = counting rate in counts/sec,
R = range of the proton (in cm polystyrene),
g = Geiger's law constant,
•b-, = distance from source to radiator,
bp = distance from radiator to collimator,
S = number of hydrogen atoms/cc in radiator,
a = radius of collimator (the counter itself acts as a collimator),
t^ = radiator thickness (in cm polystyrene),
t2 = uncertainty in end of range (in cm polystyrene),
E^ = average energy of range.,
<T = hydrogen cross section at energy E.
The expression for N(e) is derived for an ideal point source but
should hold reasonably well for the experimental setup used in obtaining
the Van de Graaff data. Since the Van de Graaff generator was monitored
with a calibrated long counter, the total neutrons per second emitted
from the target can be calculated. For example, at 3-Mev the long
counter gave 2.62 x 10^ neutrons per second emitted from the target.
Now, by integrating the area under the 3-Mev peak, it is found that the
spectrometer data gives 2.42 x 10? neutrons per second emitted from the
target. A comparison of these results shows the spectrometer data to
be 9.250 low. Checking the other calibration energies at 2 Mev and 4
Mev gives equally good comparison.
(ll) Neutron Cross Sections, "A Compilation of the AEC Neutron CrossSection Advisory Group," AECU-2040 (May 15, 1952).
- 29 -
The statistical fluctuation at each data point is calculated by:
~2 _ ^N = Ql - 02
Ql + Q2
where Oh = total foreground counts plus background counts and Qg =
total background counts, both taken in the same time interval.
SPECTRAL MEASUREMENTS
Since the spectrometer gave good results when checked with the Van
de Graaff generator, it has been used to measure the more complex
spectra of a number of neutron sources. These spectral measurements
(12)compare favorably with measurements made at other laboratories.v '
Figure 11 is the spectrum of PB-236 - polonium-beryllium neutron source.
This source was of 10 curies strength at the time the measurement was
made.
Figure 12 is the spectrum of PB-256 - also polonium-beryllium neutron
source. This source was three times as strong as PB-236 at the time
of measurement. Much better counting statistics could have been ob
tained had time permitted. From the four polonium-beryllium sources
recently measured at this Laboratory and spectra measurements of polonium-
beryllium made at other laboratories'^) it appears conclusive that the
polonium-beryllium spectrum varies considerably from source to source.
(12) J. H. Perlman, H. T. Richards, and Lyda Speck, The NeutronSpectra of Po-B and Po-Be. MDDC-39 (July 9, 1955).
(13) B. G. Whitmore and W. B. Baker, "Energy Spectrum df Neutronsfrom a Po-Be Source?, Phys. Rev., 78V 79$ (June I95O)."""
700
600
500
e
•5 300
Uj
^
200
100
UNCLASSIFIED
DWG. 17720
1 /"^ V —i
1 0 ) ll- ^3 )^—
^•V
1\H
1 —y5 V-" H
1—( )—11- (t )
j—L_ —g)— —1
1 ( V ' 1
•)—1
/1
1 1 i— —<r'— 1
1
11
11
11
1r
3 4 5 6 7 8 9 10 11 12
Mev
Fig.11. Po-Be P. B. 236 Spectrum.
I
oI
1.7
1.6
1.5
1.4
1.3
1.2
1 .1
1.0
~ 0.9
o
o 0.8
c
p 0.7ki
2«0.6
0.5
0.4
0.3
0.2
0.1
31-
UNCLASSIFIED
DWG. 17719
!• •"•<•!
•-
c i
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1 rtiV i -^
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1 ••
i :i
••
1 —I
2 3 4 5 6 7 8 9 10 11 12
NEUTRON ENERGY (Mev)
Fig. 12. Polonium —Beryllium Source. P.B.-256.
32 -
Thus, one cannot expect the average energy of one polonium-beryllium
source to be the same as that of another.
Figure 13 is the neutron spectrum of PB-235, polonium-boron source
used by the Health Physics Division for calibration purposes. The
spectrum drops to zero above 5 Mev as would be expected from the
Q-values of B10 (a>n) and B11 (a,n).
Figure 14 gives the neutron spectrum of PB-281, a very intense
polonium-boron source recently acquired by the Biology Division.
(l4) T. W. Bonner and L. M. Mott-Smith, "The Energy Spectra of theNeutrons from Disintegration of Fluorine, Boron, and Berylliumby Alpha Particles", Phys. Rev., 46, 258 (1934).
CD
O<U
1400
1300
1200
1100
1000
900
800
E 700o
</>
I 600•t-
0)
X 500ki
400
300
200
100
0
-33-
UNCLASSIFIED
DWG. 14678
t f 7"w
Po- B SOLJRCE NO. 2 35
1 V. i
|—(^—H
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F= —1
ST •— — 1 T>)^^
\y——
1— "t ^^"r^i
0 1/ I 5 *\ £> 15 7 £J $3 10 11 12
NEUTRON ENERGY (Mev)
Fig. 13
200
180
160
X
to
O
x 140
a>
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1 <20to
c3
% 100
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CDCO
CO
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UNCLASSIFIED
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NEUTRON ENERGY (Mev)
Fig. 14. Po-B Spectrum. Pb 281 10/6/52
- 35 -
CONCLUSION
The neutron spectrometer has been found satisfactory thus far in
fulfilling its design requirements. It mainly utilizes electronic
units commercially available with a minimum of modifications and is
reasonably stable considering its electronic complexity.
While considerable effort was made to obtain the maximum sensitivity
by optimizing the available parameters, more sensitivity would be
desirable. However, since the sensitivity is dependent on the number
of hydrogen atoms in the radiator and the hydrogen cross section, a
natural limit is established.
- 36
ACKNOWLEDGEMENTS
The above instrument is the result of continuing the research
initiated by Mr. B. R. Gossick and Mr. K. M. Henry in 1948.
The authors gratefully acknowledge the valuable assistance and
counsel given them by Mr. B. R. Gossick of Purdue University.
We wish to thank Mr. R. E. Zedler of the ORNL Instrument Department
for assistance in designing and constructing counters for the
spectrometer.
Thanks are due Dr. J. L. Meem for his encouragement and support.
We are also indebted to Dr. J. L. Fowler and Dr. Harvey Willard for
Van de Graaff machine time.