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UNITED STATES ATOMIC ENERGY COMMISSION
CROSS SECTION FOR THE REACTION T(d,n)He4
T. W. Bonner
Los Alamos Scientific Laboratory
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CROSS SECTIION FOR THE REACTION T(d.niHe4
By T. W. Bonner
The cross section for the reaction T(d,n)He4 has been measured at low energies by Bretscher
and French1 and later by Allan and Poole.2 Both experiments showed an unusually large cross section
for this reaction and evidence of a resonance phenomena. The first of these measurements was carried
out using a thick ice target and so is subject to inaccuracies due to the uncertainty in the energy loss
values in D)20 and to necessity of differentiating the experimentally determined curves. The measure-
ments of Allan and Poole used a thin gas target, but did not extend down below 100 key of tritium energy,
because of intensity considerations. It was the purpose of the present experiments to obtain precise
values of the cross section over a wide range of energies.
The essential idea of these measurements was to use the technique of thin targets of zirconium-
tritium which has been developed at Rice Institute. Targets were prepared by evaporating zirconium
metal onto thin polished discs of tungsten in a high vacuum. For these experiments zirconium, targets
were made with 12 Clg/cm2! and 50 Cag/cm2. The weights of the zirconium layers were determined by
weighing the targets on a microbalance before and after evaporation. These! thicknesses of zirconium
werte chosen so that the energy loss for 20-key deuterons would be approximately 1 and 5 kev.
The deuterons were accelerated with a 140O-key kenetron rectifier source of potential. The ion
source had no probe voltage and focusing electrodes with potential drops of 2.5 and 11 kev. The first
acceleration in the 2.5-key gap was in a region of high vacuum and so the spread of energy in the ion
beam is negligible due to this cause. The a-c ripple from the rectifier system was less than 1 per cent.
Accurate acceleration voltages were determined by means of a resistance voltmeter made up of 298
1-megohm wire-wound precision resistors. The milliammeters used in the voltmeter and the re-
sistors were calibrated against standards and the over-all voltage measurement should be more accu-
rate than 1 per cent.
The beam was resolved by magnetic analysis before entering the target chamber. The beam of ions
produced a spot 6 mm in diameter on the target which was in a plane which made an angle of 70 deg to
the direction of the beam. The vacuum in the acceleration system was better than 10-6 mm of Hg be-
fore deuterium was admitted to the ion source. Most of the experiments were carried out where the
pressure was 1.5 x 10-5 mm of Hg, although some measurements were tried with both lower and higher
pressures. A liquid air trap was placed immediately above the target to reduce the rate of deposition of
carbon on the target.
A proportional counter placed at 90 deg to the direction of the ion beam was used to detect the dis-
integration alpha particles. The aperture to the counter was a circular hole 0.955 em in diameter at a
distance of 6.80 em from, the center of the target. An over-all check of the counter efficiency and the
solid angle which it subtended was made by counting alpha particles from a plutonium source whose
disintegration rate was accurately determined. The plutonium source was painted over a 6-mm spot on
a thin platinum disc. The product of the solid angle times the counter efficiency was experimentally
determined to 1/858. This value was 5 per cent less than the solid angle as measured from the center
of the target. This difference was probably due to a slightly reduced aperture where the thin mica
window was attached. The proportional counter gave pulses from the alpha particles which were uniform
in size and it was always operated on the flat portion of its counting rate curve, well above the pulses
produced by X rays.
The current to the target was measured by a current integrator which was linear in the range 0.01
to 5 Clamp. A repelling potential of 300 volts was used to keep secondary electrons from leaving the
target. Counting rate curves were obtained as a function of this potential and the counting rate was
found to be constant for repelling voltages above 200 volts.
The ratio of charged particles to neutral particles at the target was determined by placing a
large permanent magnet near the target which deflected all charged' particles away from it. At operat-
ing pressures this ratio was always greater than 123 to 1 which was the value obtained at the lowest
The proportional counter was sensitive to the high energy neutrons which results in the disinte-
gration of tritium. In order to determine their contribution to the counting rate a shutter was placed
across the aperture to the counter so that no alpha particles could enter. The counting rate was de-
ereased by a factor of 9000 when the alpha particles were excluded. Counts from the d(d,p)T reaction
were found to be negligible from an experiment with a blank target which contained no tritium.
The number of tritium atoms per square centimeter in three targets (50 Crg/cm2 thickness) was
determined by comparison with a tritium gas target. Such a comparison was made by observing the
relative counting rate in a "long neutron counter" when the Zr-T targets or a tritium gas sample was
bombarded by 1.3-Mev protons. The number of counts per microcoulombs was first obtained from the
Zr-T targets placed at the end of a gas chamber 1.2 cm long when filled with 7 cml of helium. Then the
helium was pumped out and 7 cm of tritium put in its place. Enough tritium was added in order to in-
crease the counting rate by a factor of about 10. The background counting rate, observed when the
chamber was filled with He and with a tungsten blank at the end position, was less than 2 per cent of the
rate from the Zr-T targets. The number of tritium atoms per square centimeter from the three targets
was found to be 1.76 x 1017, 2.40 x 1017, and 1.31 x 1017.
Experiments were made with each of these three calibrated targets over a range of deuteron ener-
gies of from 15 to 130 kev. Experiments above 70 key were made with the mass two ion beam and at
lower energies the mass 3 and mass 4 spots were also used. The ion source gave a large ratio of
molecular (mass 4) to atomic ions (mass 2) which usually was about a factor of 10/1. The mass 5 spot
was undetectable and the triatomic (mass 6) spot was much weaker than the molecular spot. Under
these circumstances the mass 2 spot had an appreciable amount of H + ions in the beam. This per-
centage was calculated from the relative intensities of the mass 2, 3, and 4 spots and was fon to be
as great a 15 per cent. Data obtained with the mass 2 spot were corrected for the H2+ which was
present. Such corrections were made by determining from overlapping data of the mass 2, 3, and 4
spots the amount of H2+ in the mass 2 spot. No contaminants in detectable quantities were observed in
the mass 3 or 4 beams.
Figure 1 shows the experimental curve of the cross section of the reaction as a function of den-
teron energy. Bretscher and French have shown that the alpha particles are spherically symmetrical
at 35 and 75-key energy and Poole3 has obtained similar results up to 200 kev. The excitation curves
for each of the cross sections at the peak of the curves were respectively 6.0, 7.2, and 6.9 barns. The
points on the curve with the 50 Cpg/cm2 targets are plotted at energies corresponding to the calculated
center of the target. These calculated target losses4,5 varied from 10 key for 120-key deuterons to
4 key for 10-key deuterons. Figure 1 also gives the excitation curves for the alpha particles from a
12-Clg/cm2 target. The value of the cross section at the maximum was taken to be 6.7 barns, the aver-
age of the values obtained with the three calibrated sources. All the data indicated a maximum cross
section at 110-key deuteron energy. The curves for the 50 and 12-yLg/cm2 targets differ beyond the ex-
perimental error, especially in the region of 50 kev. In this region the thick target curve is displaced
about 2 key from the thin target curve. The two sets of data can be made to agree if the energy loss in
the Zr-T targets is assumed to be about 50 per cetnt greater than the calculated value in this region.
Table 1 gives the results taken with the thin target. The counting rate from this target was a factor of
20 smaller than for the thicker targets. The thin target data are subject to extremely small errors due
to the uncertainty in the energy loss in zirconium because the thickness at the lower energies is less
than 1 kev. The absolute value of the cross section is believed to be determined to an accuracy better
than 5 per cent for energies above 20 kev. Below 20 key the measurements are subject to some statis-
tical counting errors as well as errors due to a carbon deposit on the target. The data given in Table 1
were taken in three different runs. An appreciable carbon deposit would have been detected by a de-
creasing yield on successive runs at low bombarding energies.
Figure 2 gives a logarithmic plot of the cross section as a function of energy for both the thick
and thin targets. These data give a cross section for the reaction that is greater than that obtained by
Bretscher and French at all energies and is increasingly larger at the lower energies. The peak cross
section is ia fair agreement with the results of Allan and Poole6 who obtained a cross section of sdbout
'We are indebted to Drs. Taschek, Hemmendinger, Argo, and Agnew, and to H. T. Gittings for the
calibratitutof the tritium targets and other valuable? assistance, and to A. Lillie and R. S.Iith at Rice
Institute for their aid in the preparation of the tritium targets.
1. IBretscher and French, Phys. Rev., 75:1154 (1949) and AECD-2211.
2. Allan and IPoole, Nature, 164:102 (1949) and AERE-N/R-449.
3. Poole (private communication).
4. Wlarshaw, Phys. eva., 76:1759 (1949).
5. Crenshaw, Phys. Rev., 62:55 (1942).
6. Allan and IPoole, Nature, 164:102 (1949) and AER]E-N/1R-449.
Table 1---IData wide a Thin Zirconium Target (Mass 12 Clg/crn2
Deuteron energy at cen- lon Comits per (T,
energy, key ter of target, key mass 10 4L-coulombs barns
10.6 10.2 4 0.9 0.0026
13.2 12.8 4 4.5 0.0130
17.9 17.4 4 19 0.0550
23.8 23.5 3 66 0.191
23.2 22.7 4 50 0.144
30.9 30.3 3 147 0.425
46.5 45.7 2 502 1.45
29.1 28.5 4 104 0.301
38.7 38.0 3 293 0.847
58.1 57.3 2 810 2.34
34.5 33.9 4 194 0.561
45.8 45.0 3 492 1.42
68.9 68.1 2 1310 3.79
52.9 52.1 3 660 1.91
79.5 78.4 2 1750 5.06
60.0 59.2 3 915 2.89
90.1 89.2 2 2050 5.92
103 102 2 2230 6.45
112 111 2 2300 6.65
120 119 2 2150 6.22
128 127 2 1995 5.77
133 132 2 1834 5.31
7-a5) m/m agt
/ o 2~L m/c2 tageo
0 IO 20 30 40 50 60 70 80 90 100 110 120 130 140
DEUTERON ENERGY (KEV)
Fig. 1 -Experimental curve of the cross section of the reaction as a function of deuteron ene rgy.
50 4L gm/cma target
a12 4L gm/cma target
F Ig. 2 -Logarithmic
10 100 1000
DEUTERON ENERGY (Kev)
curve of the cross section as a function of energy for both the thick and thin targets.
END OF DOCUMENT
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