by
!
.
.
-
-
:
T OF I
ORNL P
1375
.
an :
.:
:...
.
.
.
:...
..
.
EEEEEEEE
3.6
I
I
MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS -1963


21
.
,
.
.
bitnowe
Como
.
.
.
.
.
:
*:
- ---
-
*:
-
---
*:*::..."07*;
-
-
-
.
rini
.
.
--- .......
---
----,
"
LEGAL NOTICE
This report was prepared as an account of
Government sponsored work. Neither the
United States, nor the Commission, nor any
person acting on behalf of the Commission:
A. Makes any warranty or representa-
tion, expressed or implied, with respect to
the accuracy, completeness, or usefulness /
of the information contained in this report,
or that the use of any information, appa-
ratus, method, or process disclosed in this
report may not infringe privately owned i
rights; or
B. Assumes any liabilities with respect
to the use of, or for damages resulting from
the use of any information, apparatus,
method, or process disclosed in this report.
As used in the above, "person acting on
behalf of the Commission" includes any em-
ployee or contractor of the Commission, or
employee of such contractor, to the extent
that such employee or contractor of the
Commission, or employee of such contractor
prepares, disseminates, or provides access
to, any information pursuant to his employ-
ment or contract with the Commission, or
his employment with such contractor.
V
*
M
enn
.
.
..-
.
- on... -
-
-
- -
-
-
-
one
..
...
..
Si
1
OF.
T
15
.
XXX
AL
Is
14.
AL
1
W
S
V
ORNG-P1345
(Paper to be presented at International Conference on the Study of
Nuc.lear Structure with Neutrons, Anjwerp, Belgium, July 19-23, 1965)
/- حاکم-
JUL 01965
LEGAL NOTICE –

The report wompardun nom of Goran peran warte, Melchor the Valle
Hatus, nor de Comunaton, www porta notting a wall of the Contactos
A. Wake uw or ropanatation, grand or lupted, no naprot to do MOCH
ruey, completeness, of wat ons of tubormation contatod u the report, or that the we
of my laformation, voorata, wchod, as prooros duroloned to the report may not afringe
privately owned mot of
1. As we lewe wa mwepet to the won or for deres te trou the
w owy taformation, wine, wschod, or poons civoloond ta do reports
de wed to the shore, o n not on ball of the Counder" mahdo wym.
plesne a contractor of the Constantan, ar aplogna o mal contrator, to the audio that
moll oployee or controtor a the Cousteaton, or play o mal contratar parente
dlaenutamaton, a promises to bo, wy tedarnation per m o s e contract
viu the Cornalon, or we amployment with mecha contractor.
Session II
(R. B. Weinberg)
On the Absolute Value and Energy Dependence of the He(n,p)T Reaction
R. L. Macklin and J. H. Gibbons
Oak Ridge National laboratory
Oak Ridge, Tennessee, USA
Introduction
.
o
The reaction 'He(n,p)T has been studied for neutron energies from
www.
thermal to above 14 MeV by a variety of techniques (1-10).
The cross
section follows a 1/v shape up to En > 10 eV (10), but then apparently
departs from 1/v to a steeper slope in the keV range, arriving at a
value about 50% less than a 1/v cross section extrapolation for
E ~ 300 keV before increasing to a peak near 1.7 MeV. The precise
behavior of the cross section in the range 1 < E (kev) < 500 is of
Tatiminizm when seamlederumunu,+. .... ir
that in
interest for two reasons:
1) This reaction is frequently used in neutron ?lux measurements,
entati
w
which are notoriously difficult to measure accurately in the keV region.
2) The exact shape of the cross section is important since the
non-1/v behavior apparently implies either a state in *He below but
near the neutron binding energy or some cusp behavior, due perhaps to
k
exit channel interactions.
PATENT CLEARANCE OBTAINED. RELEASE TO
THE PUBLIC IS APPROVED. PROCEDURES
ARE ON FILE IN THE RECEIVING SECTION.
The main body of experimental data for 1 <E (keV) < 300 has
been obtained by reciprocity from measurements of the total cross section
for T(p,n) He [3]. However one study has been reported by Bergman
et al. [11] on a measurement of the ratio of He(n,r))/olºti(n,x)) in
the range 0.5 < E. (keV) < 50. Bergman et al. fitted their results by
assuming a state in *He at an excitation energy corresponding to a few
hundred kev below zero neutron energy. The experimental results of
Bergman et al. were in reasonable agreement with our results obtained
from T(p,n), but neither set of data gives a totally convincing picture
.
-
-.
-.
-
.
.
-
-.
.
.
.
-
- -
of the precise cross section behavior,
..
--.
.
.
Later studies, for example, of proton-tritium scattering [12]
indicate resonance-like structure a few hundred keV below the T(p,n)
threshold; thus it seemed desirable to obtain better data on the
733
section behavior over the energy range in question.
Experimental
We used the ORNL 5 MV Van de Graaff as the proton source. Some
runs were taken with one half of the acceleration column shorted out,
but most were made using the full accelerator length. Special care was
taken in obtaining as great a bean stability and energy resolution as
possible. Beam currents of from 0.05 to 1.0 ua were used. The total
proton energy spread from the accelerator was about I kev. The
tritium targets were composed of tritiated titanium thin films
Fabricated by the ORNL Isotopes Division's Target Preparation Center.
'
TRT
(14-25 ugm/cm?) on a variety of metal backings. The 4971(p,n)
threshola limited the proton energies used to about 1.4 Mev. Target
thickness varied from 2 to 4 key (calculated from titanium thickness)
for 1 MeV protors. The graphite sphere 41 neutron detector was used as
described in earlier accounts [3]. The neutron yield curves were
measured under a variety of focus and magnetic analyzer cycling conditions
to obviate as many systematic errors as possible. Other chec'ss for
systematic errors included variations of beam collimator biasing, target
vacuum conditions, and beam spot size and position on the target.
The
final yield results were converted to 'He(n,p) by detailed balance and
normalized to a velue of 1980 mb at 100 keV. The detailed balance
conversion was as follows.
E
= 1.000211 E. - 1.0201,
(1)
Omplopn = 0.999645 E_/(1.000211 E - 1.0201).
Analysis
To deduce the cross section as a function of energy from our
measured yield curve, detailed information on the overall resolution
function was needed. In our earlier work (13) with a thicker target
the resolution was dominated by the tritium distribution in the somewhat
thicker target and this was measured by a neutron time-of-flight technique.
With the much thinner targets of the present experiments, proton energy
spreads from the ion source, analyzing magnet collimation and control
and other sources could also be important contributors to the overall
.
resolution. With several backing materials, particularly Ta, tritium
'absorbed in the backing was a problem. In this situation it was
important to determine the overall resolution under the conditions of
each particular experiment. This was done by analyzing the toe of each
yield curve, i.e. about the first five keV of the yield data for the
resolution function.
The resolution function analysis rests upon the assumption that
the T(p,n) cross section can be represented as nearly proportional to
the neutron velocity for the first five keV above the threshold.
Equivalently, assuming reciprocity,
(pon) ? k(E-Eth) (+ 1/2 - 0)
(3)
where & 18 small compared to 1/2 and indeed has been shown to be
essentially zero at lower energies from the 'He(a,p) measurements (10)
and reciprocity. The present results correspond to 6 = 0 up to about
10 keV.
Figure 1 shows a typical resolution histogram from the stepwise
solution on an electronic computer of the equations:
Yg = R $1 + R2$(1-1) + --- +R$1
(4)
where y, is the ooserved yield at the 1 th proton energy step, R, is
the height of the ith step in the resolution function (R, --- R(4-2).
having already been determined) and S, is the integral of equation (3)
over the 1th proton energy step.
"Control Data Corporation 1604A.
Statistical fluctuations in the data tend to produce excursions
in the resolution histogram but these remain manageable if the histogram
energy step intervai (200-280 eV here) is not chosen too small. The
oscillatory excursions are also quite sensitive to the choice of position
of the threshold within the first energy step.
The solid curve in figure 1 18 a smoothed curve through the
histogram derived for < x 0. The dashed curve shows the effect of
assuming c = 0.06, the maximum suggested by earlier She(n,p) data
near 100 keV (3). Using the solid curve with the higher energy yield
data from the same run led to a 1/v SHe(n,p) cross section (6 - 0),
whereas the dashed curve (corresponding to ¢ * + 0.06) led to a flatter
SHe(n,r) cross section (negative c and therefore inconsistent with
the 6 = +.06 assumption) near 5 kev, approaching the 1/v dependence
near 10 keV. Thus the yield data appear consistent with the strict 1/v
dependence up to about 10 keV.
All yield data were corrected for backgrowid (chiefly from cosmic
ray neutrons) and small dead time losses (always < 10% and usually < 25).
The overall counter dead time was determined by measuring yield as a
function of beam current and with the two source technique. Cross sections
at the mean of the resolution function were derived for the data at
higher energies (above 4 or 5 keV depending on the maximum width of the
. .
.
.
resolution function for different runs). The average yield over the
resolution histogram differed from the delta function yield at the mean
energy by no more than 0.4% even at the lowest energy points shown in
.
-
-
-
--
--
figure 2.
Discussion
The results reported here link the cross section extrapolated
from low energies (by 1/v) to the various direct measuremer.cs of the
cross section obtained at higher energies. Clearly the (a,p) cross
section departs from 1/v behavior for E > 10 keV and perhaps as low as
a few kev, in essertial agreement with the conclusions of Bergreen
et al (1). The agreement between our new results and those ve reported
earlier 18 satisfactory. The disagreement with a 2/v extrapolation from
thermal (straight line of figure 2) and with direct Sheln,p) cross
section measurements above 100 keV rewins. Errors in the absolute
cross section normalization could account for one or the other. A further
possibility would be traces of tritium even in the best target backings
(Cu) we have used. If present this effect vould tend to account for the
disagreements at both the lower and higher energies mentioned above.
These results clearly imply that the cross section shape cannot
be accounted for on the basis of non-resonant compound nucleus formation
or direct reaction. Bergmen et al. (11) arrived at a satisfactory fit by
assuming a new level in "He below but near the neutron threshola.
Another possibility, as pointed out by G. Breit ana A. Ez (14) 18 that
"channel" interactions G. ur between the neutron and "He at separation
distances greater than generally supposed.
References
1. J. H. Coon, Phys. Rev. &, 488 (1990).
2. R. Batchelor, R. Ave, and T. H. R. Skyr me, Rov. Sci. Instr. 26,
1037 (1955).
3. J. H. Cibbona and R. L. Macklin, Phys. Rev. 114, 571 (1959).
4. M. D. Goldberg, J. D. Anderson, J. P. Stoerring, and C. Wong,
Phys. Rev. 122, 1510 (1961).
5. C. F. Bogdanov, N. A. Viasov, S. P. Kalinin, R. B. Rybakov,
L. N. Samellov, and V. A. Sidorov, Zhur. Fxsptl'i Tucoret. F12.
36, 633 (1959); JETP 2, 140 (1999).
6. J. E. Perry, Jnr., E. Haddad, R. L. Henkel, 6. A. Jarvis, and
R. J. Smith, reported in J. D. Seagrave's article in Proceedings
of Conference on Muclear Forces and the few Nucleon Problem, London
1999, Volume II, p. 583, Pergamon Press, Oxford (1960).
7. A. R. Sayres, K. W. Jones, and C. S. We, Phys. Rev. 122, 1853 (1961).
8. J. H. Coon and R. A. Nobles, Phys. Rev. 15, 1358 (1949).
9. L. D. P. King and L. Goldstein, Phys. Rev. 29, 1366 (1949).
10. J. Als-Nielsen and C. Dietrich, Phys. Rev. 133, 1925 (1964).
11. A. A. Bergren, A. I. Isakov, IV. P. Popov, end F. L. Shapir, J. Exptı.
Theoret. Phys. (USSR) 33, 9 (1957) (translation: Soviet Phys. JETP
6, 6 (1958)).
12. N. Jarmie, M. G. Sildert, D. B. Smith, and J. S. Lids, Phys. Rev.
130, 1987 (1963).
13. R. L. Macklin and J. H. Gibbons, Phys. Rev. 109, 105 (1958).
14. G. Breit, private communication (1957); A. I. Baz, Advances in Physics
8, 349 (1959).
.
.
.
i, -
.
.a
*4
*
.
Figure Captions
Fig. 2. Resolution function using a 28 kg/cm' Ti-T on Cu tritium
target. The solid curve 18 a smooth curve through the
histogram, which was derived assuming a 1/v He(n,p) cross
section. The dechod cur ve indicates the negligible change
produced by assuming a 12% steeper SHe(a,p) cross section.
Fig. 2. The present Heln,p) cross section results normalized to 1980
mb at 100 key. The solid line indicates tile 2/v extrapolation
from the thermal cross section. Previously published results
are also indicated.
... me
r
av
noe..
...
.
.... a' con
me. ,
'm...
.
.
.
ORNL-DWG 65-5030
INTENSITY (arbitrary units)
***
********
. **
AVERAGE TARGET THICKNESS FROM
WEIGHT AND DE/dx
****..
-10
-3
-2
-1
0
AE, (kev)
Resolution Function from Toe of T (pin) Yield Curve Near Threshold;
28 ug/cm2 Ti + T Target.
Fig. 1

ORNL-DWG 65-5294
^ BATCHELOR et al. 1955
• GIBBONS AND MACKLIN 1958
0 •PRESENT RESULTS 1965
.
......
-
-
Onep (barns)
- t
to.
.
...
..
:-
...
2 5 10 20 50 100 200 500
En (Kev)
The 'He (np) T Cross Section from 4 to 300 Kev.
Fig. 2
..

.
Uw
.
LO
TIN
'
END
DATE FILMED
10/65