This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Bloch, C.

Bowyer, S.M.

Bowyer, T.W.

Jacobs, W.W.

Jolivette, P.

Meyer, H.O.

Pate, S.F.

Pickar, M.A.

Pierce, E.

Sowinski, J.

Vigdor, S.E.

Whiddon, C.

Wissink, S.W.

Xu, G.

English

IUScholarWorks

EVALUATION OF ABSOLUTE CROSS SECTIONS FOR 
THE p'(ii,d)y REACTION AT T, 183 MeV 
C. Bloch, S.M. Bowyer, T.W. Bowyer, W.W. Jacobs, H.O: Meyer, 
S.F. Pate, E. Pierce, J. Sowinski, S.E. Vigdor, C. Whiddon, 
S.W. Wissink, and G. Xu 
Ind iana  Univers i ty  Cyclotron Facility, B loomington ,  Ind iana  47408  
P. Jolivette 
Hope College, Holland,  Michigan 4 9 4 2 3  
M.A. Pickar 
Univers i ty  of Ken tucky ,  Lexington,  K e n t u c k y  40506  
The data acquisition for E328 (Spin Correlation and Analyzing Power Measurements 
for Neutron-Proton Radiative Capture at Tn = 183 MeV) was completed in February 
1992. Since then detailed analysis of the data has been performed, and the results for 
the spin observables were published.1 Our measurements for the spin observables A,, A,, 
and CNN agree well with theoretical predictions by Jaus and Woolcock2 and by Schmitt 
and Arenh~vel ,~ demonstrating great recent progress in the quantitative understanding of 
meson exchange current (MEC) and isobar current (IC) effects in neutron-proton radiative 
capture at intermediate energies. 
Although the primary goal of this experiment was to measure the spin correlation 
coefficient and analyzing powers, an accurate determination of the absolute cross sections 
for this reaction can be achieved from the acquired data, and can provide further constraints 
on theoretical model calculations. The results of our current analysis for the cross sections 
are shown in Fig. 1, together with data from previous  measurement^.^-^ In the figure both 
st atistical and systematic errors (except an overall normalization uncertainty of 10% due 
to uncertainty in the elastic n-p scattering cross sections used to extract the neutron flux) 
are included by adding them in quadrature. The cross sections and their errors are also 
summarized in Table I. 
We deduce the cross sections for n-p radiative capture and its inverse, photodisinte- 
gration, respectively by 
doC 
and 
where Y is the p(n,d) y yield (background- and accident al-coincidence-subtracted) for a 
given Pb-glass detector stack (for a detailed description of the detector setup, see Ref. 1); 
J is the Jacobian factor converting the lab cross section to the center-of-mass cross section; 
AS2 = 0.1267 sr is the lab solid angle of each Pb-glass stack front face with respect to the 
target, NH = 3.29 x 1022/~m2 is the number of free hydrogen atoms per unit area in the 
polarized proton target (PPT); F = (3.66 f 0.37) x lo6  neutrons/second is the average 
x H. 0. Meyer et al. 
x J. Cameron et al. 
0 De Sanctis e t  al. 
This experiment 
Figure 1. Extracted d(y,p)n cross section (squares) at E, E 95 MeV. Also shown are data 
from previous  measurement^,^-^ and calculations by Jaus and Woolcock using the Paris 
potential in an impulse approximation, including relativistic corrections (dashed curve), 
and in addition, including MEC and IC effects (solid curve). 
neutron flux used; q, = 0.86 f 0.04 is the photon detection efficiency for a Pb-glass detector 
stack; v d  = 0.978 f 0.001 is the combined deuteron detection efficiency for the four wire 
chambers; qs = 0.868 f 0.015 is the efficiency of software cuts used to select the p(n,d)y 
events; Ldy = 0.9175 is the electronic live time for the dy event stream with the PPT; P, 
is the probability that a photon from the p(n,d)y reaction survives attenuation along its 
path from the PPT to the Pb-glass detectors; T = 3.895 x lo6  s is the data acquisition 
time for the PPT. In equation (2), I(, and I<, are the proton and photon c.m. momenta, 
respectively. 
The estimation of Y, F, q,, qs and P, involves certain non-trivial analyses and calcula- 
tions, as discussed below. The p(n,d)y yield Y was extracted with a software cut, among 
others,' on the neutron time-of-flight T O F  = Trf - TaE +ATd, where Trf and TaE are the 
time for the RF signal and AE scintillator, respectively, and ATd is the calculated flight 
time of deuterons from the PPT to the AE scintillator, which is a function of gamma angle. 
The cut selects incident neutrons in the energy range 170-193 MeV (average 183 MeV). 
An analogous cut was used to select events from the prescaled sample of forward-going 
energetic protons that we collected at the same time as the dy events. From these proton 
events we determined the detected yield of n-p elastic scattering events from the PPT, in 
order to deduce the absolute neutron flux F by normalization to known n-p differential 
cross  section^.^ Most of the observed proton yield arose from reactions on contaminant 
nuclei in the PPT, rather than from free n-p scattering. It was thus critical to perform a 
proper subtraction of the background using a dummy target to isolate the free-scattering 
contribution. As a crosscheck on our knowledge of the relative target thicknesses of PPT vs. 
dummy target, we followed a similar procedure to extract the neutron flux from runs with 
CH2 and C targets of more precisely known thicknesses. The two procedures agreed, giving 
F = (3.66 f 0.37) x lo6 n/s. The systematic error here is dominated by the uncertainty in 
absolute n-p elastic scattering cross sections extracted from the program SAID-91 (Ref. 7): 
(do/dfl),,=(31.2f 3.1) mb/sr averaged over our acceptance. 
The software cut efficiency for p(n,d)y events can be determined by looking at the 
coplanarity dopen = $hd - $ ,  distributions satisfying all but one of the software cuts on other 
variables. The peak centered at 180" in this spectrum was used to determine the effective 
loss of p(n,d)y events by comparing peak areas with and without specified cuts. We found 
that the deuteron particle identification cut caused the dominant loss (5.0%, probably 
arising from the reaction tail in the E scintillator). All other cuts combined caused a loss 
of 8.2%. The total software cut efficiency is then estimated to be r) ,  = 0.868 f 0.015. 
The photon detection efficiency r),  for each Pb-glass detector stack has been estimated 
so far by simple calculations taking account of the path length available through Pb-glass 
for each y incident angle of interest. This path length determines the photon conversion 
probability and the probability of collecting sufficient Cerenkov light from the subsequent 
electromagnetic shower to surpass the hardware threshold set at E, - 18 MeV. The coun- 
ters were sufficiently deep (50 cm) to give 100% detection efficiency near the middle of the 
stack over the entire E, range of interest. However, edge effects reduce the overall calcu- 
lated efficiency of each stack to r),  = 0.86 41 0.04, in good agreement with the measurements 
made with a tagged photon beam at the University of I l l in~is .~  
Photons generated in the target had a significant probability (up to 22% for the worst 
case) for converting in material between the event vertex and the Pb-glass detectors. This 
attenuation factor has been calculated for each y angle bin. However, some fraction of the 
generated e+e- pairs from these intermediate conversions would still be detected in the 
Pb-glass. The fraction that would still trigger np-tdy events is complicated by bending of 
the e+ and e- in the PPT holding field. We are currently working on a detailed calculation 
of the attenuation losses. In the meantime, we have assumed that 1 - P, for each Pb-glass 
stack is half (f half) of the calculated intermediate conversion probability. The uncertainty 
in P, dominates the angle-dependent systematic errors shown in Table I. 
Our cross section results are in good agreement with previous measurements of p(n,d)y 
(Ref. 4-6), and also in good agreement with theoretical calculations by Jaus and Wool~ock.~ 
Table I 
p(n,d)y cross sections at Tn=183 MeV and their errors. 
Note: The first error in dor ldf l  is statistical, the second is systematic. The 10% systematic 
uncertainty in neutron flux F (primarily due to the uncertainty in n-p scattering cross 
sections extracted from SAID-91) is not included. 
1. S.F. Pate, C. Bloch, G. Xu, S.M. Bowyer, T.W. Bowyer, W.W. Jacobs, H.O. Meyer, 
E. Pierce, J. Sowinski, S.E. Vigdor, C. Whiddon, S.W. Wissink, P.L. Jolivette, and 
M.A. Pickar, Phys. Rev. Lett. 70, 3205 (1993). 
2. W. Jaus and W.S. Woolcock, Nucl. Phys. A473, 685 (1987), Nucl. Phys. A480, 573 
(1988). 
3. K-M. Schrnitt and H. Arenhovel, Few-Body Systems 11, 33 (1991). 
4. H.O. Meyer, J.R. Hall, M. Hugi, H.J. Karwowski, R.E. Pollock, and P. Schwandt, 
Phys. Rev. C 31, 309 (1985). 
5. J.M. Cameron, e t  al., Nucl. Phys. A458, 637 (1987). 
6. E. De Sanctis, e t  al., Phys. Rev. Lett. 54, 1639 (1985). 
7. R.A. Arndt e t  al., SAID (Scattering Analysis Interactive Dial-in), Fall 1991 version. 
8. C. Bloch, S.M. Bowyer, T.W. Bowyer, S.F. Pate, and M. Pickar, 1989-1990 IUCF Sci. 
and Tech. Rep., p. 105 . 
0, (lab) 
116.5 
101.5 
86.5 
71.5 
56.5 
41.5 
p~ 
0.925 & 0.075 
0.905 & 0.095 
0.889 rt 0.111 
0.948 rt 0.052 
0.975 & 0.025 
0.980 & 0.020 
0, (c.m.) 
48.9 
62.0 
76.0 
91.2 
107.7 
125.5 
doy/dfl (pb/sr)  
6.69 rt 0.18 & 0.64 
7.00 & 0.12 & 0.81 
7.17 rt 0.13 & 0.96 
5.96 & 0.14 & 0.44 
5.13 k 0.11 & 0.28 
4.04 rt 0.09 & 0.21 
Jacobian J 
1.409 
1.232 
1.058 
0.900 
0.766 
0.662 
Yield Y 
2445 rt 68 
5730 rt 104 
6716 & 108 
3491 & 78 
3630 rt 77 
3328 rt 74 


Evaluation of Absolute Cross Sections for the p(n,d)γ Reaction at T_n = 138 Mev

https://scholarworks.iu.edu/dspace/bitstream/2022/508/1/iucf1992-1993_6-9.pdf

Evaluation of Absolute Cross Sections for the p(n,d)γ Reaction at T_n = 138 Mev

Abstract

Similar works

Full text

Available Versions

IUScholarWorks