This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440

Byrd, R.C.

Foster, C.C.

Friesel, D.L.

Hanna, S.S.

Martoff, C.J.

Pocanic, D.

Rapaport, J.

Taddeucci, T.N.

Wang, K.

English

IUScholarWorks

STUDY OF (n,p) REACTIONS BETWEEN 100 AND 200 MeV 
C.J. Martoff, D. Pocanic, K. Wang, and S.S. Hanna 
Stanford University, Stanford, California 94305 
J. Rapaport 
Ohio University, Athens, Ohio 45701 
R.C. Byrdt, C.C. Foster, D.L. Friesel, and T.N. ~addeuccit 
Indiana University Cyclotron Facility, Bloomington, Indiana 47405 
Charge-exchange reactions are very important that. effectively all degraded neutrons could be removed 
probes for the study of giant resonances and spin-flip from the beam by a time-of-flight requirement. 
states in nuclear physics. In conjunction with (y,X), An important feature of the setup is that the 
(e,el) and (p,pl) reactions, charge exchange reactions reaction products [principally protons from the (n,p) 
can be used to isolate isovector from isoscalar reaction] are measured with high purity Ge telescopes. 
transitions. The (p,n) reaction has been studied A HPGe detector has an energy resolution of 
systematically1 since the late 1970's. This report approximately 60 keV for energetic protons. As the 
describes an experiment performed at the Indiana neutron flux is not large, counting rate considerations 
University Cyclotron Facility (IUCF) for the study of dictate the use of relatively thick (n,p) targets. 
neutron-induced-charge exchange reactions. Therefore, the overall energy resolution is determined 
A neutron facility for (n,p) studies was by the thicknesses of the two targets: the production 
constructed in the late 1970's at the Crocker Nuclear and the (n,p) target. The ideal situation is for the 
Laboratory at the University of Calif onia at Davis .2 production target and the (n,p) target to make equal 
The energy resolution of the neutron beam, less than contributions to the resolution. 
1 MeV FWHM, has made possible many significant (n,p) Figure 1 shows the experimental setup. The 
studies, but the low neutron energy, limited to 67 MeV, primary proton beam from the IU Cyclotron strikes the 
precludes strong excitation of important Gamow-Teller 7 ~ i  production target mounted on a target ladder inside 
and other spin flip resonances. a suitable target chamber. After passing through the 
Three targets are the leading candidates for 
producing neutron beams from proton beams: 7 ~ i ,  3H, 
and 2 ~ ;  for technical simplicity we chose 7 ~ i  for the 
initial facility. The intrinsic neutron energy spread 
in the 7~i(p,n)7~e reaction is due to the excitation of 
the first excited state of 7 ~ e  at 0.43 MeV. At 120 MeV 
a 300 mg/cm2 target of 7 ~ i  contributes about 1.6 MeV to 
give a total width of the neutron beam of about 1.7 MeV 
which is acceptable for initial nuclear structure 
studies. In order to achieve the desired 
monochromaticity of the neutron beam the (n,p) target 
was placed -2 m from the neutron production target so 
self-supporting target, the proton beam is swept by a 
magnet into a beam pipe leading to a beam stop some 
5.5 m beyond the target. This beam dump provides an 
effective Faraday cup for collecting the beam. A 
concrete wall, 178 cm thick, separates the production 
area from the (n,p) experimental area. 
A collimator mounted in this wall delineates the 
neutron beam. The neutron collimator consists of a 
steel cylinder filled with lead in which a section of 
copper waveguide is embedded. This assemby in turn is 
embedded in the concrete shielding wall. The wave 
guide which serves to collimate the neutrons has a 
HOLLOW WALL 
CINDER BLOCK 
. . . . .  
PRI 
TA 
DETECTOR ARRAY 
EFLECTlON MAGNET 
Figure 1. Schematic drawing of the experimental setup for (n,p) measurements at IUCF. 
rectangular aperture 2.22 cm wide by 3.33 cm high. The deflecting magnetic field is turned off. Analog 
collimator is sealed by mylar at both ends and kept signals from each HPGe detector are first amplified in 
evacuated to about 10'~ Torr. When a neutron beam is a specially modified Ortec 142B preamplitifer, which 
produced a lead slab 19 mm thick is placed over the drives a short cable leading to a standard timing 
entrance to the collimator to absorb protons scattered filter amplifier. This in turn drives a cable about a 
by the production target. At a distance of -1.7 m hundred meters long. The pulses are eventually 
behind the (n,p) target a neutron monitor3 is set up to 
measure the intensity and energy spectrum of the 
neutron beam. 
The (n,p) protons are bent out of the neutron beam 
by a dipole magnet. The maximum field is -0.8 T at a 
current of -500 A. The effective length of the field 
is 47.6 cm. With this magnet, protons of 110 MeV 
emitted at 0' can be bent at an angle of about 12'. 
This arrangement introduces a correlated variation of 
momentum and reaction angle at any given detection 
angle. A detector 2.5 cm in diameter selects a 
momentum bite of 19% and an angle bite of 1.8'. 
The protons are detected simultaneously in three 
telescopes each composed of three hyperpure Ge 
detectors. The telescopes are mounted and cooled to 
liquid nitrogen temperature inside an evacuated 
cryostat. They are arranged so as to accept protons 
emitted typically at 7.5O, 12.5' and 17.5' when the 
digitized in a peak sensing ADC and signals from the 
three elements of a telescope are summed in software to 
obtain the proton energy spectrum. The preamplifier 
modification consists simply of reducing the feedback 
resistor value by a factor of -2000 which permits 
operation at counting rates up to lo6 sec". 
In addition to the shielding discussed above, a 
shield was provided for the Ge telescopes. The 
detector shield consists of an outer layer of 3.9 cm 
thick Pb, and an inner layer of 6 ~ i 2 ~ 0 3  powder 5 mm 
thick. The 6 ~ i  nuclei absorb thermal neutrons without 
producing gamma rays. Background measurements showed 
that this shield lowers the ungated counting rate in 
the Ge detectors by a factor of about five under 
typical running conditions. Copper collimators 
inserted into the Pb shield reduce the Ge detector 
appertures to 20 mm to exclude particles which would 
leave the telescopes before stopping. 
In front of each Ge telescope of 1 mm thick AE 80 
plastic scintillator is placed to provide a fast 
tn 
trigger in coincidence with the signals in the first t- z 
3 
detector of the corresponding telescope. Particle 0 U 
timing information is obtained by starting a TDC on a & 40 
a 
W 
valid fast trigger pulse and stopping it on a m 
5 
3 
phase-stabilized beam pickoff signal. In addition, the 
timing difference between AE's and Ge detectors is 0 
recorded and monitored. Constant fraction 
discriminators are used to generate all fast logical 
En=118 MeV 
I I 1 I I 
!50 300 350 400 450 
CHANNEL NUMBER 
pulses. 
The data acquisition and analysis were performed 
using the XSYS code (written at TUNL, modified at IUCF) 
on VAX computers. Proton spectra were obtained from 
those events which satisfied cuts on AE timing vs. 
total energy E, AE vs. E, and El vs. E, where AE and El 
are energies deposited in the AE detector and the first 
detector in a given telescope, respectively. 
The overall performance of the whole setup is 
illustrated in Fig. 2 which shows a typical '~(n,~) 
spectrum obtained with a 98 mg/cm2 CH2 target and a 
correspondingly thin (100 mg/cm2)7~i primary target. 
The observed energy resolution, -1.4 MeV FWHM, is 
approximately consistent with the target thicknesses 
and the kinematic broadening. 
Measurements of (n,p) cross sections have been 
performed with 6 ~ i ,  7 ~ i  and 13c targets. Preliminary 
6~i(n,p) results are presented in a companion paper in 
this annual report. Experience with the latter two 
targets indicates that further improvement of the 
neutron beam purity would be needed for measurements of 
medium and heavy targets with smaller cross sections. 
This could be achieved by inserting an additional 
sweeping magnet over a portion of the collimator, and 
by setting up thin (perhaps MWPC) veto counters with 
low or no hydrogen content at the exit of the 
Figure 2. A typical (n,p) spectrum from a CH2 target. 
Targkt thicknesses: 7 ~ i  (primary) - 100 mg/cm2; CH2 
(secondary) - 98 mg/cm2. 
collimator. Additional MWPC's near the proton 
telescopes would complete a ray-tracing system which 
would provide improved definition of the proton 
telescope acceptances and target inlout ratios. 
Further desirable improvements include a new deflection 
magnet with enhanced maximum field to permit 
measurements at higher energy, and better telescope 
geometry (larger HPGe detectors for higher acceptance). 
The authors wish to acknowledge many useful 
discussions with the UC Davis (n,p) group, as well as 
the use of the Davis neutron monitor detectors. This 
work was supported by grants from the U.S. National 
Science Foundation. 
?present address: P-2 Division, MS D456, Los Alamos 
National Lab, Los Alamos, NM 87545. 
1) C.D. Goodman and S.D. Bloom, in Spin Excitations in 
Nuclei, edited by F. Petrovich, G.E. Brown, G.T. 
Garvey, C.D. Goodman, R.A. Lindgren and W.G. Love, 
Plenum, New York, 1982, pp. 143-159. 
2) F.P. Brady, G.A. Needham, J.L. Ullmann, C.M. 
Castaneda, T.D. Ford, N.S.P. King, J.L. Romero, 
M.L. Webb, V.R. Brown and C.Hi Poppe, J. Phys. G; 
Nucl. Phys. 10, 363 (1983). 
3) J.L. Romero, M.W. McNaughton, F.P. Brady, N.S.P. 
King, T.S. Subramanian and J.L. Ullmann, Phys. Rev. 
C 17, 468 (1978). 


Study of (n,p) Reactions Between 100 and 200 MeV

https://scholarworks.iu.edu/dspace/bitstream/2022/919/1/iucf_1986_36-38.pdf

Study of (n,p) Reactions Between 100 and 200 MeV

Abstract

Similar works

Full text

Available Versions

IUScholarWorks