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

Bowyer, S.M.

Bowyer, T.W.

Brooks, W.K.

Jacobs, W.W.

Kim, Y.

Pierce, E.

Rinckel, T.

Sperisen, F.

Vigdor, S.E.

von Przewoski, B.

Xu, G.

English

IUScholarWorks

CHARGE EXCHANGE 
REPORT ON CE-19: 160(p,n)16F(~-) IN THE IUCF COOLER 
T.W. Bowyer, S.E. Vigdor, S.M. Bowyer, W.W. Jacobs, Y. Kim, 
B. von Przewoski, E. Pierce, T. Rinckel, F. Sperisen, and G. Xu 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 74 08 
W.K. Brooks 
University of Pittsburgh, Pittsburgh, Pennsylvania 15269 
The relatively weak 160(p,n)16F(~-) transition is of great interest because it carries 
the pion's quantum numbers and is thus sensitive to potential many-body effects of the 
pion field in a nucleus. Previous experimental studies relied on neutron time-of-flight in 
order to obtain adequate energy resolution to distinguish the (ground state) 0- from the 
(E,,~200 keV) 1- state of 16F. The required energy resolution limited these studies to 
bombarding energies < 80 MeV, where reaction mechanism ambiguities severely compli- 
cated the interpretation of the measurements. The goal of CE-19 was to study this reaction 
at the more suitable energy of 300 MeV in the IUCF Cooler by exploiting an ultra-thin gas 
target to detect the low-energy ( ~ 0 . 5  MeV) decay protons from 16F as a high-resolution tag 
in coincidence with prompt neutrons (detected over a relatively short flight path).1 Two 
test runs were performed in the spring of 1991 to study the feasibility of this experiment 
in the IUCF Cooler. 
Since ultra-thin internal targets must be used in the Cooler, a gaseous form of oxygen 
was desired for this experiment. Due to the corrosive and potentially damaging effects O2 
would have on the turbomolecular pumps2 of the existing jet target, pure 0 2  could not be 
used. The only stable gas which contains oxygen without contaminants that might mock 
up the signature or create a background for the CE-19 experiment is HzO vapor. A water 
vapor jet for use in CE-19 was developed, tested and reported earlier.3 
In the test runs, neutrons were detected over the angle range 0 ~ ~ ~ 1 " - 9 "  in a 
14-element scintillator hodoscope developed for the CE-03 experiment.4 The hodoscope 
was positioned 4.8 m from the target and covered a laboratory solid angle of -34 msr. 
Low-energy protons were detected in coincidence with the neutrons in one of two cooled, 
4 cmx6 cm, 500 pm thick silicon microstrip detectors. These detectors were placed ~5 cm 
from the beam axis in order to maximize the solid angle for the 16F decay protons, which 
for the 0- state are emitted isotropically in the ion's rest frame. The detector layout is 
shown in Fig. 1. 
Luminosity Scintillators 
5 cm Nozzle 
Figure 1. Experimental layout for the CE-19 test runs. 
In order to monitor luminosity, we set up to acquire simultaneously p-p elastic scatter- 
ing events from the hydrogen in the water vapor target. Those p-p coincidences involving 
a backward low-energy proton were monitored both by the neutron hodoscope-microstrip 
combination and by a second detector combination comprising forward plastic scintilla- 
tors on the beam left side at laboratory angles ~ 9 . 5 '  to 14O, and a position sensitive 
silicon detector (PSD) for the recoil proton on beam right. The latter combination al- 
lowed software reconstruction of the jet profile in a manner similar to previous Cooler 
e ~ ~ e r i m e n t s . ~ j ~  Both arms should also provide an absolute normalization via p-p scatter- 
ing of the l6 0(p,n)16F(~-)  cross sections measured simultaneously. In order to avoid over- 
whelming n-p coincidence contamination by p-p scattering events, we placed four charged- 
particle veto (CPV) scintillators in front of the neutron detectors, and only allowed a small 
sample (1%) of p-p data to be written to tape. 
The first test run went smoothly though the low beam current (100-150 pA average) 
and low Cooler acceptance (~47-r mm-mrad) limited our luminosity to only 
E ~ X  ~ m - ~ s - '  for p-p events. This was far too small to get a statistically significant 
number of 0- (p,n) events above background. Nonetheless, the analysis of the first test 
run was encouraging, since the strongly populated 2- state ( E , ~ 4 0 0  keV) in 16F clearly 
showed above the background in the decay proton spectrum with excellent ( ~ 9 0  keV) en- 
ergy resolution. Fig. 2a shows the corrected pulse height spectrum from the silicon strip 
detectors: the 2- state is seen roughly at channel 200, corresponding to approximately 
900 keV proton energy. This spectrum was gated on neutron time-of-flight, RF time and 
neutron-proton time difference. 
Channel Number 
Channel Number 
Figure 2. Spectra of silicon microstrip pulse height gated on neutron time-of-flight, RF 
time and n-p time difference. (a) First test run (CE-19A). (b) Second test run (CE-19B), 
obtained with 300 MeV protons on H 2 0  vapor jet target. 
The goal of the second test run was to run with an improved luminosity in order to 
gain enough statistics to see the 0- state above background. The average beam current 
was increased in the second test run to about 500-600 PA, increasing the luminosity to 
-2x ~ m - ~ s - '  for p-p events. Unfortunately, the singles counting rates in the silicon 
microstrips increased by much more than an order of magnitude. This increase came from 
real particles correlated with the detected neutrals, all below 1 MeV energy deposition in 
the microstrips. We found that the singles rate without a target depended strongly on the 
amount of current injected into the Cooler and seemed to decrease with a half-life of x 4  s 
after beam cooling. There was also a long-lived component of the singles rates which had 
a half-life of x15 minutes. We surmise that the increased rate of low energy deposition 
particles may have come from activation of the copper nozzle from the vapor jet target 
and/or pumping apertures in the target box. We tried without significant success to adjust 
the beam tuning to eliminate the apparent activation problem. 
The analysis of the data from the second test run showed that the 2- state was com- 
pletely swamped by the low energy tail in the microstrips. Fig. 2b shows the microstrips' 
energy spectrum from the second test run with the same gating conditions as Fig. 2a. Ob- 
viously, there was no chance to see the relatively weak 0- ground state in this case. The 
results from the test runs suggest that the technique we employed for measuring observ- 
ables in the 160(p,n)16F(~-) reaction works in principle; however, coincidences between 
neutral particles and very low-energy charged particles offer no opportunity to ray- t race 
to a reconstructed event vertex, and hence make the technique very susceptible to back- 
ground from non-target sources. One thus needs far better control and reproducibility of 
beam tuning than presently exists for the T-section of the Cooler. The experiment will be 
deferred until appropriate advances in beam tuning have been made. 
1. T.W. Bowyer and S.E. Vigdor, spokespersons, "Study of the '60(p,n)16F(~-) Reac- 
tion at Tp = 300 MeV in the IUCF Cooler," IUCF Proposal No. 90-07, June 1990. 
2.. R. Hellmer , Balzer's High Vacuum Prod., private communication. 
3. T. W. Bowyer, et al., IUCF Sci. and Tech. Rep., May 1990-April 1991, p. 166. 
4. W. Daehnick, et al., IUCF Sci. and Tech. Rep., May 1990-April 1991, p. 52. 
5. H.O. Meyer, et al., Phys. Rev. Lett. 65, 2846 (1990). 


Report on CE-19: 16-O(p,n)16-F(0-) in the IUCF Cooler

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Report on CE-19: 16-O(p,n)16-F(0-) in the IUCF Cooler

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