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

Bacher, A.D.

Beene, J.

Berg, G.P.A.

Bertrand, F.

Bowyer, S.M.

Chang, S.

Franklin, W.

Halbert, M.

Hicks, K.

Lisantti, J.

Liu, J.

Mueller, P.

Olive, D.

Stephenson, E.J.

Varner, R.

Wells, S.P.

Wissink, S.W.

English

IUScholarWorks

MEASUREMENTS OF (5, p") AND (p', ply) OBSERVABLES 
FOR 1+ STATES IN 12C AT 200 MeV 
S.P. Wells, S.W. Wissink, A.D. Bacher, G.P.A. Berg, S.M. Bowyer, 
S. Chang, W. Franklin, J. Liu, and E.J. Stephenson 
I n d i a n a  Un ive r s i t y  Cyc lo t ron  Facili ty,  Bloornington,  I nd iana  4 7 4 0 8  
J. Beene, F. Bertrand, M. Halbert, P. Mueller, D. Olive, and R. Varner 
Oak R idge  Na t iona l  Laboratory, Oak  Ridge ,  Tennes see  37831 
J. Lisantti 
C e n t e n a r y  College of Louis iana ,  Shreveport ,  Louis iana  71134  
K. Hicks 
Ohio  Univers i ty ,  A thens ,  Ohio  4 5 7 0 1  
The study of ($,$I) spin observables has historically provided valuable information 
about the NN interaction inside the nuclear medium.' There are, however, only eight in- 
dependent quantities that can be determined in singles ($, p") reactions. For transitions 
to discrete states of non-zero spin J, additional information can be obtained by study- 
ing the polarization of the recoil nucleus. This is possible through measurements of the 
angular correlation between the scattered proton and the particles emitted in the nuclear 
de-excitation. For transitions with the simple spin sequence 0+ 7- I+, measurements of 
the type (5, ply), in which the polarizations of the outgoing proton and photon are not 
detected, when combined with the complete sets of (p',p") observables discussed above, 
provide sufficient information to completely specify the scattering amplitude.2 Using the 
K600 spectrometer and its associated focal plane polarimeter (FPP) to detect the scat- 
tered protons, and a system of BaF2 detectors3 surrounding the K600 target to detect the 
coincident y-rays, we have performed simultaneous measurements of ($,$I) spin-transfer 
observables as well as (5, ply) coincident observables for the two dominant 1+ states in 
12C (T=l at 15.11 MeV, T=O at 12.71 MeV) at a proton bombarding energy of 200 MeV. 
Because (p', 5') measurements provide at most eight pieces of information, yet eleven 
are required for a complete determination of the scattering amplitude (for O+ + 1+ transi- 
tions), it can be shown2 that (6, ply) observables contain information about the transition 
which is not accessible in (p', p") singles measurements. The full scattering amplitude can 
be divided into two parts: terms involving the proton spin coupled to unit vectors in the 
reaction plane, and terms where the proton spin couples to the unit vector normal to the 
scattering plane. It is the relative phases between these two sets of terms that can be 
measured via (p',ply) observables and are not accessible in (p',p") measurements. It can 
also be shown2 that if the coincident y-rays are detected at some angle out of the reaction 
plane, yet not normal to it, then the photon polarization tensor will have negative parity. 
In this case, the sideways and longitudinal analyzing powers, Dos and DoL, which are 
identically zero in singles ($,$I) measurements due to parity conservation, may be non- 
zero. Contained in these asymmetries are the relative phases just discussed. By placing 
three of the BaF2 arrays at an angle of 45" out of the scattering plane, we were able to 
map out the angular distribution of these previously unmeasured asymmetries. 
Because O+ + 1+ transitions are of unnatural parity, they are dominated by an orbital 
angular momentum change of AL = 0. As such, the differential cross sections for exciting 
these states are very forward peaked in the laboratory frame. To obtain appreciable 
yields for these studies, it was therefore necessary to place the entrance to the K600 
spectrometer at small laboratory scattering angles. Moreover, since the BaF2 detectors 
were positioned very near the target chamber, it was necessary to deposit the unscattered 
beam in a well-shielded beam dump far downstream from the target. To meet both of 
these requirements, we utilized the septum magnet mode of the K600 ~ ~ e c t r o m e t e r . ~  In this 
mode, we were able to detect cleanly coincident y-rays associated with protons scattered at 
laboratory angles as small as 8, = 5". In order to minimize the attenuation of the coincident 
y-ray flux and to ensure that this attenuation was independent of 8,) a special thin-walled 
scattering chamber compatible with all septum magnet mode hardware was installed for 
these measurements. This scattering chamber was cylindrical in shape, with the symmetry 
axis perpendicular to the scattering plane. The aluminum chamber had 0.25" thick walls 
and an outer diameter of 10". A 0.25" thick upper lid was also used since, for reasons 
discussed below, one BaF2 array was placed directly above the target to detect coincident 
photons emitted along the normal to the scattering plane. With this choice of geometry for 
the scattering chamber, and given the positioning of the BaF2 arrays (discussed below), the 
attenuation of the emitted y-ray flux was independent of photon angle and approximately 
4% for E, = 15 MeV. 
In April 1993, we were given 28 shifts of beam to carry out measurements of (p', p") 
singles spin observables and (p',p1y) coincident observables for the 1+ states in 12C at 
200 MeV. Measurements were made with the BaF2 arrays in two different configurations. 
In the first configuration, using an incident proton beam polarized normal to the scattering 
plane, three BaF2 arrays were positioned on beam right lying in the scattering plane at 
angles of 8, = 60") 100°, and 140" with the front face of each array at a distance of 
56 cm from the target. Due to the large charged-particle flux emitted at forward angles 
from the 93.8 mg/cm2 N A T ~  target, it was necessary to place 3.6 cm of aluminum in 
front of the BaF2 array at 6, = 60°, which was sufficient to stop protons of ~ 1 0 0  MeV. 
In this configuration, we made measurements of the singles observables daldfl, A,, P ,  
and DNNt, and simultaneously measured the coincident cross section d2a/dfl,dfl,(i) and 
-+ 
. - .  . 
coincident asymmetry Ay(k)  as a function of photon angle in the scattering plane. (Here 
is the momentum of the emitted photon.) Using the BaF2 array placed directly above the 
target, we also measured the coincident cross section and asymmetry, both of which can 
be related,2 in a model independent way, to the singles spin-transfer observable DNNt. 
Since we were simultaneously measuring this observable in the FPP, we thus obtained 
a valuable internal consistency check on the data. Measurements were made at proton 
scattering angles of Op = 5", 8", 11°, and 15". Shown in Fig. 1 is a schematic diagram of 
the configuration used for these measurements. Walls of lead were stacked both upstream 
and downstream of the BaF2 detectors to shield from y-rays emitted from beam-line 
elements and the external beam dump, respectively. 
In the second configuration, where we used an incident proton beam which had its 
polarization vector rotated into the scattering plane, we supported three BaF2 arrays at 
an angle of 45" out of the scattering plane. In this case, the BaF2 arrays were positioned 
Figure 1. Schematic diagram showning the position of the four BaFz arrays relative to the 
thin-walled scattering chamber, septum magnet hardware, and K600 entrance quadrupole. 
Distances and angles of the arrays are given in the text. Walls of lead were stacked both 
upstream and downstream of the arrays to shield them from radiation generated from 
beamline elements and the beam dump, respectively. 
on beam right at angles of 8, = 41°, 88.3", and 140". Due to space limitations, we were 
forced to place the centers of the arrays at distances of 79 cm, 66 cm, and 66 cm from 
the target, respectively. In this configuration, we measured two linear combinations of the 
four singles spin-transfer coefficients DLL1, DLS/, DSLlr and Dsa as well as the induced 
polarization P (which is independent of the incident beam polarization) in the FPP. The 
BaFz arrays were used to map out an angular distribution of the photon yield asymmetries 
for the measurement of the sideways and longitudinal analyzing powers Dos and Doh In 
order to separate the sideways and longitudinal pieces, it is necessary to have at least 
two orientations of the incident proton spin in the scattering plane. To provide internal 
consistency in these measurements, we chose to make measurements with the incident 
proton polarization in three different orientations, at in-plane angles of Q = 53", 117", 
and 169" with respect to the incident beam direction. Again, data were taken at proton 
scattering angles in the lab of 8, = 5",  8O, 11°, and 15". 
In our experiment, three signals were read out by CAMAC for each photon detector 
that satisfied a low threshold discriminator in hardware coincidence with the detection 
of a proton in the K600 focal plane: fast timing information, relative to the K600 focal 
plane scintillators; a wide-gate integrated pulse height, for good photon energy resolution 
over the energy range of primary interest; and a short-gate pulse height, the amplitude of 
which is dominated by the fast (ultraviolet) component of the scintillator response.4 These 
three signals provide sufficient information to cleanly separate pulses generated by photons 
from those generated by fast neutrons and charged particles.5 These latter types of parti- 
cles can be identified by exploiting the lower relative intensity of the fast component that 
these types of particles generate. Figure 2a shows the correlation between the fast com- 
ponent and the total light output for all coincident events in the BaF2 array positioned at 
19, = 100". Charged and neutral particles are located in two distinct bands in this corre- 
lation. Shown in Fig. 2b and 2c are the correlations between the total light output (the 
wide-gat e integrated pulse height) and the relative p-y time-of-flight difference for the 
two distinct bands shown in Fig. 2a. Because at Op = 8' the yields for the 4.44 MeV and 
15.11 MeV states dominate the proton energy spectrum (with the p+12C elastic peak ve- 
toed in hardware), one expects an enhancement of the coincident photon yield at these two 
energies. This is clearly shown in Fig. 2b where, at the shortest relative time-of-flight, two 
maxima are visible at photon energies of 4.44 and 15.11 MeV. There is also some fraction of 
coincident photons which generate pulses of identical shape as those produced by charged 
particles (the origin of these events is still under investigation). With the time-of-flight 
vs. energy correlation, however, these events can be separated from charged particles, as 
shown in Fig. 2c. While this preliminary offline analysis indicates that (p', p'y) events from 
the 15.11 MeV, 1+ state in l2 C can be easily identified, further analysis (such as accidental 
beam-burst subtraction) is required to extract any coincident (5, observables. 
1. See, for example, H. Bagheai, e t  al., Phys. Rev. Let. 69, 2054 (1992), and references 
therein. 
2. J .  Piekarewicz, e t  al., Phys. Rev. C 41, 2277 (1990). 
3. M. Thoennessen, J.R. Beene, F.E. Bertrand, and J.L. Blankenship, Oak Ridge Nat. 
Lab. Prog. Rep. (1989). 
4. G.P.A. Berg, e t  al., this report. 
5. R. Novotny, e t  al., Nucl. Instrum. Methods A 262, 340 (1987). 
pi::::: a * . . .  . . 0 , ::om
Figure 2. (a) Intensity distribution of the fast light component versus the total light out- 
put of all BaFz crystals in the 19-pack placed at By = 100". Requirements for incrementing 
this histogram are given in the text. Charged particles are discriminated from photons 
and neutrons as indicated by the solid line. (b) Intensity distribution of the relative time- 
of-flight versus total y-ray energy for photons and neutrons as identified in (a) (above the 
solid line). (c) Same as (b) for charged particles and photons (below the solid line). 


Measurements of (p,p) and (p,p'γ) Observables for 1+ States in 12-C at 200 MeV

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Measurements of (p,p) and (p,p'γ) Observables for 1+ States in 12-C at 200 MeV

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