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

Berg, G.P.A.

Chang, S.

Crawley, G.M.

Galonsky, A.

Ieki, K.

Liu, J.

Mercer, D.J.

Stephenson, E.J.

Yoo, G.H.

English

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SEARCH FOR FRAGMENTED MI STRENGTH IN 48Ca 
D.J. Mercer, G.M. Crawley, A. Galonsky, K. Ieki, and G.H. Yoo 
M i c h i g a n  S t a t e  Univers i t y ,  E a s t  Lans ing ,  M i c h i g a n  4 8 8 2 4  
G.P.A. Berg, S. Chang, E.J. Stephenson, and J.  Liu 
I n d i a n a  U n i v e r s i t y  C y c l o t r o n  Faci l i ty ,  B l o o m i n g t o n ,  I n d i a n a  4 7 4 0 8  
The quenching of MI (AL=O, A JT=l+) strength remains one of the outstanding 
problems of nuclear physics. If configuration mixing within and across major shells is a 
dominant mechanism for the quenching, then one should in principle be able to locate 
fragmented M1 strength in states close to where the main strength occurs. In the case of 
48Ca(p,p1), 30% of the predicted total strength has been found concentrated in a single 
1+ state with excitation energy 10.21 MeV.'j2 No other 1+ states in 48Ca have yet been 
positively identified using the (p,pl) probe, but a 48Ca(e,e1) experiment suggests that 
eighteen small J" = 1+ states exist with Ee,=7-13 MeV, each having strength about 
1-3% as large as that seen in the 10.21-MeV state.3 IUCF experiment E269 was designed 
to look for small 1+ states such as these, making use of the good energy resolution of the 
K600 spectrometer. 
Data were collected at IUCF during Jan. 22-28, 1993. The beam was 200 MeV 
unpolarized protons, an energy chosen for its high selectivity for MI transitions. The 
primary target was a 4.0-mg/cm2 self-supporting foil of 97.69 Atom% 48Ca, manufactured 
at IUCF. The K600 was used in septum mode, with two x-position and two y-position 
wire-chamber detectors placed near the focal plane. Data from the 48 Ca(p,p') reaction 
were collected at nominal laboratory angles of 5", 6", 8", and 10". Angle calibrations were 
performed by determining the kinematic crossover angle of the lH(p,pl) and 12C(p,p1) (4.44 
MeV) reactions from a CH2 target, from which it was determined that the true angles were 
0.88" smaller than the nominal spectrometer settings as read from the angle encoder. This 
large deviation from previous calibrations was verified by moving the spectrometer to an 
angle small enough to stop the beam on the entrance aperture of the septum, which is 
designed both to stop the beam and read the current. Energy dispersion at the focal plane 
was found using the positions of the 7.65 and 15.11 MeV 12C peaks from the CH2 target 
(yielding 4.68 keV per 0.1-mm channel), and excitation energy was extrapolated from the 
known value of the strong 48 Ca 1+ peak at 10.21 MeV. Replayed spectra appear in Fig. 1. 
The FWHM energy resolution obtained is about 34 keV. No background was subtracted in 
the spectra. Only standard conditions were set, e.g., particle identification, and acceptance 
limits in vertical position and scattering angle. 
The signature of an MI transition is a cross section which increases as the scattering 
angle approaches zero. Other transitions have a cross section predicted to decrease as the 
angle approaches zero (or at least not to increase as fast as an MI transition). In the 4.1" 
spectrum, only the 10.21 MeV JT=l+ peak remains strong, and contributions from higher 
multipoles are greatly diminished. 
An expansion of the 4.1" spectrum reveals evidence for a number of small peaks 
(labelled "a-k", "m-o" in Fig. 2). The majority of these can be eliminated as potential I+ 
6 8 10 12 14 16 
excitation energy (MeV) 
Figure 1. Spectra showing the range of 48Ca(p,p') data collected in the present exper- 
iment. The 10.21-MeV 1+ state is plainly visible at all angles, while states from higher 
multipole transitions are diminished at the smaller angles. 
4.1 (detail) 
400 
7 8 9 10 11 
excitation energy (MeV) 
Figure 2. Detail from one of the panels of Fig. 1. Lower case letters serve to indicate 
visible peaks, a few of which are candidates for fragmented M1 strength. 
states on the basis of their angular distribution. For example, "b" and "d" are certainly not 
I+, since they clearly decrease in proportion to the 10.21 MeV state as the angle decreases 
from 9.1" to 4.1". The best candidates for 1+ states seem to be "k" and "m", at 10.12 and 
10.33 MeV, which maintain their proportion to the 10.21 MeV state at all angles. If these 
are indeed 1+ states, then they have MI strengths of about 3.5% and 2.5% as large as that 
found in the 10.21 MeV state. The above mentioned (e,el) reference reports 1+ states at 
10.14 and 10.33 MeV with similar ~ t r e n g t h . ~  On the other hand, many of the other states 
reported in the (eye1) reference are not evident in Fig. 2. 
Data at smaller laboratory angles are needed to complete the investigation. If any of 
the small peaks persist at zero degrees, then they likely can be identified as fragmented 
M1 strength. To achieve such small angles, the K600 spectrometer must be operated 
in transmission mode. A test run of the transmission mode took place Jan. 29-Feb. 1. 
Although significant progress was made in achieving the necessary angular and energy 
resolution and background suppression, no 48Ca data were collected during this test. 
1. G.M. Crawley, N. Anantaraman, A. Galonsky, C. Djalali, N. Marty, M. Morlet, A. 
William, and J.-C. Jourdain, Phys. Lett. 127B, 322 (1983). 
2. Y. Fujita, M. Fujiwara, S. Morinobu, T. Yamazaki, T. Itahashi, H. Ikegami, and S.I. 
Hayakawa, Phys. Rev. C 37, 45 (1988). 
3. W. Steffen, H.-D. Graf, A. Richter, A. Harting, W. Weise, U. Deutschmann, G. Lahm, 
and R. Neuhausen, Nucl. Phys. A404, 413 (1983). 


Search for Fragmented M1 Strength in 48-Ca

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Search for Fragmented M1 Strength in 48-Ca

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