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

Cassagnou, Y.

Dayras, R.

Kwiatkowski, K.

Legrain, R.

Norbeck, E.

Pollacco, E.C.

Viola, V.E.

Volant, C.

Wile, J.L.

Yennello, S.J.

Yoder, N.R.

English

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SEARCH FOR MULTIFRAGMENTATION NEAR THRESHOLD 
IN THE 3He 4- Ag REACTION 
S.J. Yennello, K. Kwiatkowski, N.R. Yoder, J.L. Wile, V.E. Viola 
Indiana Urriversity Cyclotron Facility, Bloomington, Indiana 4 74 05 
E.C. Pollacco, C. Vola.nt, R. Dayras, Y. Cassagnou, R. Legrain 
CEN SA CLAY, Gzf-sur- Yvette, France 
E. Norbeck 
University of Iowa, Iowa City, Iowa, 5224.2 
Coincidence measurements of intermediate mass fragment (3 < Z < 15) production 
from the 3He + natAg system at 900 MeV and 3600 MeV have been ma8de at the SATURNE 
I1 accelerator in Saclay, France. These mea.surements are a followup on previous inclusive 
measurements1 of this system that extended the existing data2 fro111 threshold through the 
proposed onset of multifragnle~ltation. 
Inclusive nieasurements indicate that the IMF formation mechanism undergoes a, 
change between 900 MeV and 1.8 GeV.' This is evidenced by (1) a large increa.se in 
the overall cross section; (2) a flattening and then ~a~turation f the high-energy tails of the 
Pnergy spectra, accompanied by a, broadening and a shift to lower energies of the Coulomb 
pea2k, and (3) a saturation of the fragment charge distribution. 
The experimental setup included 36 detector telescopes covering about 20% of 47r. 
Thirty-two detectors were gas ion chamber (GIC)/500 pm silicon pstrip AE/E tele- 
scopes capable of detecting fragments from Z=3-15 with good energy resolution. The 
remaining four detectors were GIC/220 pm silicon pstrip/(300 pm or 500 pm) silicon 
pstrip/scintillator logrithmic telescopes capable of detecting light charged particles as well 
as IMFs. Several different scintillators were used in an attempt to determine which would 
he most suitable for future work: these included CsI, BGO, and orange plastic, with the 
resulting light detected by photodiodes in all cases. 
Shown in Fig. l a  is the raw multiplicity (M) distribution for Z 2 3 fragments at 
3600 MeV. Although few in number, there are events of charge 3 or more up to M i m f = 5 ,  
accounting for at least 30% of the total charge of the target and projectile. This has been 
corrected for accidentals, but not for solid angle or angular distribution. Fig. I b  compares 
the multiplicity distribution rela.tive to singles at 900 MeV and 3600 MeV. Although singles 
dominate at both energies, the ratio of triples to singles shows an 8-fold increase for the 
higher energy. 
IMF mult 
multiplicity relat ive singles 
mult ipl ic i ty  Lie 
Figure 1 .  (left): Relative raw multiplicity of Z 2 3 fragments formed in 3600 MeV 3He + 
Ag reaction; (right): Fraction of events relative to singles for Z 2 3 fragments observed in 
3He + Ag reaction at 3600 MeV (dashed line) and 900 MeV (solid line). 
The charge distribution can best be quantified by fitting the yields to a power law, 
A T value of approximately 2.1 has been suggested as an indication of m~l t i f r a~men ta t ion .~  
In Fig. 2 we show how the tau parameter is affected by requiring a coincidence with a par- 
ticular trigger detector. The solid and dashed lines are the values of the tau parameter 
from the inclusive data. For both energies, requiring a coincidence with a backward detec- 
tor produces relatively more heavy fragments than requiring a coincidence with a forward 
detector. These axe more likely to be associated with central collisions4 which deposit 
more energy and thus are more likely to produce multifragmentation events. 
t a u  comparison 
Figure 2. Value of charge distribution parameter T for fragments coincident with trigger 
detectors at various angles. Trigger detectors are defined as follows: 2(25"), 3(63"), 4(117") 
and 5(14g0). Damtan 8t 900 (3600) MeV are indicated by crosses (diamonds) and the average 
7 value is given by solid (dashed) line. 
1. S.J. Yennello, et al. to be published in Phys Lett. B. 
2. L.G. Sobotka, et al., Phys. Rev. Lett. 51 (1983) 287; K. Kwiatkowski et al., Phys. 
Lett. I3171 (1986) 41. 
3. N.T. Porile, et al., Phys. Rev. C 39 (1989) 1914. 
4. M. Fatyga, et  al., Phys. Lett. B222 (1989) 356. 


Search for Multifragmentation Near Threshold in the 3-He + Ag Reaction

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Search for Multifragmentation Near Threshold in the 3-He + Ag Reaction

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