In-beam gamma-ray spectroscopy of target fragmentation Page: 2 of 6
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EXPERIMENTAL METHOD
A target fragmentation experiment was performed at the 88-Inch Cyclotron at LBNL with a 12C beam at 30
MeV/A (3 pnA) on a 40 mg/cm2 51V target. This energy is lower than that of typical fragmentation reactions (E/A >
50 MeV), which may enhance the population of high-spin states in neutron-rich products. Most of the fragments
were stopped within the target; however, the 12C beam is not stopped but only loses 1.5 MeV/A through the target.
Prompt gamma rays with energies up to 3 MeV were detected by the Gammasphere5 array and analyzed using
RadWare6.
An advantage of the target fragmentation method, compared with beam fragmentation, is that the gamma rays are
not affected by Doppler broadening as long as their lifetimes are longer than the stopping time of the fragments (1
ps). This permits an accurate identification of new gamma-ray transitions. The fragments are produced at high
excitation energy. Some of the excitation energy is removed by evaporation of nucleons and, after the evaporation
process has finished, de-excitation of the nucleus is continued by a series of gamma-ray decays. To identify nuclei
produced, detected gamma rays were sorted into an Er - E7 correlation matrix which contained the energy and
coincidence relationships between the gamma rays emitted within ~50 ns of each other. The intensity of these
coincidences provided the basis for determining the yields of different product nuclei, their decay pathways, and the
probability of spin population.
Since each transition generally removes no more than two units of angular momentum, the number of gamma
rays emitted in coincidence (the multiplicity M) is directly related to the initial spin. In an array of detectors such as
Gammasphere, the M coincident gamma rays in an event produce K hits on the array (where K is the fold) and, in an
ideal array, K and M would be nearly equal. For each multiplicity value M, a distribution N(K) in K is actually
observed, and these distributions are given by the response functions of the array.
To estimate the input angular momentum, we used the following procedure. From the data, the K-spectrum for a
given reaction is determined. By computation we de-convolute the measured K-spectrum of the reaction into its
corresponding M-spectrum. Also, we obtain by this process the distribution specifying the M-content of each K-
value. The centroid of such distributions is then the experimental M-value that we wish to determine.
Once the M value is obtained, the spin J needs to be estimated. Here we used the expression J = 2 (M - 4) + 4
which is generally valid for fusion-evaporation reactions and might not be applicable in fragmentation reactions.
The formula assumes that four gamma rays out of M carry a AL = 1 and that the remaining M - 4 gamma-ray
transitions carry a AL = 2, so J = 2M - 4.
We were also able to obtain a measure of the spin input from the maximum spin observed in the discrete
transitions. This value is expected to be lower than the actual input spin due to the presence of an unresolved
continuum of feeding transitions.
RESULTS AND DISCUSSION
Figure 1 represents the product distribution observed in this experiment using a 51V target. More than 70
different isotopes from F to Fe (Z=9-26) were identified and new excited states found in 17 nuclei (38Ar, 43K,
41,42,43,44,45,46Ca, 44,485c, 45,52Ti, 47V, 50'52Cr, 51'52Mn). Since the gamma-ray spectrum contained transitions from such a
large number of fragments, the fragment identification was only possible due to the high efficiency and high energy-
resolution of Gammasphere.
In previous experiments using targets of 40 Ca, 48Ca and 50Ti, gamma-rays were detected by the 8x array7, and the
average gamma-ray coincidence fold of the discrete transitions determined by the BGO innerball had values of 6-12,
which implied that a maximum input angular momentum of 20 is possible in some of these nuclei. The higher
sensitivity of Gammasphere, when compared to the 8x array, allows the study of neutron-rich nuclei with smaller
production cross sections.
According to calculations the 48Ca target should give higher yields for neutron-rich nuclei, but this is not what
was observed: 48Ca and 51V seemed to produce similar fragment yields.
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Rodriguez-Vieitez, E.; Lee, I. Y.; Ward, D.; Fallon, P.; Clark, R. M.; Cromaz, M. et al. In-beam gamma-ray spectroscopy of target fragmentation, article, October 6, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc784769/m1/2/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.