Nuclear fragmentation cross sections for NASA database development Page: 4 of 5
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1400 _NI ts - I ,rk%7
1200
10f..'s.64) I He
800 50O 1
lii 60 115 0 Li
0 5 10 15 0 200 400 600
AE PSD2 (not scaled) Pulse ht. Strip Y10
FIGURE 4. Left: scatter plot of AE in a single strip of the SSD vs. AE in an unsegmented PSD placed just downstream of the
SSD for a 400 MeV/nucleon 12C beam incident on a composite target. Events with a carbon ion are off-scale; the boxed region
indicates the region populated by events with either a Li fragment or two He fragments. Using the PSD alone (i.e., taking a
projection onto the x-axis) would yield a single peak of "ambiguous" events; taking a projection onto AE in the strip shows clear
separation of the two event types.
In a recent article on the fragmentation of 20Ne (7), a method for resolving some ambiguities in particle detection
was described; the method makes use of the correlations between signals in the last silicon detector and the far-
downstream plastic scintillator and does not depend on spatial segmentation of the detectors. It was shown that, at
least for a Ne beam, a majority of apparent "charge 4" events seen in d3mm5/6 were due to the detection of four He
fragments. In the same article, we described in detail the methods for extracting charge-changing and fragment
production cross sections from histograms analogous to those in Fig. 3. The same methods are applicable to all the
data sets discussed here. For fragment species with Z's near the beam charge, little fall-off was seen in the cross
sections obtained at 70 compared to those obtained at 2.40; this is because the fragment angular distributions are
strongly forward-peaked when only a few nucleons are removed from the projectile, in keeping with the predictions
of Goldhaber's statistical model of fragmentaton (8). As fragment charge and mass decrease, angular distributions
broaden and significant decreases are seen in the cross sections obtained at small angles. The loss of fragments at
small acceptance causes the number of events at Z = 0 (no charged fragment inside the detector acceptance) to
increase, as can be seen clearly in the Fig. 3 histograms. The same effect is also observed in preliminary results
obtained with all other beams.
Results: Charge-Changing and Fragment Production Cross Sections, Model Comparisons, Angular Distributions
The methods used to extract the cross sections have been described in detail (7). Several corrections to the raw
counts are needed, and these grow in importance as target depth increases. The corrections contribute to the overall
systematic errors, which are typically about 3% for charge-changing cross sections, about 5% for fragment
production cross sections for Zrmg > Zbeam/2, and 10-20% for the lighter fragment species. The light-fragment cross
sections have generally not been reported in earlier fragmentation work. An example showing preliminary data is
given in Fig. 5 for 600 MeV/nucleon 28Si on aluminum and carbon targets; the results are compared to the semi-
empirical nuclear fragmentation code NUCFRG2 (9), which was found to give good agreement with measured cross
sections for 56Fe on a variety of targets (10). The model is not very accurate below charge 12, and does not
reproduce the observed enhancement of even-Z fragments and the relative suppression of odd-Z fragments.
Part of the large systematic uncertainties for the light fragments is due to uncertainties in the modeling of
fragment angular distributions. Our beamline simulation code (7) has a single free parameter, 6o, which controls the
widths of the Gaussians that describe (as a function of Armg) the distributions. This parameter is not known a priori
and the existing experimental data - which have been summarized by Tripathi and Townsend (11), leading to a
parametrization for 6o - are sparse. The acceptance-angle dependence of the cross sections we obtain is an indirect
measure of this parameter over a wide range of beam and target masses. To obtain more direct information about
fragment angular distributions, a program of measurements with silicon detectors placed at specific angles off the
beam axis (2.50, 40, 5.50, 7.50, and 100) has also been begun, using 600 and 800 MeV/nucleon 28Si beams and a 600
MeV/nucleon 20Ne beam. These data will yield d6/dO at specific values of 0; fitting these results to Gaussian
distributions will determine the appropriate values of 6o and allow us to make a direct test of the angular
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Zeitlin, Cary J.; Heilbronn, Lawrence H.; Miller, Jack; Fukumura, Akifumi; Iwata, Yoshi; Murakami, Takeshi et al. Nuclear fragmentation cross sections for NASA database development, article, August 24, 2001; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc716657/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.