H.M. Xu

The MSU Miniball 4π fragment detection array

Abstract A compact and highly granular charged particle detection array, covering 89% of 4π in solid angle, is described. In its present configuration, the array consists of 188 fast-plastic CsI(Tl) phoswich detectors arranged in 11 rings coaxial about the beam axis. Each phoswich detector is comprised of a 40 μm thick plastic scintillator foil and a 2 cm thick CsI(Tl) crystal selected for good scintillation uniformity. The detectors are read out by photomultiplier tubes. Elemental identification up to Z ≈ 18 and isotopic identification of H and He nuclei is achieved by exploiting pulse shape discrimination techniques.

Multifragment Emission in the Reaction 36Ar + 197Au at E/A = 35, 50, 80, and 110 MeV

Abstract Multifragment emission in the reaction 36 Ar + 197 Au at E A =35, 80, and 110 MeV has been measured with a low-threshold 4π detector array. Over this broad range of incident energies, the mean values and variances of the intermediate mass fragment (IMF: 3 ⩽ Z ⩽ 20) multiplicity distributions exhibit an approximate scaling with the total charged particle multiplicity. The measured multiplicities of light charged particles and intermediate mass fragments are compared with both a model involving statistical decay of an expanding compound nucleus, and with a model involving microscopic quasi-particle dynamics. The statistical decay model predictions are sensitive to the low-density nuclear equation of state.

Resolution tests of CsI(Tl) scintillators read out by pin diodes

Cylindrical CsI(Tl) scintillators of 38 mm diameter and 100 mm length read out with PIN diodes of 400 mm2 area were tested with respect to their response to medium energy light particles (p, d, t, α). Resolutions of better than 1% were achieved for 50 MeV protons and 90 MeV α-particles. For many crystals the resolution was found to be limited to 2–3% by local crystal nonuniformities which caused variations of the light output efficiency of several percent. A bench test is described which allows the detection of inhomogeneities to better than 0.5% accuracy. The quality of particle identification obtained with ΔE-E and pulse shape discrimination techniques are investigated as a function of count rate.

Nuclear temperature measurements and feeding from particle unbound states

Abstract Inclusive fragment cross sections and cross sections of coincident γ -ray transitions from the decay of 8 Li, 7 Be, 10 B, 12 B and 13 C fragments were measured for 32 S induced reactions on Ag at 715 MeV. These measurements are compared to quantum statistical model calculations which include sequential feeding from particle unbounds states of heavier fragments. When the uncertainties in the sequential feeding calculations are taken into account, although the measured γ -ray fractions are consistent with temperature, T ⪖ 4 MeV; the calculations demonstrate that the investigated γ -ray transitions cannot be used to determine the temperature of highly excited systems.

Thermalization in nucleus-nucleus collisions☆

Abstract Impact parameter dependent excited state populations of intermmediate mass fragments are investigated for 36 Ar induced reactions on 197 Au at E / A = 35 MeV. Population inversions, indicative of non-thermal excitation mechanisms, are observed in peripheral collisions characterized by low associated charged particle multiplicities. These population inversions disappear for collisions with larger associated charged particle multiplicities, consistent with a more complete thermalization for more complex final states. Discrepancies, observed in central collisions, suggest that the limit of local thermal equilibrium has not yet been observed.

Reaction filters. Charged-particle multiplicity and linear momentum transfer

Abstract The relation between charged-particle multiplicity and linear momentum transfer to heavy reaction residues has been investigated with a 4π charged-particle detector for the reactions 36Ar+238U at E A =35 MeV and 14N+238U at E A =50 MeV . The multiplicity of charged particles at backward angles (θ > 35°) incrreases linear momentum transfer while the multiplicity of charged particles in the forward direction is almost independent of the linear momentum transfer.

A position sensitive high resolution hodoscope for particle unstable intermediate mass fragments

Abstract A position sensitive high resolution hodoscope was developed to detect particle unstable intermediate mass fragments and to resolve the individual excited states of these nuclei. The hodoscope consists of 13 telescopes, four of these telescopes were designed to isotopically resolve fragments with 3 ≦ Z ≦ 10 and the other nine to resolve hydrogen and helium isotopes. In order to optimize the excitation energy resolution of the hodoscope, each telescope contains an x − y position sensitive gas proportional counter. A position resolution better than 0.5 mm is obtained for 5.8 MeV α-particles. This position sensitivity also allows compensation for the thickness nonuniformity of the nonplanar silicon detectors incorporated in the nine light particle telescopes. For proton decay channels of the particle unstable nucleus 14 N ∗ produced in 14 N induced reactions on nat Ag at E A = 35 MeV , the hodoscope provides an excitati energy resolution of about 60 keV (FWHM).

Entrance channel effects and the formation of hot nuclei

Abstract With an improved Boltzmann-Uehling-Uhlenbeck code, we have calculated the excitation energies and the angular momenta of heavy composite residues formed in 40 Ar + 27 Al collisions. At E A ⩾35 MeV , the maximum residue angular momenta are predicted to be significantly smaller than the static limits predicted by the liquid drop model. The excitation energy of residues produced in central collisions is predicted to increase with incident energy, reaching a value of about 5–6 MeV/nucleon at E A =40 MeV .

Complex fragment emission in the E/A=60-100 MeV 14N+natAg, 197Au reactions

Abstract Excitation functions have been measured for complex fragments with Z =3–15 emitted in collisions of E/A=60, 80 and 100 MeV 14 N ions with targets of nat Ag and 197 Au. The results are analyzed in terms of a three-source model: projectile fragmentation, non-equilibrium emission from the composite system, and statistical decay of an equilibrated heavy residue. Enhanced heavy fragment production is found for reactions with the nat Ag target above E / A ⩾60 MeV. Above E / A ≅50 MeV the ratio of equilibrium to non-equilibrium cross sections is found to be nearly independent of bombarding energy. Fit-parameter systematics are presented.

Two-proton correlation functions for equilibrium and non-equilibrium emission

Abstract Two-proton correlation functions are compared for equilibrium and non-equilibrium emission processes investigated, respectively, in “reverse kinematics” for the reactions 129 XE+ 27 Al and 129 Xe+ 122 Sn at E A = 31 MeV and in “forward kinematics” for the reaction 14 N+ 197 Au at E A = 75 MeV . Observed differences in the shapes of the correlation functions are understood in terms of the different time scales for equilibrium and preequilibrium emission. Transverse and longitudinal correlation functions are very similar.