Direct evidence of “washing out” of nuclear shell effects

Abstract

Constraining excitation energy at which nuclear shell effect washes out has important implications on the production of super heavy elements and many other fields of nuclear physics research. We report the fission fragment mass distribution in alpha induced reaction on an actinide target for wide excitation range in close energy interval and show direct evidence that nuclear shell effect washes out at excitation energy ∼ 40 MeV. Calculation shows that second peak of the fission barrier also vanishes around similar excitation energy. One of the major areas that have generated unprecedented interest among contemporary nuclear physicists and chemists is the synthesis of super heavy elements (SHE). It is known from the Liquid Drop Model (LDM) of the nucleus [1] that if the two fundamental nuclear parameters, the attractive nuclear surface potential and the repulsive coulomb forces are taken into account, then our nuclear chart may end at around element number 104. This is simply because, nuclei with Z ≥ 104 immediately fission as there is no barrier to prevent their decay. However, elements have been synthesized beyond that atomic number [2].The observed stability of these heavy elements is believed to originate from the microscopic shell effects in nuclei. While LDM predicts the bulk properties of nuclei and explains their collective behavior, nuclear Shell Model [3] explains these shell gaps and the single-particle nature of nuclear states. Both the bulk properties and the shell properties of nuclei can be incorporated by adding a shell-correction term to the liquid-drop model energy. Strutinsky [4,5] considered the shell effect as a deviation from uniform liquid drop model prediction and used the shell averaged single particle energy as a correction term to the liquid drop model energy. The liquid drop barrier height diminishes smoothly with the increase in atomic number as the nuclear fissility increases. However, as the shell correction term retains the fluctuations in the shell model energy, it is found that the incorporation of this shell correction alters the fission barrier and in fact, causes to develop large barrier to decay that can increase alpha or fission half-lives by several orders of magnitude for the heavy elements. Thus shell effects play a central role in determining the stability of the super heavy elements. Many important nuclear phenomena such as the fission isomers [6], super deformed nuclei [7] and new magic numbers in the exotic nuclei [8] are the consequences of the shell effect. * tilak@vecc.gov.in It is generally believed that shell effects are washed out at higher excitation energy [9]. For the production of the super heavy elements by heavy ion bombardment on actinides targets, the compound nuclei are always formed with an excitation energy exceeding a few tens of MeV. Judicious choice of the excitation energy is critical as the production cross section of the SHE may be increased by a few orders of magnitude if the beam energy is increased by few MeV. Therefore, constraining the exci...

Relative mass distributions of neutron-rich thermally fissile nuclei within a statistical model

“…al. [44] at large excitation energy, which contradict the recent prediction of large asymmetric mass distribution of neutron-deficient Th isotopes [45]. These two results [44,45] along with our present calculations confirm that the symmetric or asymmetric mass distribution at different temperature depends on the proton and neutron combination of the parent nucleus.…”

“…[44] at large excitation energy, which contradict the recent prediction of large asymmetric mass distribution of neutron-deficient Th isotopes [45]. These two results [44,45] along with our present calculations confirm that the symmetric or asymmetric mass distribution at different temperature depends on the proton and neutron combination of the parent nucleus. In general, both TRMF and FRDM predict maximum yields for both symmetric/asymmetric binary fragmentations followed by other secondary fragmentations emission depending on the temperature as well as the mass number of the parent nucleus.…”

Study of hot thermally fissile nuclei using relativistic mean field theory

“…The value of this point depends on the ground state shell structure of the nucleus. The, experimental evidences of washing out of the shell effects at and around 40 MeV excitation energy has also been pointed out in Ref [56]. Shell correction obtained from the intercepts on the E * − axis in Figure 3 The values for all three parameter sets are almost same with a little difference in NL3 model (see Figure 3).…”

Into the Fission Valley of 'magic Nucleus' Polonium

“…However, to produce SHE one would require to find out the best combinations of nuclei (target and projectile) to be fused in the laboratory [6,7] . Also it is required to be ensured that the SHEs are produced at right excitation energies at which nuclear shell effects survives [8].…”

“…The fission fragment mass distributions can be used as an experimental probe to look for the presence or absence of shell effects in nuclei [8]. Therefore mass distributions of the nuclei 210,206 Po were studied [9] by the researchers at Variable Energy Cyclotron Centre.…”