| Nuclear reaction plays a key role in nuclear astrophysics for understanding of the early universe and evolution of the stars. For this reason the cross section(σ(E)) of an important astrophysical reaction at the relevant thermal energy must be known accurately. However, the direct measurements of reaction cross section between charged particles at an astrophysical energy region are severely hindered by the presence of the Coulomb barrier. Since the cross section drops steeply at energy E far below the Coulomb barrier, it is advantageous to transform the cross section into the astrophysical S(E) factor.The cross section of nuclear reaction between two bare nuclei is called the bare cross section (σbare) and the corresponding S-factor is called the bare S-factor (Sbare(E)). However, for the reaction studied in the laboratory, the target nuclei and projectiles are usually in the form of neutral atoms, molecules or ions. Consequently, the charged particles surrounding the interacting nuclei screen the repulsive Coulomb potential between the bare nuclei. This leads to an enhanced cross section (σscreened(E)) and a screened S-factor (Sscreened(E)). Due to the screening effect, the cross section is enhanced by a factor f(E, Us), where Us is a screening energy provided by the environment, and f(E, Us) increases exponentially as the energy decreases.The Sbare(E) for reactions with light nuclei, is deduced experimentally by assuming a polynomial function of E, whose coefficients are usually determined at higher energy region (E/Us>>100) where the screening does not affect the cross section. Then, the values of Us are determined from lower energy data so as to explain the observed enhancement. Recently, it has been reported that there are surprisingly large screening effects for the D+d and the 6Li+d(or 7Li+p) reactions in metal environments. In metal, the screening due to the conduction electrons should be considered in addition to the bound electrons. However, observed values of Us for the D+d reaction are more than 300 eV in most of metals, which is much larger than predicted with the Thomas-Fermi model (several tens of eV). For the 6Li(d,a)4He and 7Li(p,α)4He reactions, reported values of Us are sometimes anomalously larger than 1000 eV; they are Us=1400±480 eV and 3790±330 in PdLix target, and Us=1280±60 eV in solid lithium metal target.These abnormal observations have promoted us to study the screening effect due to metal environments more deeply. In this work, the solid and liquid lithium targets have been developed for two purposes:The first purpose is to observe the phase-dependece of screening effect by comparing the screening energies provided by molecular (data from Engstler et al.), solid and liquid target. The other one is to measure the temperature-dependence of screening effect by comparing the screening energies provided by liquid lithium with different temperature conditions.This work was processed in the Department of Nuclear Science, Tohuku University, institute for low energy accelerator platform. The thick target yields of the charged particles emitted from the 6Li(d,a)4He and 7Li(p,a)4He reaction were measured, by using the low energy (22.5-70 keV) proton (or deuterium) beam bombardment on the pure lithium metal, with the changement of target condition, i.e., the target phase or the environmental temperature.Comparing the screening effect in different environments, i.e., solid and liquid environment as well as gas environment reported previously, we found that:Comparing the solid and liquid environments, we found that the discrepancy of screening effect of the 6Li(d,a) He reaction is 74±68 eV, for the 7Li(p,a)4He reaction the discrepancy is 98±176 eV; By comparing the gas and liquid environments, the discrepancies of screening effects were 235±63 eV and 140±82 eV for the 6Li(d,a) He and 7Li(p,a)4He reactions, respectively.For the measurement of the temperature dependence of screening effect, by comparing to the screening effect of the 6Li(d,a)4He reaction occurred in gas environment, we found that the screening effect would be affected by the environmental temperature obviously. For instance, when T～222℃,ΔUsliq.-mol.=297±40 eV; T～259℃,ΔUsliq.-mol.=260±32 eV; T～303℃,ΔUsliq.-=223±36 eV and T～327℃,ΔUsliq.-mol.=119±35 eV.However, it is impossible to deduce the absolute values of screening energies due to the uncertainty of Sbare(E) factor. By comparing the modified S(E) factor (S*(E)) obtained in various target conditions, the discrepancy of screening energies can be unambiguously deduced. The experimental results show:1). The screening energy is affected by target phase (Uliquid> Usolid> Ugas); 2). The screening energy is affected by environmental temperature (Us(Tlow)> Us(Thigh)). The present work observed these phenomenon, for the first time. |
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