素粒子論研究

In order to investigate the reaction mechanisms of Intermediate Mass Pragments(IMF) as well as Fission Products(FP) from a highly excited compound nucleus, we have carried out the experiment using the RIKEN Ring Cyclotron for ^<84>Kr+^<27> Al at 10.6 MeV/u in which the complete fusion is still dominant. The distributions of nuclear element Z and its isotope mass are analyzed extensively in terms of Extended Hauser Feshbach Method(EHFM) and it is concluded that the sequential decay is playing an important role in accounting for those distributions and the fully equilibrated compound states of ^<111>In at 〜 200 MeV do exist with the angular momentum state of 83 ± 7h above the limit of rotating liquid drop model of 68h. A large difference observed in the previous experimental Z-distributions for ^<84>Kr+^<27>Al at 5.83 MeV/u and ^3He+Ag at 30 MeV/u, which form the nearly same compound nucleus In at 〜 100 MeV, is interpreted consistently in terms of EHFM as being due to the effect of angular momentum windows involved.

抄録全体を表示

我々はQMD計算を用いて、^<16>O+^<16>O系の約1MeV/uから数100MeV/uの広いエネルギー領域に於いて入射エネルギーの変化に応じて反応機構の移行する様子を調べることができた。まず、反応の過程で生ずるフラグメントの質量数分布によってその概観を得ることができ、特に、比較的高い入射エネルギーについては、放出される核子の運動エネルギーの時間変化を見ることで、反応に特徴的な時間変化のパターンを見つけ、フラグメント質量数分布と絡めた議論を行い、次のような結果を得た。低エネルギー高エネルギーb=Ofm:融合反応→Explosion b=3fm:融合反応→Participant-Spectator的 b=5fm:融合反応→DIC的→Quasi Elastic DIC的な反応や、P-S的反応については、その特徴である大フラグメントに関する考察をする必要がある。特にDICでは核間運動のエネルギーが核内励起に使われ減少するが、このエネルギーの散乱角依存性(Wilczynsky Polt)を再現するかが興味深い。P-S反応では、spectator zoneは運動エネルギーを殆ど失わずに出射される。また、特に入射エネルギーが高くなると、この大フラグメントの運動エネルギー分布(運動量分布)の幅はGoldhaber modelによってよく再現されるが、QMD計算による大フラグメントの運動エネルギー分布を調べることにも大きな意味があろう。

抄録全体を表示

完全にLorentz共変的なRelativistic QMDを用いてE_<lab>=84,800及び2100MeV/Aでの^<12>C-^<12>C衝突の計算を行い従来の非相対論的なQMDによる計算との比較を行った。84MeV/Aでは中心部最大密度に対し両者は殆ど同じ結果を与えるがエネルギーが高くなるにつれてRQMDの方がより大きな値を与え、それがより早く減衰する。これは初期状態の密度分布のLorentz収縮の影響である。Proton inclusive spectraに対するRQMDとQMDの計算結果の間には殆ど差が無い。しかしながら例えば重イオン衝突によるしきい値近傍での重中間子(Kやη)生成のような過程は反応の初期に起こると考えられているので、初期の段階での密度を正しく記述するためにRQMDが必要とされると思われる。

抄録全体を表示

抄録を表示する抄録を非表示にする

We have analysed subthreshold pion production on the basis of the VUU transport theory and the incoherent production mechanism with free production rates. Within the semi classical VUU formulation we have taken into account the high momentum components as described by HF ground state momentum distributions on the mean-field level by introducing a Gaussian discretization in momentum space for the one-body phase-space distribution f(r,p;t). It should be noticed that the high momentum components described above are not a direct result of short range correlations nor of many-body correlations beyond the mean field level, but a consequene of the many-body effects through the mean field and the effective interaction. Therefore their inclusion does not contradict the VUU description and the incoherent treatment of particle production. The present results with the high momentum components show a remarkable agreement with the data for and the angle integrated energy spectra of π^<0'>s in the laboratory frame down to 44 MeV/u. They also reproduce well the recent data of the invariant energy spectrum of π^^- at 95° in the NN center-of-mass frame from ^<14>N + ^<12>C at 42 MeV/u incident beam energy. Furthermore, the exponential slope parameter of π^- spectra at 90° in the center-of-mass frame is analyzed over a wide range of beam energy. Our results follow well the anomalous behaviour of the experimenta data, i.e. the slope parameter decreases monotonically with beam energy down to around 100 MeV/u and turns to be flat below 100 MeV/u. Since this anomalous behaviour can be easily understood in our model based on the basic dynamical effects of nucleus-nucleus collisions in this energy region, the general trend of the slope parameter obtained in the present study may not be wiped out by the more detailed analysis including the dynamical treatment of the pion reabsoption, the medium effects, or the momentum dependent force. Though the experimental trend is not so clear due to the large error bars for T_<0'>, this anomalous slope parameter might be an additional clear evidence for the incoherent production mechanism for pions in nucleus-nucleus collisions above 40 MeV/u: The slope parameter for π^0 angle-integrated energy spectrum in the laboratory frame as obtained in the cooperative model does not show the anomaly but monotonically decreases below 100 MeV/u. The experimetal situation will become more clear by forthcoming new measurements.

抄録全体を表示

PDF形式でダウンロード (698K)

J-STAGEへの登録はこちら（無料）