Alpha-decays of At-195g,m and Fr-199

Abstract

The alpha decay of At-195 was reinvestigated in order to clarify its complicated nature. The alpha decays from both the ground state (g) and isomeric states (m), which are produced in the Tm-169(Ar-36,alpha 6n) reaction, were successfully identified. In addition, the first evidence for the alpha decay of Fr-199 produced in the Tm-169(Ar-36,6n) reaction has been found. Metallic Tm-169 targets evaporated onto aluminum foil were fixed on a rotating disk. The pulsed Ar-36 beam, which had the intensity of 1.0 x 10(exp 12)/sec on the average and the energy of 215 +/- 5 MeV, was irradiated. Reaction products, which recoiled from the target, were separated from the primary beam in a Gas filled Recoil Isotope Separator (GARIS), and were detected with a two-dimensional Position Sensitive Detector (PSD) placed at the focal plane of the GARIS. A Micro Channel Plate (MCP) assembly was placed at upstream of the PSD. In the analysis, the PSD signals in coincidence with MCP signals were regarded as the Evaporation Residues (ER), while the signals, which were not in coincidence with MCP signals, were treated as the possible alpha decay events. Three events in time sequence, which are corresponding with ER, first alpha decay, and second alpha decay, were searched. The alpha decay properties of At-195g,m were assigned based on the observed correlation among ER, At-195g and Bi-191g (ER, At-195m and Bi-191m) and on the agreement of the alpha decay energy and lifetime of Bi-191g (Bi-191m) with literature values. Assignment of Fr-199 was based on the observed correlation among ER, Fr-199 and At-195g and on the agreement of the alpha decay energy and lifetime of At-195g with the measured values in this experiment.

At-195のα崩壊の複雑な特性を明確にするため、このα崩壊の追試を行った。基底状態(g)から、およびTm-169(Ar-36,α6n)反応において生成した異性体状態(m)の両状態からのα崩壊を明らかにした。また、Tm-169(Ar-36,6n)反応において生成されるFr-199のα崩壊を初めてとらえた。Alに蒸着した金属Tm-169の複数のターゲットを、回転するディスクに固定し、そこにビーム強度が平均1.0×10(exp 12)/秒、エネルギーが215±5MeVのAr-36のパルスビームを照射した。ターゲットから反跳した反応生成物は、ガス充填反跳同位体分離器(GARIS)により1次ビームから分離し、GARISの焦平面に配置した2次元的な位置敏感検出器(PSD)で検出した。またマイクロチャネルプレート(MCP)をPSDの上流に配置し、解析においては、MCP信号と合致するPSD信号はすべて蒸発残留物(ER)とみなし、MCP信号と合致しないPSD信号をα崩壊事象とた。ER、1次α崩壊、2次α崩壊に相当する時間的に連続した3つの事象を調べた。At-195g,mのα崩壊は、測定したER、At-195g、Bi-191g(ER、At-195m、Bi-191m)の相関、およびBi-191g(Bi-191m)のα崩壊エネルギーと寿命が文献値とが一致することにもとづいて決定した。Fr-199のα崩壊は、測定したER、Fr-199、At-195gの相関、およびAt-195gのα崩壊エネルギーと寿命にもとづいて決定した。