Intense high-energy proton beams from Petawatt-laser irradiation of solids

An intense collimated beam of high-energy protons is emitted normal to the rear surface of thin solid targets irradiated at 1 PW power and peak intensity 3x10(20) W cm(-2). Up to 48 J ( 12%) of the laser energy is transferred to 2x10(13) protons of energy >10 MeV. The energy spectrum exhibits a sharp high-energy cutoff as high as 58 MeV on the axis of the beam which decreases in energy with increasing off axis angle. Proton induced nuclear processes have been observed and used to characterize the beam.     

Synthesis of superheavy nuclei in the reactions of 244Pu and 248Cm with 48Ca

This paper presents results of the experiments aimed at producing long-lived superheavy elements located near the spherical-shell closures with Z ⩾ 114 and N ⩾ 172 in the ²⁴⁴Pu + ⁴⁸Ca and ²⁴⁸Cm + ⁴⁸Ca reactions. The large measured α-particle energies of the newly observed nuclei, together with the long decay times and spontaneous fission terminating the chains, offer evidence of the decay of nuclei with high atomic numbers. The decay properties of the synthesized nuclei are consistent with the consecutive α-decays originating from the parent nuclides ^{288, 289}114 and ²⁹²116, produced in the 3n and 4n evaporation channels with cross-sections of about a picobarn. The present observations can be considered as experimental evidence of the existence of the “island of stability” of superheavy elements. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: utyonkov@sungns.jinr.ru     

Gammasphere上裂变丰中子核核结构若干前沿领域的新进展(Ⅱ:第三和第四部分)(英文)

使用Gammasphere多探测器系统对252Cf裂变源瞬发γ射线进行γ-γ-γ和γ-γ(θ)符合测量,A～100(Z～40,N56)丰中子核区关于形状过渡、形状共存和形状突变的深入研究获得了新的进展。奇-Z核的系统研究揭示了从Z=39(Y)同位素的轴对称大四极形变到Z=43(Tc),44(Ru)和45(Rh)同位素中具有中等四极形变之大三轴形变的过渡。Nb(Z=41)同位素具有过渡核的特征。根据邻近同位素和同中素带交叉信息的系统性及推转壳模型(CSM)计算,Tc和Rh偶-N同位素中观察到的带交叉产生于一对h11/2中子的转动顺排。首次观察到100Nb(Z=41,N=59)的高自旋能级纲图和形变态,从而证实了100Nb中的形状共存,并确认,在Nb同位素链上,基态形状突变发生于N=58(球形)到N=59(大形变);而在Sr(Z=38),Y(Z=39)和Zr(Z=40)各同位素链上,基态形状突变均发生于N=59(球形)到N=60(大形变)。100Nb具有大形变基态,同现有理论预言相矛盾,其大形变的基态及转动带所具有的很大的和不规则的转动惯量具有重要的理论意义。对奇-质子Cs(Z=55)和La(Z=57)丰中子同位素八极激发的详细研究,发现了141,143,144Cs同位素链上电二极矩D0随中子数增加而减小的急剧变化,此前在145,147La中观察到的电二极矩D0的较缓和的减小似乎亦很陡地下延至148La。在Cs和La同位素中观察到的电二极矩D0的急剧下降很可能同偶-质子核146Ba和224Ra中观察到的电二极矩D0的急剧下降相类似,后者已获反射非对称平均场壳修正理论的成功解释。实验证明,141Cs具有很大电二极矩D0。141Cs和142Xe中观测到的很大的电二极矩D0和仅在141Cs中观察到的电二极矩D0的simplex量子数相关性需要进一步的理论研究。根据转动频率比值ω-(I)/ω+(I)的检定,同Xe同位素一样,认定Cs同位素中包括号143Cs的八极激发为八极振动,而在La同位素中存在八极形变。     

Gammasphere上裂变丰中子核核结构若干前沿领域的新进展(Ⅰ:第一和第二部分)(英文)

(第一和第二部分):使用Gammasphere多探测器系统对252Cf裂变源瞬发γ射线进行γ-γ-γ和γ-γ(θ)符合测量,裂变丰中子原子核核结构若干前沿领域的深入研究获得了新的进展。高达5.7×1011以上的三重和更高重符合事件的数据统计,以及更少压缩的三维数据为宽广未知丰中子核区的寻找和研究提供了有利的条件。在具有重要物理意义的若干丰中子核区首次建立,或显著扩展了一批包括转晕态和转晕附近能态的高自旋能级纲图。在偶-偶丰中子核110,112Ru和108Mo中鉴别出了手征对称破缺结构。丰中子110,112Ru附近核的三轴形变基态具有最低的能量,在它们之中已确认了接近最大值的三轴形变。在这些Ru和Mo同位素中观察到的手征双线能带展示出手征破缺的一切特征,特别是其理想的能量简并,表明它们在迄今已报道的手征破缺结构中,具有最好的手征特性。研究了手征结构从具有γ软度的108Ru到具有大三轴形变的110,112Ru的过渡。斜轴推转(TAC)和随机相近似(RPA)理论计算成功地拟合了在这些偶-偶丰中子核中观察到的手征双线能带的特性,并指定其为软手征振动态。在这些偶-偶核中观察到的手征破缺不可能归纳为奇-奇核中那样的简化的几何图像。前者来自闭壳外所有中子的相互作用。对双幻核132Sn附近N=83同中素链的系统研究为这个极富吸引力的核区提供了大量新的谱学信息。N=83同中素135Te(Z=52),136I(Z=53),137Xe(Z=54),138Cs(Z=55)和139Ba(Z=56)的最新能级信息,特别是首次建立的138Cs高自旋能级纲图和壳模型理论计算表明,Z=50质子闭壳外少数g7/2价质子激发同N=82中子闭壳外之唯一f7/2价中子的耦合对该核区能级结构具有关键作用。观察到了132Sn和208Pb附近核区谱学信息的相似性和相对应的三粒子和五粒子态。在135Te中观察到了磁转动,这是在双幻核132Sn附近观察到的首例磁转动。     

Evolution of chirality from γ soft Ru to triaxial Ru

Rotational bands in ^108,110,112Ru have been investigated by means of γ–γ–γ and γ–γ(θ) coincidences of prompt γ rays emitted in the spontaneous fission of ²⁵²Cf. The positive parity bands are described by different versions of IBA, where ¹⁰⁸Ru is best described as a γ-soft nucleus whereas ^110,112Ru are more like rigid triaxial rotors. New ΔI=1 negative parity doublet bands are found. In case of ^110,112Ru, these are interpreted as soft chiral vibrations. Many of the experimental findings can be explained by microscopic calculations that combine the TAC mean-field with random phase approximation but a simple geometrical explanation is not apparent.     

New insights into the 243Am + 48Ca reaction products previously observed in the experiments on elements 113, 115, and 117

Results of a new series of experiments on the study of production cross sections and decay properties of the isotopes of element 115 in the reaction (243)Am+(48)Ca are presented. Twenty-one new decay chains originating from (288)115 were established as the product of the 3n-evaporation channel by measuring the excitation function at three excitation energies of the compound nucleus (291)115. The decay properties of all newly observed nuclei are in full agreement with those we measured in 2003. At the lowest excitation energy E*=33 MeV, for the first time we registered the product of the 2n-evaporation channel, (289)115, which was also observed previously in the reaction (249)Bk+(48)Ca as the daughter nucleus of the decay of (293)117. The maximum cross section for the production of (288)115 is found to be 8.5 pb at E*≈36 MeV.     

E2 transitions between positive- and negative-parity states of the ground-state alternating-parity bands

Experimental transition probabilities between states of the ground-state alternating-parity bands of ¹⁴⁴Ba and their theoretical analysis are presented. Lifetimes of states in ¹⁴⁴Ba have been measured using the recoil distance method following spontaneous fission of ²⁵²Cf. The experiment was performed at the Lawrence Berkeley National Laboratory employing the Gammasphere array and the New Yale Plunger Device. The experimental data show a significantly larger value of the E2 transition probability between the negative-parity states compared to the positive-parity ones. It is shown that this effect can be explained by a higher weight of the deformed component in the wave functions of the odd-I states. In the framework of the cluster approach it is explained by a higher weight of the alpha-cluster component in the wave function of the negative-parity states compared to the positive-parity ones. In the framework of the traditional collective model with the quadrupole and octupole degrees of freedom the same effect is explained by a higher value of the quadrupole deformation at the minima of the potential energy as a function of β₂₀ and β₃₀ compared to its value at the top of the barrier separating two physically equivalent minima, having opposite signs of the octupole deformation. Additionally, the dependence on parity of the E2 transition probability is analyzed qualitatively in nuclei with a minimum at β₃₀ = 0 in the collective potential energy and compared to experimental data for ¹⁴⁸Nd.     

First chemical on-line separation and detection of a subsecond α-decaying nuclide, 224Pa

Subsecond ²²⁴ Pa (T _1/2 = 0.85 s) was produced via the ²⁰⁹ Bi(¹⁸ O,3n)²²⁴ Pa reaction at the 88 inch cyclotron at the Lawrence Berkeley National Laboratory. After production it was transported via a gas-jet system to the centrifuge system SISAK 3. Following on-line extraction with trioctylamine/scintillation solutions from 1M lactic acid, ²²⁴ Pa was detected applying on-line -liquid scintillation counting. Unambiguous identification was achieved using time-correlated --decay chain analysis. This constitutes the first chemical on-line separation and detection of a subsecond -decaying nuclide, 0.85-s ²²⁴ Pa with the fast extraction system SISAK 3.