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Nuclear Isomer Research with Photons

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Nuclear Isomer Research with Photons JP0250492 JAERI-Conf 2002-007 4. Nuclear isomer research with photons Toshiyuki SHIZUMA Advanced Photon Research Center, Japan Atomic Energy Research Institute 2-4 Shirakata-Shirane, Tokai, Ibaraki 319-1195, Japan We describe the research plan of nuclear isomer physics at the Advanced Photon Research Center in the Japan Atomic Energy Research Institute. Photons generated by ultra-high peak power laser, free electron laser and Compton backscattering of laser lights will be used to excite and de-excite the isomers. Related topics of the nuclear isomer research using the light sources are described. Keywords: Nuclear isomers, Photons, Nuclear astrophysics, Atomic-nuclear in• teraction, Isomeric decay 1. Introduction Comparatively long-lived excited states in nuclei are called "isomers" [1], arbitrarily defined as having a half-life greater than 1 ns. When an isomer decays, the stored energy is usually released by emission of a, /3 or 7 rays. Gamma-ray transitions, which occur within the same nucleus, usually have transition strength larger than the a or fi decay. However, if the 7-ray half-life is long, the a- or /3- transition can pioceed in competition with the 7 decay. This is commonly seen at the lower excitation energy. The spin and parity selection rule governs the decays for this type of isomers (spin isomer). In case that the transitions involve a larger spin difference and/or parity change, the isomer half-life increases, depending on the branching ratios (including the internal conversion) and the energies of transitions decaying out of the isomer. For axially deformed nuclei, an additional selection for the K quantum number, defined as the projection of the total angulai momentum onto the nuclear symmetry axis, is applied to a transition which involves a K change. This is known as the K selection rule, and the associated isomer is called K isomer. Another type of isomer known as a fission isomer arises due to a large difference in the shapes of the initial and final states, therefore called a shape isomer. The fission isomer is trapped in the second energy minimum with a large elongated shape in the multi-dimensional nuclear shape coordinates. Due to the small overlap of wave functions between the states at the first and second minima, the 7-decay strength is weak compared to the fission into two fragments. Table 1 summarizes the classification of isomers. Table 1. Classification of nuclear isomers. Type of isomers Spin isomer K isomer Fission isomer spin/parity selection arise due to spin/parity selection & large shape change K selection Since isomers arise due to specific reasons as shown above, and details on nuclear structure infor• mation can be obtained by measuring the half-life of the state interested, isomers have been a probe in nuclear structure studies. In experimental studies, isomeric events can be separated from intense prompt -13- JAERI-Conf 2002-007 events by the delayed coincidence technique, off-beam measurements (with pulsed beams), or geometrical configuration (e.g., the recoil shadow method), which facilitates observing certain nuclear phenomena. A potential application of isomers is usage as a storage medium of nuclear energy which may be released by irradiation of photons [2]. Furthermore, the existence of /3-decaying isomers in heavy elements presents opportunities to deduce the quantities of astrophysical interest. In this report, we describe the research plan for nuclear isomers using light sources at the Advanced Photon Research Center (APRC) of the Japan Atomic Energy Research Institute (JAERI). 2. Photon sources Photons to be used in the present studies are described below, and their characteristics are sum• marized in Table 2. 2.1 Ultra-high peak power laser An ultra-high peak power laser has been developed at APRC in JAERI. The peak power has exceeded 100 TW with sub-20 fs pulse durations and an average power of 19 W at a 10 Hz repetition rate [3], Applications of this kind of lasers at intensities of 1019 ~ 1020 W/cm2 to nuclear physics have been demonstrated by two research groups at the Rutherford Appleton Laboratory [4], and at the Lawrence Livermore National Laboratory [5], They observed laser-induced nuclear reactions of (7,n) and U(7,fission) using bremsstrahlung beams generated by energetic electrons produced by laser-matter interactions. The intense, short-pulse nature of the radiation might be useful for measuring the population of isomeric states with life-times in a range inconvenient for accelerator- or reactor-based experiments [4]. Other applications of the high peak power laser to nuclear physics are the study of atomic-nuclear interactions (/ « 1015 W/cm2 is required), such as nuclear excitation by electron transition (NEET) [6], nuclear excitation by electron conversion (NEEC or inverse internal conversion process), in a plasma. These atomic processes may play a role in stimulating isomer de-excitation [7]. 2.2 Free Electron Laser Free Electron Laser (FEL) uses a relativistic electron beam as its lasing medium to generate coherent radiation differing from the conventional lasers in which bound atomic oi molecular states are used. The JAERI-FEL has been developed to produce a high-power laser at infrared wavelength using the superconducting rf linac. So far, a stable kW-level lasing has been achieved [8], If the energy of the FEL reaches the X-ray regime, the characteristics of FEL; high intensity, tunable wavelength and sharp line width may benefit the nuclear physics research. 2.3 Laser-Compton backscattering photons Experimental studies with MeV-photons have been performed by mainly using electron bremsstrahlung beams with high photon intensities. However, there are serious drawbacks such as the continuous photon energy spectrum and exponentially increasing background with decreasing photon energy. The Laser- Compton Scattering (LCS) photons produced in collisions of a laser with relativistic electrons alleviate these problems. At the storage ring TERAS in National Institute of Advanced Industrial Science & Tech• 4 5 _1 nology (AIST), the LCS photons with 1 < Eph < 40 MeV and photon intensity of 10 ~ 10 s have been produced [9]. Its small energy spread may be useful for the measurement of excitation functions for the (7,n) reaction yields which is important for production of p-nuclei in the universe. - 14 - JAERI-Conf 2002-007 Table 2. Characteristics of photons used in the present nuclear isomer research. Photon sources Ultra-high peak FEL LCS photon power laser Energy regime e V~ ke V (P lasma) ~keV 1 ~ 40MeV ~MeV(Bremsstrahlung) Feature intense, short pulse intense, tunable lin. polarized, tunable small energy spread small energy spread Facility JAERI JAERI AIST Status running planned running 3. Related topics 3.1. Acceleration of isomeric decays The 31-yr 16+ isomer in 178Hf shown in fig. 1 has re• ceived much attention due to its possible usage as the storage of energy that may be prompt-released by photons. The accel• erated decay of this isomer was observed with bremsstrahlung beams from a dental x-ray device [11]. The endpoint energy of bremsstrahlung beams was as low as 60 keV, peaked at 30 keV. The integrated cross section from the resonant absorption was measured as 1 x 10-21 cm2keV. Recently, similar experi• ments were performed [12] using x-ray beams from an undula- tor insertion device at the Advance Photon Source in the Ar- 178 gonne National Laboratory. Although they used the incident Fig.l. A partial level scheme of Hf beams with intensities that were over 4 orders of magnitude taken from Ref. [10]. than those used in Ref. [11], the results showed no signal. However, interactions which couple atomic and nuclear states may enhance isomer decays [7]. An example is a process called the nuclear excitation by electron transition (NEET) in which a nucleus is excited by a virtual photon generated during de-excitation of an atomic level [13]. The NEET occurs under the conditions of 1) closely matching of the transition energies in the atom and nucleus, 2) same multipolarity for the atomic and nuclear transitions. The isotopes of 1890s [14], 197Au [15] and 235U [13] fulfill the above conditions. In a plasma produced by the high peak power laser, the NEET process is expected to have the largest contribution to the nuclear excitation in 235U [16]. Measuring the NEET rates as a function of plasma temperature is of interest. Among the above candidates, the 1890s and 235U nuclei are suitable for such a measurement, since they include the isomers (6-h 9/2_ state in 1890s and 27-min l/2+ state in 235U) which are populated in NEET, and therefore the delayed transitions can be detected off-line. -15- JAERI-Conf 2002-007 3.2. Nuclear astrophysics The odd-odd nucleus 180mTa is famous for two aspects that W 180 181 182 —4 183 j—> it is the only naturally occurring isomer and is the nature's rarest isotope. The stellar production of 180mTa has been a challeng• v_ * 179 181 182 ing astrophysical problem [17], since the population of this iso• Ta tope is bypassed by the slow neutron-capture (s-)process, and fur• s-process Hf I—" r~~i I—I 1—1 thermore shielded from the /?-decay chains following the rapid 178 ™-4 179 f-4 180 I—* 181 neutron-capture (r-)process as shown in fig. 2. One possible way to reach 180mTa is 7-process [18] which takes place during 7 burst r-process in type II supernovae. In order to explore the isomer's produc• unstable stabie tion yield along this reaction path, cross section measurements D of the 181Ta(7,n)180mTa reaction using LCS photons have been performed using AIST-LCS photons under the Konan University, Fig. 2. The reaction path of the AIST and JAERI collaboration.
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