叩山山川町，川川団川問問川ハ川畑川畑町ノι間山間柵柵川山m間山山門 剛山

QU JPU 5 4 JAERI-Conf JAERI-Conf 2002-007

4. 4. Nuclear isomer research with photons

Toshiyuki SHIZUMA Adv α nced Photon Research Center ，J αpα nA t: om:ic En 併:q y Rese α rch Institnte

2-4 2-4 Shimk αfα -Shir αηe， ，Tok αi ，Ib ω'aki 319-1195) Jαpαη

We describe the resean:h plan of nuclear isomer phy 邑ics at the Actv 乱nc 吋 Pho もon Research Center in in the Japan Atomic Energy Research Institute. Photons generatecl by 1l 1tra-high peak power laser ，free electron electron las ♂r anct Cornpton backscattering of laser lights will be used to excite 州 d de-excite the Isomers.

Relatedωρ1CS 吋 the nucleω ・isomer research using the light sources are described.

Keywords: N uclear isomers ，Photons ，N uclear astrophysics ，Atomic 明 nuclear in- teraction ，Isomeric decay

1. 1. Introduction

Comparatively Comparatively long-lived excited states in nuclei are called "isomers" [1] 、 arbitrarily defi 田 d as having having a half-life 忌日也，ter 七han 1 ns. When an isomer decays ， the stored energy i己 usually releωed by emission emission of α，s orγrays. Gamma-ray transitions ，which occur within the same nucleus ， usually have trall 首ition strength larger than the αor s decay. However ， if the γ'-ray halιlife is long ，theα- orβ transition transition can proceed in competition with the γdecay. This is commonly seen at the lower excitation energy. energy. The spin and parity selection rule governs the decays for this type of isomers (spin isomer).

In In case that the tran 邑itions involve a larger spin differe 恥 e andjor parity change ， the isomer half-life increases ，depending on the branching ratios (including the internal co 即時I 自ion) 叩 d the energies of transitions transitions decaying out of the i弓omer. For axially deformed nuclei ，an additiollal selection for the K qnantum number ， defined as the prυjection of the total angular Ul omentum 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 associated isomer is called K isomer. Allother Allother type of isomer known as a fission isomer arises due to a large difference in the shapes of the the initial and final states ，therefore called a shape isomer. The fission isomer is trapp 刷 1 iu the second energy energy minimum with a large elongated shape in the multi-dimensional nuclear shape coordinates. Due to to the small overlap of wave functions between the states at the first and second minima ，theγ-decay strength strength is weak compared to the fission into two 企agments. Table 1 summariz 創出 e classification of lsomers. lsomers.

Table Table 1. Classification of nuclear isomers. Type of isomers Spin isomer K isomer Fission isomer spir 山 arity selection arise arise due to spinjparity selection & large shape change K selection

Since 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 nuclear structure studies. In experimental studies ， isomeric events can be separated from intense prompt

- 13 一 JAERI-Conf JAERI-Conf 2002-007 events events by the delayed coincidence technique ， off-beam measurements (with pulsed beams) ， or geometrical co 凶 guration (e.g. ， the recoil shadow method) ，which facilitates observing certain nuclear phenomena. A potential potential application of isomers is usage as a storage medium of nuclear energy which may be released by irradiation irradiation of photons 開[2 司2J. Fu 抽rt仕出h削e opportunities opportunities to deduce the quantities of astrophysical interest. In In this report ，we describe the research plan for nuclear isoiners using light sources at the Advanced Photon Research Center (APRC) of the Japan Atomic Energy Research Institute (JAE Rl).

2. 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 2.1 Ultra-high peak power laser An ultra-high peak power laser has been developed at APRC in JAER I. The peak power has exceeded exceeded 100 T 羽T with sub-20 fs pulse durations and an average power of 19 W at a 10 Hz repetition 19 19 20 2 rate rate [3J. Applications of this kind of lasers 抗 intensities of 10 "-' 10 W /cm to nuclear physics have been demonstrated by two research groups at the Rutherford Appleton Laboratory [4J ，and at the Lawrence Livermore National Laboratory [5 卜 They observed laser-induced nuclear reactions of (γ ， n) and U (r， fission) using bremsstrahlung beams generated by energetic electrons produced by laser-matter interactions. interactions. The intense ，short-pulse nature of the radiation might be useful for measuring the population of of isomeric stat 白 with life-times in a range inconvenient for accelerator- or reactor-b 間 ed experiments [4J. Other applica 七ions of the high peak power laser to nuclear physics are the Rtudy of atomic-nuclear 15 2 interactions interactions (1 勾 10 W/cm is required) ， such as nuclear excitation by electron transition (NEET) [6 ]， nuclear 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 [7J

2.2 2.2 Fr ee Electron Laser Free Free Electron Laser (FEL) uses a relativistic. electron beam as its lasi 時 medium to generate coherent coherent radiation differin 。g:仕 om the conventional lasers in which bound atomic OI molecular states are used. used. The JAERI-FEL has been developed to produce a high-power laser at infrared wavelength using the the s叩 erco 凶 ucting rf linac. So far ，a stable kW 品 vellasing h出 been achieved [8J. If the energy of the

FEL reaches the X 回目.y regime ， the characteristics of FEL; high intensity ， tunable wavelength 叩 d sharp line line width may benefit the nuclear physics research.

2.3 2.3 Laser-Compton backscat 七ering 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 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 these problems. At the storage ring TERAS in National Institute of Advanced Industrial Science & Tech- nology nology (AIST) ， the LCS photons with 1 三 Eph 壬40 Me V and photon intensity of 10 4 "-' 10 5 s-1 have been produced [9 卜It s small energy spread may be useful for the measurement of excitation functions for the the (γ ， n) reaction yields which is important for production of p-nuclei in the universe

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Table Table 2. Charac 七eristics of photons used in the present nuclear isomer research. Photon sources | Ul tra-high peak FEL LCS photon power laser

Energy regime eVrvkeV(Plasma) rvkeV 1 c"v 40MeV rv rv Me V (B remsstral 山 ng) Feature Feature intense ，shor 七pulse intense ， tunable lin. polarized ， tunable small small energy spread small energy spread Facility Facility JAERI JAERI AIST Status Status runnmg planned runnmg

3. 3. Related topics lr lr 16'" 2446 31y-isomer 31y-isomer 3. 1. Acceleration of isomeric decays 12- The 31-yr 16+ isomer in 178Hf shown in fig. 1 has re- 回 11 ceived ceived much attention due to its possible usage as the storage 10: of of energy that may be prompt-released by photons. The accel- 9- erated erated deelly of this isomer was observed with bremss 灯油lung 1141 beams from a dental x-r 可 device [11]. The endpoint energy of of bremsstrahlung beams was as low 回 60 keV ，peaked at 30 ke V. V.ke The integrated cross section from the resonant absorption tz was measured as 1x 10- 21 cm 2 keV. Recently ，similar experi-

ments were performed [12] using x-ray beams 仕om an undula- g.s.b tor tor insertion device at the Advance Photon Source in the Ar- Fig. l. A partiallevel scheme of 178Hf gonne N ational Laboratory. Although they used the incident taken 仕om Ref. [10]. beams with intensities that were over 4 orders of magnitude than those used in Re f. [11 ]， the results showed no signa l.

However ，interactions which couple atomic and nuclear states may enhance isomer decays [可. An example is a process called the nuclear excitation by electron transition (NEET) in which a nucleus is excited excited by a virtual photon generated during de-excitation of an atomic level [1 司. The NEET occurs under under the conditions of 1) closely matching of the transition energies in the atom and nucleus ， 2) sar 同 multipolarity multipolarity for the atomic and nuclear transitions. The isotopes of 1890s [14 ]，町 Au [15] and 235U [1 司 ful 五11 the above conditions. In a plasma produced by the high peak power laser ， the NEET process is expected expected to have the largest contribution to the nuclear excitation in 235U [16]. Measuring the NEET rates 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 1/2+ state in 235U) which are populated in NEET ，and therefore the delayed transitions can be detected off-line.

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3.2. 3.2. Nuclear astrophysics The odd-odd nucleus 180 m Ta is famous for two aspects that it is is it the only naturally occurring isomer and is the nature's rarest w 日日 isotope. isotope. The stellar production of 180mTa has been a challeng-

ing ing a町 ophysical problem [1 可， since the population of this iso- Ta 回哩 tope is bypassed by the slow neutron-capture (s- )process ，and fur- thermore shielded from the s-decay chains following the rapid Hf

neutron-capture neutron-capture (r- )proce 総括shown in fig. 2. One possible way

1; 0 reach 180mTa is 今'-process [18] which takes place during γburst ¥¥¥ ¥ r-process r-process in in type II supernovae. In order to explore the isomer's produc- 口unst 枇 tion tion yield along this reaction path ， cross section measurements n stable of of the 181Ta Ci， n)180mTa reactionusing LCS photons have been performed using AIST-LCS photons under the Konan University ， Fig. 2. The reaction path of the AIST and JAERI collaboration. The detailed results will appear s- and r-process in 七he region of the elsewhere elsewhere [19]. tantalum isotopes.

4. 4. Summary The nuclear iRomer research planned at ACPR in JAERI was described. The photons generated by the the ultra-high peak power laser ，FEL and LCS can be used to study nuclear physics ， nuclear astrophysics and atomic-nuclear interaction with a probe of the nuclear isomers.

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