Level and isomer systematics in even tin isotopes from 108Sn to 118Sn observed in Cd(α, xn) reactions

  • Published on
    21-Jun-2016

  • View
    214

  • Download
    0

Embed Size (px)

Transcript

<ul><li><p>1.E.I: Nuclear Phystcs A134 (1969) 81 --109; (~) North-Holland Pt~bhshtng Co, Amsterdam </p><p>I.E.4: 3,A Not to be reproduced b) photoprmt or mtcrofilm ",~thout v*rttten permission from the pubhsher </p><p>LEVEL AND ISOMER SYSTEMATICS IN EVEN TIN ISOTOPES FROM 18Sn TO 118Sn OBSERVED IN Cd(~, xn) REACTIONS </p><p>T. YAMAZAK1 -t and G. T. EWAN *t Lawrence Radtatmn Laboratory, Unirerstty of Califorma, Berl~eley, CahJornia </p><p>Recewed 13 March 1969 </p><p>Abstract: The gamma-ray spectra observed ~ hen separated cadmium ~sotopes (A = 106-116) are bombarded by 28-50 MeV ~-pamcles from the Berkeley 2.2 m c~clotron have been stu&amp;ed with Ge(L0 detectors Levels m even tm isotopes produced by (~, xn) reactions are reported The p~ompt and delayed spectra have been stu&amp;ed using nsec time analysis and the natural beam bunching of the cyclotron to gwe a zero of t~me. Isomers have bcen observed in ~SSn ~ ltb t_ I_ = 20 nsec, 230 nsec and &gt; 500 nsec, m ~ ~6Sn with t~ 350 nsec and &gt; 500 nsec and m J J4Sn ~th t &gt; 500 nsec and m 11ZSn with t~ = 14 nsec. The level and ~somer systematlcs m the e~en tm ~sotopes are &amp;scusscd Levels m odd tm ISOtOpes for A -- 113 to A ~ 119 are also discussed </p><p>NUCLEAR REACTIONS 106. 108. 110. 112. 114, I 16Cd(ct, xn?,~, 160(~, p), (~, n), (~, pn), E : 20-50 MeV; measured Er, c~T-dela2r 107. Joy, l,.lSn deduced E~, </p><p>~08, 1~0. ~x2. ~3. ~4. t~_~, ~16. ~7, xJSSn deduced levels, I, ~, T~. xs, 1OF, lONe level deduced T) Enriched targets </p><p>1. Introduction </p><p>I somer ic states popu la ted in nuc lear react ions ha~e been extensively studied in </p><p>recent years. Usua l ly such exper iments have been per fo rmed us ing pu lsed beams f rom </p><p>an accelerator , and the range of l i fet imes covered depended on the character is t ics o f </p><p>the acce lerator be ing used. D iamond, S tephens and co -workers have d iscovered many </p><p>new isomers wi th half- l ives 1 msec us ing a heavy- ion l inear acce lerator 1.z) </p><p>Brand i et al. 3) have stud ied i somers w i th hal l - l ives ~" 5 Ftsec produced by (7, n ) react ions us ing a betat ron . I somers w i th half - l ives in the range 5-1000 ~tsec have been </p><p>stud ied us ing pulsed beams of p ro tons , deuterons and ~-pamcles f rom cyc lot rons 4 - 7). </p><p>I somers in the Itsec range have also been studied us ing a pu lsed beam f rom a tandem </p><p>acce lerator 8) In all these measurements , the ha l f -hves have been in the / tsec range </p><p>or longer. It wou ld be an obv ious improvement if the range cou ld be extended to </p><p>shor t half - l ives enab l ing h igher -energy t rans i t ions and t rans i t ions o f lower mul t ipo le </p><p>o rder to be studied. </p><p>In recent letters 9.10), we repor ted a s imple method for the systemat ic observat ion </p><p>of i somer ic states wi th l i fet imes longer than a few nsec popu la ted in (part ic le, xn) </p><p>react ions The method depends on the natura l phase group ing of beam pulses f rom </p><p>t Now at Department of Physics, Umverslty of Tokyo, Bunkyo-ku, Tokyo, Japan. *t Permanent address: Chalk River Nuclear Laboratory, Chalk Rwer, Ontario, Canada </p><p>81 </p></li><li><p>82 T. YAMAZAKI AND G. T. EWAN </p><p>a cyclotron and the fast timing properties of Ge(Li) detectors. We have used this method to study isomeric states in a number of Sn and Po isotopes 9, l1-33). In the present paper, we report a survey study of the level and isomer systematics of even Sn isotopes revealed in Cd(c~, xn) reactions with separated cadmium isotopes. </p><p>Previously energy levels in 116Sn and 1 a asn populated following radioactive decay have been studied by several workers 14-a 7), and the levels in 1 a 2Sn ' 1 ~4Sn and 1 a 6Sn populated in (3He, 3n) reactions have been studied by Betigeri and Morinaga as). A detailed study of the 1 a6Sn isomeric state has been performed by Chang, Hageman and Yamazaki ~9). A brief description of our results on Sn isotopes from 108 to 118 has appeared earlier 13). In this paper, the experimental methods, the analysis of experimental data and the comparison with theoretical predictions are described in more detail. </p><p>2. Experimental method </p><p>2.1. ELECTRONIC ARRANGEMENT </p><p>In our experiments, the natural beam bunching in a cyclotron is used to give a zero of time, and the time distribution between the pulses of ),-rays from a target is studied with Ge(Li) detectors. Although this method has not previously been used for ),-ray studies, a similar technique has been used in neutron time-of-flight measure- ments where the neutron distribution is studied between beam pulses. Such measure- ments have been reviewed by Bloom 2o). </p><p>In the present experiments, the external beam from the Berkeley 2.2 m sector- focussed cyclotron has been used. The beam bunches from the cyclotron are typically 4 nsec wide, and the repetition interval is equal to the inverse of the r.f. frequency. This is in the range 100 to 170 nsec depending on the particle being accelerated and on the energy. In most of our experiments, 28-50 MeV s-particles were used to </p><p>bombard the targets. The ~-rays were studied with a Ge(Li) ),-ray detector. The fast timing properties </p><p>of Ge(Li) detectors have been discussed by several workers 2a, 22,23). TO obtain fast timing, it is essential to use leading-edge rather than cross-over timing. This is because the pulses from Ge(L 0 detectors do not have a uniform rise time. The time resolu- tion obtainable depends on the energy of the ),-ray being studied and on the size of the Ge(Li) detector. Wath 3 mm thick planar detectors, FWHM of 2.7 nsec has been obtained for 511 keV 7-rays 24). The half-slope on the edge of the curve can be as low as 1 nsec, although at ~ ~ peak height, there is usually a tail with a greater slope. </p><p>A simphfied block diagram of the present electronic arrangement is shown in fig. 1. The ?-ray signal fi'om the Ge(Li) detector after amplification by a fast pream- plifier was split into one pulse for energy analysis and another for time analysis. After further amplification by a I nsec fast amplifier 25), the latter was used to trigger a fast discriminator to give a timing signal The threshold of the dascriminator was set slightly above the noise level. V, qth the planar detectors used in our experiments, this </p></li><li><p>Cd(~, xn) REACTIONS 83 </p><p>trigger level corresponded to about 10 keV. For the large volume coaxial detectors, it corresponded to about 20 keV. The fast signal was used to generate the start signal for a time-to-height converter (EG &amp;G Model TH200A). The stop signal was generated from the cyclotron r.f. oscillator using a fast discriminator and a suitable delay. For convenience, only every second pulse from the discriminator was used to generate a stop signal. This gives r~se to two prompt peaks with a time separation of the spacing of the beam bunches as the observed 7-rays could be associated with either beam bunch. The predicted time distribution is shown schematically in the reset to fig. 1. </p><p>' 2 -~ ' ~ i ' d .... . . . . . . . . . ' l </p><p>,' t- q~: I ' I . . . . "o'o l ' ' A o,o,00 A i </p><p>Time pulse he~cjht </p><p>(300 nsec F.S) </p><p>F~g. l. Slmphfied block dmgram of the electromc apparatus used m the present experiment. A schematic time distribution ~s also sho~n </p><p>The output of the time-to-height converter was fed to a mixer amplifier, where a correction was applied to compensate approximately for the time "walk" with pulse-height associated with leading-edge timing. This correction was not critical as we were studying time distributions of individual y-ray peaks, and the "walk" only caused a shift in the position of "zero-time" for different energies. The energy signal from the Ge(Li) detector system and the time-signal u.ere analysed in a two- dimensional analyser usually with 256 energy channels and 16 time channels. </p><p>2 2. EXPERIMENTAL ARRANGEMENT </p><p>A schematic diagram of the experimental arrangement used with the cyclotron is shown in fig. 2 The analysed beam struck a thin target in a simple target chamber, and the transmitted beam was stopped in a well-shielded beam catcher. The ?,-ray </p></li><li><p>84 T. YAMAZAKI AND O. T. EWAN </p><p>L[ </p><p>$~TTFR~NG </p><p>DEF'NING ~mmm / Y: t , " </p><p>1 </p><p>i </p><p> t . - ~ </p><p>1 LEAD SH,EL D </p><p>. . . . . </p><p>,02 ...... (" Irl ' ' '~ </p><p>TARS[ - _ </p><p>COUNTER </p><p>SCATT-RING </p><p>ALU~ NUM w~co,~ h 2m m ih,ck </p><p>\L'~O SHI E~p_ </p><p>DETAIL </p><p>BEAM TO HI GHINTENSrTy C~VE </p><p>ANAL~ SING SLITS_ i I 2mm~ 8mp </p><p>BEAM BENDING M%GNBT ~E AM H~r'ZQ'T%I ~'IDTI~ LI%~ITING COLLIMATOR </p><p>i 6m~ BEAM VERTICAL HE'GHT LIMITING COLLIMATOR </p><p>,6ram ' QLISP~ r':tE __ LLh5 I , DCUBLET </p><p>STF~I_ S~IELnING </p><p>. .''/\ Po.~ ",CE ' 1 </p><p>. BEA,~ ~LALY:'S MAGN_ET / </p><p>C~CLOTR^N MArNET t rG </p><p>me'e o I 2 3 ~ 3 E b~ ~ J </p><p>so , l , </p><p>F*g. 2. Schematic diagram of the experimental arrangement used at the Berkeley 2.2 m cyclotron. </p></li><li><p>Cd(~, xn) REACTIONS 85 </p><p>spectrum was studied with a Ge(Li) detector surrounded by a thick lead collimator to reduce the background from y-rays from sources other than the target. In the experiments described in this paper, either a 7 cm 2 z 10 mm thick planar detector or a 30 cc coaxial detector was used. The detector was usually placed at a distance of 12 cm from the target and at an angle of 126" to the beam direction in order to obtain 7-ray intensities free from angular &amp;strlbutlon effects described by the P2 (cos 0) term. </p><p>The Cd targets used were prepared by depositing enriched isotopes of metallic Cd on 800 izg/cm 2 thick mylar foil. The targets were typically 10-20 mg/cm 2 thick. With these thicknesses bombarded with a beam of a few nA, the background counting rate from sources other than the target was less than a few per cent of the total cotmt- </p><p>mg rate from the target. </p><p>2.3. EXPERIMENTAL PROCEDURE </p><p>The y-ray spectra were initially surveyed either with 2048 channels for energy analysis and two time channels (prompt and ~ 50 nsec delayed) or with 1024 chan- nels for energy analysis and four time channels (prompt, 25, 50 and 75 nsec delayed). These runs immediately revealed the existence of isomers although the half-li~es were not measured. The lower limit of observable half-lives in these runs was 7 nsec. </p><p>When delayed y-rays were observed in these survey scans, a more detailed time analysis was performed. The spectra were recorded with 256 channels for energy analysis and 16 channels for time analysis. The lower limit of observable half-lives in these runs was 5 nsec. The time spacing of the cyclotron beam bunches (~ 150 nsec) places an upper limit of 300 nsec on the measurable half-life. Longer lived isomers are easily identified but their half-hves cannot be readily determined. </p><p>The total counting rate in these runs was kept to about 3000 counts per sec and the usual counting time was approximately 30 min for each run. </p><p>2 4 BACKGROUND PEAKS IN THE 7-RAY SPECTRA </p><p>In the y-ray spectra, there are some peaks which are due to the mylar backing of the targets, neutrons produced in the reaction and activity produced m the target Some of these peaks are present both in the prompt and delayed spectra. Some of these are identified in e.g. fig 4. The relative intensities depend on the bombarding energy. </p><p>When mylar backing is used, peaks are observed both m the prompt and delayed spectra. Mylar contains hydrogen, carbon and oxygen. The peaks in the delayed spectra are due to reactions with 160. These arise from the 160(~, p)~F, a60(c~, n)t9Ne and 160(c~, pn) 18F reactions. The transitions which appear in thc delayed spectra have energies of 184, 197, 238 and 939 keV. The locations of these transitions in the level schemes are shown on the right of fig. 3. The time spectra for the 197 and 238 keV y-rays are shown on the left of fig. 3 The half-hves measured in our experiment are consistent with previous determinations 26-2o). The measured half-hfe of the 184 and 939 keV transition in ~8F was ~ 130 nsec. </p></li><li><p>86 T. YAMAZAKI AND G. T. EWAN </p><p>In all prompt spectra, a peak at 511 keV is observed. This is due to annihilation quanta emitted when high-energ~ 7-rays are absorbed by surrounding material by pair production. A peak at this energy is also usually present in the delayed spectra. This is due to the production of positon-emitting radioactive isotopes. For some heavy metallic targets such as Au and Pb used without backing material, the 511 keV y-rays have no delayed component. </p><p>I0 </p><p>I0 </p><p>2 </p><p>o ~10 </p><p>"- - -T ime (nsec} I00 50 0 </p><p>' ' ' I ' ' ' ' ] ' ' ' ' I r </p><p>160 (,.,, p ) 19 F </p><p>1 9 7 ~ . </p><p>from 7 h igh energy ?" </p><p>from ,8 +,~ l </p><p>, _~- ! ,_ ~ ~ I_ , , _k_ 8 12 16 </p><p>Channel number </p><p>160(,-, pn)18F </p><p> 150 nsec 5* % 1123 </p><p>3+- -~ 939 939 E2 </p><p>I+- -~- 0 18 f </p><p>160(a, p) 19F" 160(a,n) 19Ne </p><p>88 nsec 52+ ~ 238 </p><p>I/2+- ~E20 1/2 +-~E2 0 </p><p>Fig. 3. Ttme distributions of low-energy delayed y-rays resulting from oxygen m mylar backing of targets. The relevant level schemes are shown on the right </p><p>The broad peak around 693 keV is characteristic of all v-ray spectra in nuclear reactions in which neutrons are produced. It is due to the (n, n') excitation of the 691 keV 0 + excited state 3o) of 72Ge in the Ge(Li) crystal. This state de-excites to the ground state and produces internal conversion electrons which are fully absorbed in the crystal. The shape of this peak is not Gaussian but skew to the high-energy side due to a contribution from the recoil of the nucleus struck by high-energy neu- trons As the half-life 31) of the 690 keV state of 72Ge is 422 nsec, this peak is also seen in the delayed spectrum. </p><p>3. Experimental results </p><p>The targets and energies of bombarding particles used in our experiments are given in table 1. For these energies, the predominant reactions are (c~, xn). The Q-values </p></li><li><p>Cd(a, xn) REACTIONS </p><p>TABLE 1 Targets and beams used </p><p>87 </p><p>Target Beam Energy Predominant Residual (MeV) reactton nucleus </p><p>1 ~6Cd e 28 (e, 2n) 118Sn 40 (e, 3n) llTSn </p><p>114Cd ~ 28 (~, 2n) 116Sn 40 (~, 3n) 11SSn </p><p>1 t 2Cd o~ 28 (~, 2n) ll,Sn ct 40 (~, 3n) x tZSn </p><p>'Cd ct 28 (ct, 2n) 1'2Sn 40 (~, 3n) 11 lSn </p><p>laCd ~ 30 (~, 2n) xlSn 40 (~, 2n) + (~, 3n) x XOSn_i_ l OOSn 50 (~, 3n) lgSn </p><p>~6Cd ~. 30 (oq 2n) tSSn cx 40 (~, 2n) + (~, 3n) ,OaSn + t O~Sn o~ 50 (0q 3n) 17Sn </p><p>HSln p 12, 14, 16 (p, 2n) HaSn 'laln p 12, 14, 16 (p, 2n) 112Sn </p><p>TABLE 2 Q-values (MeV) for Cd(g, xn) reactions </p><p>Target (g, n) (g, 2n) (g, 3n) (g, 4n) (g, 5n) (g, 6n) </p><p>ll6Cd 4.3 10.8 20.1 27.0 36.6 44.1 '14Cd 5.3 12.2 21.8 29 3 39.6 47 4 l'zCd 6.2 13.7 24.0 31 8 42.9 51.8 llCd 7.7 15.4 26.5 35.4 46 6 55 9 l8Cd 9.3 18.2 29 4 38 7 50.3 60.2 l6Cd 11.1 20.4 32.1 41.9 54.1 64 5 </p><p>for such reactions on the Cd isotopes used were estimated from the Myers-Swlatecki mass table 32) and are given in table 2. For the heavier Cd isotopes, the a-particle </p><p>energies of 28, 40 and 50 MeV give nearly the maximum yields of the (a, 2n), (ct, 3n) </p><p>and (a, 4n) reactions, respectively. For the hghter isotopes, the Q-value is greater; therefore the opt imum energy should be increased correspondingly. However, for experimental convenience, we used energies of 30, 40 and 50 MeV which were below </p><p>the maximum of the yield curves. It was still easy to identi...</p></li></ul>