A Plunger System for the Recoil Distance Doppler Shift

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Bull Inst Chem Res KyotoUniv Vol 60 No 2 1982, A Plunger System for the Recoil Distance Doppler Shift. Life Time Measurements of Nuclear Excited States,Toshiyasu HIGOt Yoshihisa IWASHITA. and Seishi MATSUKI,March 31 1982, A plunger systemfor the recoil distanceDoppler shiftlife time measurementswas constructed. The distancebetweena target and a stopper can be variedby drivinga micrometerwith a pulsed. motorfrom the outside of a vacuum chamberin a precisionof 1 pm All the systemis controlled. automatically, from a remotestation The systemdetail performanceand someexamplesof life time. measurementsare presented, KEY WORDS Recoil distance Doppler shift life times of high spin states.
plunger system,I INTRODUCTION, The importance of the electromagnetic transition probabilities of the nuclear excited. states to understand nuclear structures has been recognized for a long time Recently. the property of the high spin states of collective nature has been more and more inves. tigated to discriminate various theoretical models and thus the life time measurements. for the high spin states become one of the most important techniques Radiative life time. of excited nuclear states in the 10 15 10 9sec range can be measured by Doppler shift. methods i e the recoil distance plunger method RDDS and Doppler shift attenuation. method DSAM, The two methods both rely on the Doppler effect i e the shift in energy of a. rray when it is emitted from a moving source In the RDDS the nuclei excited by. a reaction recoil freely in a vacuum untill they are quickly stopped in 0 5 psec by. a movable stopper Since the energies of the 7 rays emitted by the stopped and moving. nuclei are different it is possible to know the time the nuclei takes to reach the stopper. and thus to obtain the life times in the range 10 9 10 sec The DSAM makes use. of the fact that the nuclei recoiling from a nuclear reaction are slowed down in about. O 5 psec in a solid material thus the Doppler shifted energy profile of emitted r rays. depends on the nuclear life time in the range of 10 12 40 15sec. The development of high resolution Ge Li detectors has brought extensive applica. tions of the RDDS and DSAM techniques to the measurement of nuclear life time. up to 10 sec The Chalk River group especially developed a method of RDDS. technique which is termed now the plunger technique firstly suggested by A E Lither. 1jEtA 4 VT YA pkti 15 Laboratoryof Nuclear Reaction Institutefor ChemicalResearch. KyotoUniversity, present address NationalLaboratoryfor High EnergyPhysics Tsukuba Ibaraki. T HIGO Y IWASHITA and S MATSUKI, In part of series of investigations for the nuclear structure of transitional nuclei. around A 70 90 such as even Se Kr and Sr 3 we have developed a simple and. versatile plunger system to study life times of collective nuclear excited states We. have particularly measured the life times of the high spin states of Sr up to the 8 in. the ground state band with the RDDS and DSAM method In the present plunger. system we used a micrometer in which the position can be read out by an attached. optical rotation encorder with the precision of 1 itm This system offers an especially simple. and still very accurate measuring apparatus for the RDDS method In this paper the. plunger system is described in somewhat detail and the performance of the system is. presented and discussed,II THE APPARATUS,II 1 General.
An overall view and a schematic diagram of the plunger system are shown in Figs 1. and 2 respectively, A stopper frame is mounted on sliding stage of a digimatic micrometer which can. be driven by a pulsed motor from the out side of a vacuum chamber The moved distance. Fig 1 An overall view of the plunger system,stoppet arget. Digimatichead,micrometer, Fig 2 A schematic view of the present plunger system. made by Mitsutoyo Seiki Co Ltd, A Plunger for Recoil Distance Life Time Measurements. of the micrometer can be read out by an optical rotation encorder attached to the. micrometer with an accuracy of 1 am, A target frame is mounted on a flat stage which is fixed on the base plate of the.
micrometer The absolute distance between the target and the stopper is monitored by. measuring capacity between them The vacuum chamber is made of acrylic acid resin. to reduce the absorption of emitted f rays,II 2 Target and stopper assembly. A target foil is mounted on a target frame as shown in Fig 3 The present method. for stretching the target foil is similar to the double stretching technique of Jones et al 5. The inner ring made of teflon is pushed outside to stretch the foil further with a guide. pin thus avoiding mechanical stress to the foil The whole assembly was made of teflon. except a screw for stretching and the flat stage for mounting the assembly The stopper. assembly is almost the same as the target assembly. The target and the stopper foils are adjusted to be parallel within 0 06 degrees with. an Ar laser beam Firstly the laser beam is reflected from the stopper foil without. mounting the target foil and the reflected beam spot is marked onto a screen which is. enough far away further than 5 m from the foil to get the accuracy of the parallel. adjustment Then the target assembly is mounted and the laser beam is then reflected. by the target foil The position of the target assembly is then adjusted with two. tensioned micrometer adjuster so that the reflected laser beam spot coincides with the. previous spot with the stopper foil reflection In the actual trial the spot size of the. reflected laser beam on a screen was about 3 mm in diameter and thus the parallelism. between the stopper and the target was estimated to be within 0 06 degrees. Fig 3 A target assembly for double stretching,II 3 Capacity measurement. Capacity between the target and stopper foils can be used to know the absolute. distance between them In the present plunger system the measuring circuit incor. porated into a small circuit board was put into the vacuum chamber near to the target. and stopper assembly thus reducing the effect of unwanted stray capacities The block. diagram of the circuit for the capacity measurement is shown in Fig 4 The principle. T IliGo Y IwAsHITA and S MATSUKI,At IttellrrF x piIRIVx. IIITL072axaX air TlnF,RD5AAinal1,IrefIiVref,1oOpF same asabove 0. II LS14i 2nFI 2nF, Fig 4 A schematicblockdiagramof the electronicsystemfor the capacity.
measurements, of the circuit operation is as follows Timing control pulses a b and c of 50 kHz. repitition are produced with a NE 555 Texas Instruments timing circuit Saw tooth. wave of the same repitition rate produced with a constant current source a capacitor and. an analog switch AS1 is applied to the capacitor C the capacity of which is to be measured. The resulting voltage is sampled and holded Vx and then fed to an analog to digital. converter ADC of 3 2 digit The output of the ADC is displayed with 7 segment. display tubes in the unit of pF The capacitor Cx can be replaced by a 100 pF capacitor. thus enabling a test of the accuracy of the whole measuring system. III PERFORMANCE,M 1 Absolute recoil distance, The absolute recoil distance between the target and stopper was determined by the. capacity measurements in a case of a Zn target foil and a Pb stopper The target. foil of about 3 mg cm thickness with the backing foil of natural Pb 1 mg cm2 was. made with the evaporation method The stopper thickness was about 30 mg cm The. inverse of the measured capacity 1 C is plotted as a function of the micrometer. reading with the optical rotation encorder as shown in Fig 5 From the extraporation. of the linear portion of the plot the zero point of the distance can be determined in. terms of the micrometer reading, E 2 Life time measurements of the 2 9 0g and 4 8 E 2 gE. transitions in the ground,state band of Sr, A Plunger for Recoil Distance Life Time Measurements. 8 Sr 29 09,20 30 40 50 60,Micrometer reading Jim CHANNEL.
Fig 5 The inverse of the measured capacity Fig 6 A portion of the 7 ray spectra. versus the micrometer reading forin the vicinity of the 385 7 keV. a 66Zn target and a Pb stopper system 4 0 8 transition in Sr with. various recoil distances,z 40 7 Ps8 Sr 1 0 7594,5 5 tI 5ps. 0 10020030060050060010 20 30 40 50 60 70,0RECOIL DISTANCE. Fig 7 The ratio I of unshifted versus Fig 8 The ratio I for the 594 8 keV. sum of the shiftedand unshifted41 4 transitionin 80Sr. peak yields for the 385 7keV,transitionin 80Sr, The 0 beam of 55 MeV from the INS SF cyclotron was used to bombard enriched. Zn target and 7 rays from the Zn 0 2 nr Sr reaction were measured with a 60. cm Ge Li detector at 0 with respect to the beam direction 5 The target was made. by evaporating metalic Zn on natural Pb backing and doubly stretched as described in. the preceding section, Typical 7 rays spectra in the vicinity of 385 7 keV 7 ray corresponding to the. Sr 4 4 E transition are shown with various recoil distances in Fig 6 The deduced. ratio of unshifted peak yield to the sum of unshifted and shifted peaks I U U S. is plotted as a function of the target stopper distance in Fig 7 After correcting the. feeding time from higher excited 4 2 state the resulting life time of the 385 7 keV. state was determined to be 40 7 psec This value is in good agreement with the result. T HIGO Y IWASHITA and S MATSUKI, by Nolte et al 6 The ratio I for the transition 594 8 keV is also shown in.
Fig 8 Preliminary result of the life time for this transition is estimated to be 5 1 psec. from this ratio,IV DISCUSSION, When the 160 beam was on the target in the above mentioned life time measurements. the capacity between the target and stopper slightly increased in short distances This. may be due to thermal expansion of the target and or stopper No critical examination. of this problem however has yet been tried, The main cause of the finite limit of the close distance between the target and. stopper is due to local contacts which may arise from local irregularity of the surface of. the foils or from the attaohed dust It is thus important to refine further the method. of stretching the foil in order to get more close distance easily. ACKNOWLEDGEMENTS, The authors would like to thank Prof T Yanabu for his interests and continuous. encouragement throughout the work,REFERENCES, 1 T K Alexander and J S Forster in Advances in Nuclear Physics 10 197 1978 and refe. rences cited therein, 2 T K Alexander and K W Allen Can J Phys 43 1563 1965.
3 S Matsuki et al Nucl Phys A370 1 1981 and references cited therein T Higo S Matsuki. and T Yanabu to be published, 4 T Higo T Shiba T Ohsawa S Matsuki K Ogino and T Tanabe to be published. 5 K W Jones A Z Schwerzschild E K Warburton and D B Fossan Phys Rev 178 1773. A plunger system for the recoil distance Doppler shift life time measurements was constructed The distance between a target and a stopper can be varied by driving a micrometer with a pulsed motor from the outside of a vacuum chamber in a precision of 1 pm

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