Experimental techniques for the investigation of isomeric transitions in the milli- and microsecond region

NUCLEAR

INSTRUMENTS AND

METHODS 91

307-3II; ©

(1971)

NORTH-HOLLAND PUBLISHING CO.

EXPERIMENTAL TECHNIQUES FOR THE INVESTIGATION OF ISOMERIC TRANSITIONS IN THE MILLI- AND MICROSECOND REGION J. UYTTENHOVE • and J. DEMUYNCK Rljksun;versileit, Nattlllrkll/ldig Laboratorillm, I.N. W., Proeftl/i/lstraat 40, B-9000 Ghent, Belgium

Received 15 July 1970 and in revised form 15 September 1970

A measuring system for investigations on isomers in the ms and lIS region is described. Energy and decay-time of the isomeric state are determined by measurements ofy-ray spectra in successive time intervals with a Ge(Li) detector. Provision is made for compensation of the y-flash and for beam-inspection.

1. Introduction The aim of this system for measurements on isomeric transitions in the ms and jJ.S region, produced by

photonuclear reactions between the bursts of a pulsed accelerator, is to obtain simultaneously information about energy and decay-time of the excited levels. This is done by registration of the y-ray spectra in a number of successive time intervals after each beam burst. The use of a large Ge(Li) detector for good

• Research worker of the "Interuniversitair Instituut voor Kernwetenschappen ", Brussels, Belgium.

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ISOMERIC TRANSITIONS IN THE MILL!- AND MICROSECOND REGION

resolution and efficiency involves some difficulties, especially in the case of isomers with very short halflives. A system for investigations down to 10 fl.S isomers is described. 2. Experimental set-up The main difficulty in the measurement of shortliving isomers is the scattering of the y-beam on the target under investigation. The preamplifier is saturated during a time, proportional to the charge produced in the detector by the scattered y-flash. To prevent this dead time of the whole amplifying chain, some special techniques are necessary (see fig. 1). 2.1. THE COMPENSATING SYSTEM Jn order to red uce this dead time, a compensating charge is injected at the input of the preamplifier, via a small capacitor. A plastic scintillator-photomultiplier combination receives also a scattered y-flash, and the shaped anode signal provides the compensating charge, proportional to the intensity of the y-beam. The degree of compensation is adjustable by regulation of the HT supply, and/or the distance between the target and the plastic detector. The photomultiplier (56 AVP/03) is set in cut-off, except in a region of 20 fl.S containing the ')I-flash, by controlling the tension of the first dynode. The anode signal, with a decay of 1-2 ms is fed via a follower to the test input of the preamplifier (the test capacitor is increased to 10 pF). When the system is carefully adjusted, it is possible to prevent saturation of the preamplifier and to preserve the overall resolution, even by increasing the beam intensity by a factor 3 or more.

309

2.2. THE AMPLIFYING CHAIN It is quite conventional, except for gating and pulse shaping, in order to reduce the overload of the main amplifier caused by the y-flash. A linear gate, suitable for small signals, is placed between preamplifier and main amplifier. A first differentiation (1-2 fl.s) in front of the linear gate reduces the amplitude of the preamplifier signal at the end of the gating interval (duration 30 I1S, beam pulse of 2 Jis occurs at 5 JIS after the start of the gating interval). A second differentiation and an integration (1-2 JIS) in the main amplifier ensure short overload recovery time. Starting from a Ge(Li) detector with a resolution of 3.8 keY, the system resolution is 5 keY at 1.33 MeV. 2.3.

THE INSPECTION SYSTEM

A NaI detector, mounted in the y-beam, delivers a signal proportional to the beam intensity. By means of a differential discriminator, a usable intensity range is selected. The discriminator output triggers the blocking control and starts the measuring cycle. This inspection system protects the measurements against beam intensity variations and beam fall-ollt. 2.4. THE ROUTING SYSTEM By means of a routing unit, described in ref. 1, a sequence of sixteen sllccessive spectra are registered in different memory locations. 3. Performances

This experimental device enables to easily observe isomeric decays with different half-lives, produced in the same target material. Fig. 2 shows two decays with

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Fig. 3. 30 and 25 channels (out of 256) of each of the successive spectra showing the decay of the 220.0 keY (2) and 306.6 keY (3), 22.5 JlS gamma-rays in lOlll1Ru, Rnd a" Ge-Background" line (1).

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ISOMERIC TRANSITIONS IN THE MILLI- AND MICROSECOND REGION

different half-lives, produced by (y,n) and (y,p) reactions on natural ruthenium2). Using time intervals of9.80 ±O.OS J.ls, fig. 3 shows the decay of the 526.6 keY level in lOlmRu. The half-Jives of 22.8±0.7 J.lS of the 220.0 keY line and 22.2±0.7 J.lS of the 306.6 keY line are calculated by least squares, leading to an average value of 22.5 ± 0.4 flS 2). By measurement of successive y-ray spectra, one should take into account the loss of counts due to the dead time of the analog to digital convertor. Correction for this can be done by normalization to a random pulser peak or to y-lines from a calibration source as shown in fig. 4. Finally, the benefits of this method are good

311

energy resolution by using a Ge(Li) detector and accurate half-live determination by observing the decay of each y-ray in detail. The authors would like to thank Prof. J. L. Yerhaeghe for continued interest in this work. This work is part of the research program of the "Interuniversitair Instituut voor Kernwetenschappen". References 1) J. Demuynck and J. Uyttenhove, NucI. Instr. and Meth. 74

(1969) 97.

2) J. Uyttenhove et al., Z. Physik 238 (1970) 90.