A Ge (Li)-NaI(Tl) coincidence system with multichannel analysis

NUCLEAR INSTRUMENTS AND METHODS 43

(I966) 379-380; ©

NORTH-HOLLAND PUBLISHING CO.

A Ge(Li)-NaI(TI) C O I N C I D E N C E S Y S T E M W I T H M U L T I C H g N N E L ANALYSIS A. V. RAMAYYA, B. VAN NOOIJEN*, H. K. CARTER and J. H. H A M I L T O N

Vanderbilt University +, Nashville, Tennessee Received 26 April 1966

The establishment of coincidence relations plays an essential role in the investigation of nuclear decay schemes. The most commonly used system in gammag a m m a coincidence work is the combination of two NaI(Tl) spectrometers. The disadvantage of this type of spectrometer is its poor line resolution and one is therefore tempted to use high-resolution Ge(Li) spectrometers instead. The present letter describes a Ge(Li)NaI(Ti) coincidence arrangement which works satisfactorily and can be easily assembled from instruments which are commercially available. Fig. 1 shows the block diagram of the coincidence system. The Ge(Li) detector has a depletion depth of 5 mm, an area of 4 cm 2 and was manufactured by Ortec; the NaI(TI) detector is a 2 " x 2 " cylindrical crystal. The models of the rest of the equipment are indicated in fig. 1. The resolutions of the spectrometers were 6 keV for the Ge(Li) spectrometer and 9% (662 keV line in 'aTCs) for the NaI(T1) one. The pulse height analyser of the Ge(Li) spectrometer is set to accept all pulse heights above the noise level; the other single pulse height analyser is set to accept * On leave from the Technological University, Delft, The Netherlands. + Supported in part by a grant from the National Science Foundation. GE(LI)

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that part of the NaI(Tl) spectrum with which one wants to coincide. Both single channel analysers have the property that they make up for time differences introduced by pulses of different heights from one particular detector. After amplification, the pulses from the Ge(Li) crystal are fed to the input of the multichannel analyzer via a delay line (delay 4/as). The latter makes up for the time difference between the pulses from the TC 200 amplifier and those gating the multichannel analyzer. The stability of the entire system is very satisfactory; the largest shift observed in either spectrometer over a period of 24 h was 0.25%. An important question, especially if one wants to derive quantitative results from the coincidence spectra, is that of the behaviour of the coincidence efficiency as a function of energy. This point was investigated by measuring the g a m m a spectrum of 6°Co in coincidence with the 1.33 MeV gamma-ray for different values of the resolving time of the coincidence unit. It was assumed that the coincidence efficiency in the measurements at the largest resolving time (1.2 ps) was independent of the energy. These experiments indicated that at a resolving time of z = 8 x 10 - s sec, the efficiency was constant within seven per cent over the energy range from 150 to 1000 keV. The performance of the coincidence system is demonstrated in fig. 2 which shows some coincidence results obtained in our investigation 1) of the decay of ~54Eu. Curve a represents the single g a m m a spectrum, b the spectrum in coincidence with the 123 keV gamma-ray and c that in coincidence with the 248 keV g a m m a transition. The improved resolution allows new coincidence relations to be established like those between the 1492 and 123 keV lines, the 1592 and 123 keV gamma-rays, the 1246 and 248 keV transitions and the 893 and 248 keV lines. The power of the technique is seen in noting that the 1246 keV transition, which has an intensity of only 2% of that of the 1274 gamma-ray in the single spectrum, is in strong coincidence with the 248 keV line as compared with the chance coincidence 1274 keV line. Also other new transitions not seen in the single spectrum such as the 893 and 1187 keV transitions are observed in the coincidence spectra.

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Fig. 2. Some studies of the 154Eu decay. Curve a is the single gamma spectrum, b is the spectrum coincident with the 123 keV gamma ray (measuring time 24 h) and c is the spectrum in coincidence with the 248 keV transition (measuring time 24 h). T h e m e a s u r i n g t i m e s i n v o l v e d in this c o i n c i d e n c e

w o r k are n o t t o o long. F o r instance, the c o i n c i d e n c e spectra of fig. 2 were taken in 24 h.

Reference 1) A. V. Ramayya, B. van Nooijen, H. K. Carter, J. H. Hamilton, L. L. Riedinger and N. R. Johnson, to be published.