Decay of the First Excited State at 13.3 kev in 73Ge

Transcription

1 Z. Physik 243, (1971) 9 by Springer-Verlag 1971 Decay of the First Excited State at 13.3 kev in 73Ge R. S. RAGnAVAN Physik-Department der Technischen Universit~it Miinchen Received January 28, 1971 Singles and coincidence measurements of the 13.3 kev transition of 73Ge are reported. The total and K conversion coefficients of this gamma-ray and the half-life of this level have been determined. The results are: C~T= t ; ~K= 312-t-30; T1/2= 2.98 _ gs. 1. Introduction The gamma decay of the first excited state of 73Ge at 13.3 kev is of considerable interest due to its potential application to high-resolution MSssbauer spectroscopy. The availability of Si(Li) detectors with energy resolution of the order of a few hundred ev has enabled the observation of this gamma-ray in spite of its high conversion probability and its close proximity to the Ge K X-rays. In the last year Douglas 1 has made coincidence measurements to determine the half-life of this state and the K conversion coefficient c~ K of this transition. The measured value of c~ r agrees well with the theoretical prediction of Hager and Seltzer z thus establishing the multipolarity of this gamma-ray to be E2. Recently, Riegel and Vester a have reported a value for the total conversion coefficient c~ r based on a singles measurement which indicates a discrepancy of about 2070 from the theoretical value. Due to the current interest in the properties of this gamma-ray we briefly report here the results of our singles and coincidence measurements which were performed in These results are in full agreement with the theoretical prediction for c~ r and c~ K and with Douglas 1 as regards the half-life of the level. 2. Experimental Details The source of 73As used in the experiments was prepared from a batch of 20 gci activity supplied by the Radiochemical Centre, Amersham, England, in the form of sodium arsenate in aqueous solution with the following specifications: Specific activity > 1 Ci/g of As; Concentration 50 gci/ml; 74As content of 73As content; Chemical purity 1 Douglas, D. G.: Can. J. Phys. 47, 1815 (1969). 2 Hager, R. S., Seltzer, E. C.: Nucl. Data A4, 1 (1968). 3 Riegel, D., Vester, K. P. : Z. Physik 241, 188 (1971).

2 442 R.S. Raghavan: A1, Fe and Ge, all < I0 gg/ml. A small drop of the source was deposited and dried on a mylar foil and used in the measurements. The Ge content in the source used was certainly less than 1 gg. Measurements were made with a Si(Li) detector, 3 mm thick and with a rated resolution of 320 ev for 6.4 kev. Coincidence measurements were carried out in conjunction with a 1 mm thick NaI (T1) crystal placed as close to the Si(Li)detector as possible with the source sandwiched in between. The 53.4 kev gamma-ray feeding the 13.3 kev level was detected in the scintillation crystal and the coincidence spectrum as seen by the solid-state detector was recorded in a 1024 channel analyser. To measure simultaneously the random coincidence events, a time to amplitude converter (TAC) was used in the coincidence part of the electronics. The output of the TAC was fed to two single-channel analysers (SCA), one gating the delayed part of the time spectrum containing only chance events while the other was gated earlier in the time spectrum containing a large fraction of true coincidences. The outputs of these two SCA's were used to gate the amplifier output of the Si (Li) detector in different parts of the analyser memory. As a precautionary measure the time spectrum itself and the parts of it gated by the two SCA's were recorded in a second analyser. The coincidence resolving times which are effectively determined by the window-widths of the two SCA's were thus continually monitored. The true coincidence spectrum was obtained simply by subtracting the recorded chance events from the total coincidence spectrum. The lifetime measurement was performed with the same arrangement in a separate experiment by recording the time spectrum from the TAC which was started by the 53.4 kev photons detected by the scintillation crystal and stopped by the Si(Li) pulses gated on the X-rays and the 13.3 kev photons. Pulses from the SCA's was used to trigger the TAC and no effort was made to obtain the best possible time resolution. The TAC range was calibrated by using a digital delay pulse generator. 3. Results a) Total Conversion Coefficient at. Fig. 1 shows the singles spectrum of V3As together with the gamma spectrum of 57Co. The 6.4 kev K~ line of Fe and the kev line of 57Fe were used for calibration. The energies of the K X-rays of Ge are correctly identified to be 9.88 and 11.0 kev while the gamma-ray of 7aGe has an energy of 13.29_+0.4 kev. The small peak to the left of the kev line of the 57Co spectrum is probably the Si X-ray escape peak. The total conversion coefficient a T of the 13.3 kev transition may be calculated from the ratio of the background subtracted line areas of the

3 9...--~ Decay of the First Excited State at 13.3 kev in 73Ge As73 76 d 0,53s 1 / 2 _ ~ % EC 534 3ps 5/2+~ '3 9/2 + ~-~-~ "~ (stable) c lo z" [ "L'f" L.'-,,: 9."~. 7%;,/ '../. i: 14, ~ L : S[ Escape?: :,.i?!i "X,-..:<, _ I I I Fig. 1. Singles spectrum of 73As source taken with a Si(Li) detector. A comparison spectrum of a source of SVCo containing 6.4 kev K X-ray of Fe and the kev gamma-ray of SVFe is also shown gamma-ray to the X-rays of Ge since the decay scheme of 73As (see Fig. 1) is well known. Such a procedure has been adopted by Riegel and Vester 3. The K X-ray to gamma ratio is thus given by ~-~NK _ [0.89 -F {(L~)C~rl } ~K The first term in the square brackets represents the K-capture probability of the EC transition. Using the same values for ek1, (K/L+M)I of the 53.4 kev transition, (K/L+ M)2 of the 13.3 kev transition and the 30 Z. Physik, Bd. 243

4 444 R. S. Raghavan: Table. Line areas after background subtraction X-rays 13.3 kev?,-rays Singles x 104 Coincidences '1 9?'- Ge K - Xrays to E ~ ;.." 9 "..,. -..:. '... _ (' Fig. 2. Coincidence spectrum of 73As gated by the 53.4 kev photons fluorescence yield f= as Riegel and Vester 3, we obtain at = t- 55 from our measured value of (N~/N~) (see Table for relevant data). This value is in excellent agreement with the theoretical value of at= 1080 of Hager and Seltzer z for an E2 transition of this energy. The higher value of 1 310_+60 obtained by Riegel and Vester 3 is probably due to the presence of Ge in their unseparated Ge target. b) K Conversion Coefficient ak. Fig. 2 shows the gamma-spectrum in coincidence with the 53.4 kev gamma-ray after subtraction of chance

5 Decay of the First Excited State at 13.3 kev in 73Ge c.,v~-v.:~ / I I I I I I! I I! I I I I I I I I f I I I I I Fig. 3. Delayed coincidence spectrum of the ( K X-ray)cascade i i 640 coincidences. The K conversion coefficient c~ r is then simply given by the ratio of the line areas: e/(= (CK/C~ f) since the detection efficiencies are very nearly equal for the two energies. Using the data of the Table we get e~= 312_+ 30. This agrees very well with the result of Douglas 1 and also with the theoretical value of ~K=313. The measured values of a T and ek of the 13.3 kev gamma-ray are thus very consistent with each other and establish unambiguously the E2 character of this gammaray. c) Half-life of the 13.3 kev Level. Fig. 3 shows the delayed coincidence spectrum of the kev gamma-cascade. A least squares fit of the measured data, taking into account the effect of the prompt admixture in the time spectrum gave a result for the half-life T1/2 = 2.98_+0.05 gs, confirming the results of Douglas ~ and Lindstrom 4. I wish to thank Dr. H. J. K6rner and Prof. P. Kienle for their interest, This investigation was supported by the Bundesministerium ffir Bildung und Wissenschaft. 4 Lindstrom, I. E.: Diss. Abstr. 19, 2374 (1959). Dr. R. S. Raghavan Physik-Department (E 15) der Technischen Universit/it Miinchen BRD-8000 Mflnchen 2, Arcisstr. 21 Germany 30*