Evaluation the relative emission probabilities for 56 Co and 66 Ga Yu Weixiang Lu Hanlin Huang Xiaolong China Nuclear Data Center China Institute of Atomic Energy, P.O. Box 275(41), Beijing , China

Measurement:  A 136cm3 coaxial Ge(Li) detector connected to an ORTEC-919 data acquisition system operating on a PC is used in present work. Spectra were recorded by the front face of the detector 17.5cm far from the source

Full-energy peak efficiency  In the energy range from 100 to 2754keV, the primary standard radioactive sources with well-known activities were used to calibrate the absolute efficiencies of Ge(Li) detector. For example 24Na, 60Co, 54Mn, 65Zn, 137Cs and 133Ba were used to calibrate the efficiency curve in this region. At the same time, a mixed 125Sb+154Eu+155Eu multi-energy source produced by National Institute of Standards & Technology (NIST) with uncertainties in 0.6%-1.3% was used.  The I  values used in the calibration were taken from the Table of Isotopes, Eighth Edition. The uncertainties (one standard deviation) of these sources are about %. The uncertainties of full-energy peak efficiency curve are about 1-2% in this region.

Full-energy peak efficiency  Above 2754keV, there are no radioactive sources suitable for efficiency calibration. Therefore, nuclear reactions are commonly used. In present work, the efficiency curve is obtained by the calculated results using the EGS4 M-C code above 2754MeV, and calibrated at 6.13MeV using 19F(p,  )16O reaction at resonance energy point Ep=340keV.

Full-energy peak efficiency 

Measurement:  By using the efficiency curve measured above ， the new relative intensities were determined for the emitted  -rays of 56Co and 66Ga. The final results are presented in Table 2 and 3, respectively.  It’s noted that our measurements are about 2% lower than other new measurements in high energy range.

Measured relative  -ray intensities for 56 Co

Measured relative  -ray intensities for 66 Ga

Standards  -ray emission probabilities for 14N(n,  )15N reaction  The main measurements of the  -ray emission probabilities for 14N(n,  )15N reaction are T.J.Kennett et al., E.T.Jurney et al., H.Takayama and T.Belgya. The main difference among these measurements are from the level scheme. For example, the levels are 15, 19, 19 and 17, respectively and  -rays are 28, 58, 64 and 55, respectively corresponding to these measurements.  Fig. 3 shows the comparison of the  -ray emission probabilities. It’s easy to find that the measurements of H.Takayama and E.T.Jurney et al. are in good agreement in 1.7-6MeV energy region. But the ratio of the  -ray emission probabilities for Kennett/Jurney and Belgya/Jurney are exactly reverse(see Fig.4).

Standards  -ray emission probabilities for 14N(n,  )15N reaction

 From Fig.4 it’s easy to find that the measurements of Kennett are higher about 1.2% to 3.7% than Jurney’s in 2.5~4MeV and 6~8MeV range. To average of these two measurements simplely is not suitable for the standard for detector efficiency calibration.  About 58  -rays were observed in the measurements of Jurney, but only 30  -rays of Kennett. In order to keep the intensity balance(∑I  (in-out)), the measured  -ray emission probabilities of Kennett should be higher than Jurney’s.  We think that the  -ray emission probabilities of Jurney are better than Kennett because the level scheme of Jurney is reasonable and complete.

Correction  G.Molnar calibrated the detector efficiency curve using 14N(n,  )15N reaction, in which their  -ray emission probabilities were from the measurements of T.J.Kennett et al.  S.Raman calibrated the detector efficiency curve using 14N(n,  )15N reaction, in which their  -ray emission probabilities were from the average of measurements of T.J.Kennett et al. and E.T.Jurney et al.  So it’s necessary to correct the measurements of S.Raman and G.Molnar using the  -ray emission probabilities of Jurney above 2.5MeV.  In present work the level scheme suggested by Jurney is adopted to correct the  -rays intensities of 56Co and 66Ga.

Previous evaluation I  for 56 Co  The decay data of 56Co were evaluated based on 33 measured data sets from 1965 to 2002 years by C.M.Baglin et al. in  Firstly several measured values are statistical outliers(about 11%) according to the Chauvenet criterion. Secondly the measured data relied on linear extrapolations of the efficiency on a log-log plot above 3MeV were excluded. The rest measured data were processed by several evaluation methods: weight average(WM), limitation of relative statistical weight average(LWM), normalized residuals method(NR) and Rajeval method(RA). At the same time, the whole measured data, the whole measured data except the exceed Chauvenet criterion value, were also processed by these evaluation methods.  The recommended values are obtained from one of these processed values according to author’s judgments. It’s easy to find that the final recommended data are evaluated based on the whole measured data below 2598keV and 8 data sets above 2598keV.

Present evaluation I  for 56 Co  Fig.5 shows the comparison of the ratio of newly measurements of S.Raman et al. ， G.Molnar et al. and present work above 2.5MeV, present evaluation below 2.5MeV to the evaluations of C.M.Baglin for the relative  -ray emission probabilities of 56Co. Below 2.5MeV the present evaluation is in good agreement with Baglin’s within 0.4%. But above 2.5MeV, the evaluation of Baglin is larger than the newly three measurements.  The similar trend can be found for 66Ga (see the next section in detail). We think that the different detector efficiency curve may cause this discrepancy.

Present evaluation I  for 56 Co

 The modified values of S.Raman and G.Molnar using the  -ray emission probabilities of Jurney for 56Co are listed in table 2. It’s noted from table 2 that the systematic deviation among the measurements of S.Raman, G.Molnar and present work is not existed. Present measurements are in good agreement with the modified values of S.Raman and G.Molnar within 1%. Fig. 6 shows the comparison of present evaluations to the modified measurements of S.Raman and G.Molnar, and evaluations of C.M.Baglin.  The present evaluation is obtained from the average of 11 measured data sets including present measurements using LWM below 2.5MeV energy region. Above 2.5MeV energy region, the present evaluation is obtained from the unweighted average of the measurements of S.Raman et al. ， G.Molnar et al. and present work(the measurements before 2000 are rejected when evaluated due to present knowledge of detector efficiency curve).

Present evaluation I  for 56 Co  Table2

Present evaluation I  for 56 Co

Temporary evaluation I  for 66 Ga  The newly recommended relative  -ray emission probabilities for 66Ga were evaluated by E.Browne et al. based on the measurements of S.Raman et al.(quoted data from Budapest), G.Molnar et al.(Budapest) and C.M.Baglin et al.(Berkeley) in We’ve finished the measurements of relative  -ray emission probabilities for 66Ga. A temporary evaluation of the relative  -ray emission probabilities for 66Ga was given based on the measurements of S.Raman et al., G.Molnar et al.(Budapest), C.M.Baglin et al.(Berkeley) and present work.  This two evaluations are in agreement with each other within 1%, but the deviation between the evaluations and newly measurements is up to 2.5%, which is shown in Fig. 7.

Temporary evaluation I  for 66 Ga

Present evaluation I  for 66 Ga  We corrected the measurements of S.Raman and G.Molnar using the  - ray emission probabilities of 14N(n,  )15N reaction from Jurney above 2.5MeV. We also corrected the measurements of C.M.Baglin using present evaluated  -ray emission probabilities of 56Co. The modified values are given in table 3 and shown in Fig.8.

Present evaluation I  for 66 Ga

 Present final evaluations are recommended based on present measurements and modified measurements of S.Raman, G.Molnar and C.M.Baglin shown in Fig.9. The present evaluations are in good agreement with the modified measured values within 1.3%.

Present evaluation I  for 66 Ga

Conclusion  The newly recommended relative  -ray emission probabilities for 56Co and 66Ga evaluated by C.M.Baglin et al., E.Browne et al. and present work show that the old standard for detector efficiency calibration existes systematic errors (up to 30%) in high energies range. But present evaluation is lower than the evaluation of Browne and Baglin above 2.5MeV. The deviation at 3.4MeV is up to 2.7% and this deviation is consistent with the difference shown in Fig.6. The rationality of present evaluation and corrected method will be dependent upon new measurements, and more precise standard data are desirable in further.