Half-life check
Half-life measurements on the four most intense peaks were made (using the same
series of 50 s individual spectra that were used in Fig. 1), by summing corresponding spectra
(i.e., all first, all second, etc.) of these nine Ge sources, which were all taken at equal intervals
of about 50.4 s. As can be seen in Fig. 3, all four half-life determinations fall within the
accepted value [1] of (52.9±0.6) s for 77Gem.
Gamma-ray intensities
The relative efficiency curve for the detector covered with the 6.4 mm thick Pb absorber, with
Cd and Cu backings, at a source-detector distance of 4.8 cm, was obtained using a mixed
source of
60
Co+(108Agm+110Agm)+182Ta, plus standard
mixed source of
57
performed with
77
133
Ba and
152
Eu sources, and another
Co+60Co+137Cs. The extension of the resulting curve up to 2300 keV was
Ge and
124
Sb sources prepared “in situ”. In this efficiency calculations we
used the gamma-ray energies and intensities given by Lorenz [2]. The only exception to this is
77
Ge, in which case the data was taken from Ref. [3]. The fitting of the experimental points
was performed by the Deming method [4]. The estimated resulting uncertainties (one standard
deviation) for this relative efficiency curve lie between 1.2% and 2.5% in the energy interval
of our interest, i.e., 500 to 1700 keV.
The currently accepted [3] gamma-ray energies (in keV) and relative intensities (given
in parentheses) for the decay of 77Gem, with their uncertainties in italics, are: 194.8 2 (1.95 8),
215.5 1 (100), 419.5 5 (0.45 5), and 614.3 5 (0.21 3). Of these four gamma-rays we cannot
choose the 194.8 keV line for normalization of relative emission probabilities, because it is
not observed in our spectra due to its strong attenuation in the lead absorber. Nor can we use
the 419.5 keV gamma-ray for it is strongly interfered by the 419.1 keV line from
75
Ge (1.38 h), and also by the 419.75 keV line from 77Ge (11.3 h). Therefore, we are left with
only two choices for a reference line: the 215.5 and the 614.3 keV gamma-rays. The above
considerations are borne out by an inspection of Fig. 1: only the 215.5 and 614.3 keV
gamma-rays have practically vanished after 250 s, thus showing that the main contribution to
their areas are due to the decay of 52.9 s
77
Gem. The obvious reference line to choose, to
calculate relative intensities for the new gamma rays, would be the 215.5 keV line. However,
the counting efficiency for our experimental setup falls off rapidly below about 350 keV,
1
therefore the uncertainties below that energy increase rapidly. In effect, when calculating
relative intensities for the new gamma-rays, the uncertainties associated with the reference
line alone, amount to more than 50% when we chose the 215.5 keV gamma-ray as the
reference line, as compared to about 3.9% for the 614.3 keV line. And these numbers change
respectively to 51% and 19% when calculating absolute gamma-ray emission probabilities.
Therefore, our choice as a reference line is the 614.3 keV gamma-ray. This gamma-ray is part
of a doublet with the 617.7 keV gamma-ray from 75Ge (1.38 h). This doublet was resolved by
hand (graphical peel-apart technique) and the 614.3 keV area obtained by cutting and
weighing. The contribution of the 614.39 keV gamma-ray from the decay of 77Ge (11.3 h) was
taken into account. All areas were corrected for self-attenuation produced in the germanium
samples. These corrections varied from 1.8% to 3.2% for the gamma-ray energies involved.
Gamma-rays and energy levels
A short discussion regarding some of the new gamma-ray transitions and levels found in this
work which are worth commenting on, follows.
264.44 keV: As mentioned in the main article, we detect a gamma-ray with an energy of
1339.99 keV, whose origin we assign to a partial depopulation of the proposed level at
1604.65 keV leaving the
77
As nucleus in its well established excited level [3] at
(264.401±0.014) keV: (1604.65±0.10)–(1339.99±0.49)=(264.66±0.50) keV. Therefore, we
submit that in the decay of
77
Gem a gamma-ray of 264 keV is also emitted, although we are
unable to detect it with the present experimental setup. This is because this very weak
264 keV peak is strongly interfered by the 264.66 keV gamma-ray (I=11.3%) from the decay
of 75Ge (1.38 h) and also, to a lesser amount, by the 264.44 keV gamma-ray (I=53.9%) from
the decay of
77
Ge (11.3 h). In fact, a simple calculation shows that the contribution to the
264 keV peak-area from the 77Gem (53 s) decay (assuming that the proposed level at 264 keV
is fed by the 1340 keV gamma-ray, probably also by a 1412 keV gamma-ray, and no 
feeding), amounts to less than one half the uncertainty corresponding to the experimentally
measured peak-area at 264 keV (spectrum of Fig. 1a) which, as mentioned above, is mainly
due to the contributions of the
75
Ge (1.38 h) and
77
Ge (11.3 h) decays. For lack of a better
estimate, we establish lower and upper limits for the peak area corresponding to the proposed
264 keV gamma-ray.
2
503.86 keV: This peak is clearly observed in Figs.1 and 1a. Also, its decay with time is seen in
Fig. 3. After having arrived at our proposed decay scheme of Fig. 2, it came to our attention
that the existence of a level around this energy in
77
As, had already been seen by Betts et
al. [5] at (514±7) keV, by Schrader et al. [6] at (503.0±0.9) keV, and by Rotbard et al. [7] at
(503±3) keV. Betts et al. [5], and Schrader et al. [6], were looking at (3He,d) reactions on
76
Ge and from the deuteron energy spectra determined different excited levels in the residual
nucleus
3
77
As. Similarly, Rotbard et al. [7] looked at (d,3He) reactions on
78
Se and from the
He energy spectra also determined different excited states in the residual nucleus 77As. These
values are in good agreement with our proposed level at 503.89 keV.
1412.50 keV: Although small, there is some evidence for the existence of this peak. This is
because it is almost completely buried in the high energy tail of the “strong” peak at 1410 keV
which, according to our results, is about 16 times stronger. Nevertheless, in one of our partial
sum spectra we have a statistically significant indication of the existence of this peak. From
this spectrum we get the energy and intensity given in Table 1 for the 1412 keV peak. Also,
taking into account the considerations above for the 264 keV gamma-ray, it can be readily
established that the sum: -intensity to the 264 keV level plus the 1412 keV gamma-ray
intensity, is greater than (0.00000.0050)% and smaller than (0.00690.0091)%. It can be
seen that this last value agrees with the intensity given in Table 1 for the 1412 keV peak. This
would seem to indicate that the -branching to the 264 keV level is (0.00000.0094)%.
Therefore, we believe it is safe to assume that this branching lies below 0.01%, and is
probably zero if one takes into account -decay forbiddenness considerations.
1604.65 and 1676.46 keV: These are the strongest new gamma-rays we observe in our spectra
and, from energy-sum relations, their placing as energy levels in the
77
As level scheme is a
straightforward matter. Betts et al. [5] report an 77As level at (1674±7) keV. Schrader et al. [6]
report levels at (1618±5) and (1652±5) keV. Rotbard et al. [7] report levels in
77
As at
(1606±10) and (1660±10) keV. The levels reported in these experiments performing nuclear
reactions probably correspond to these two levels proposed in the present work. It is
interesting to note that M. Poncet [8], performing high-resolution -ray spectrometry, reports
the existence of both these gamma-rays in the decay of
77
Ge (11.3 h), giving the following
energy (relative intensity) values: 1605.0 keV (0.01) and 1676.6 keV (0.016), taking
I(264 keV)=100. However, in Ref. [9], p. 264, these gamma-rays are mentioned with
3
question marks, and in Ref. [3], p. 257†, it is specifically said they were not taken into account
for the listing of
77
77
As gamma-rays. In an attempt to see these gamma-rays in the decay of
Ge (11.3 h), we irradiated a 187 mg Ge sample for 3 hours in a thermal neutron flux of about
11012 cm-2 s-1. Under these conditions, we were unable to see either of these two lines in a
spectrum taken during 14.7 h after a 25 h-decay. From this spectrum, however, we can say
that the relative intensity of both these lines lies below 0.020—referred to the 264 keV line
taken as having a relative intensity of 100—for the ground-state decay of
77
Ge. These upper
values are compatible with the work of M. Poncet [8].
†
Although Ref. [3] refers to the work of C. Ythier and M. Poncet (C.R.Acad.Sci., Ser.B, 273,
(1971) 407) as originating these data, there is no mention whatsoever of these gamma-rays in
Ythier and Poncet’s paper.
4
References
[1] J. K. Tuli, Nuclear Wallet Cards, National Nuclear Data Center, Brookhaven National
Laboratory, Upton, New York, 1995.
[2] A. Lorenz, in Handbook on Nuclear Activation Data, STI/DOC/10/273 (IAEA, Vienna,
1987), p. 187.
[3] A. R. Farhan, S. Rab, and B. Singh, Nucl. Data Sheets 57 (1989) 223.
[4] P. M. Rinard and A. Goldman, A curve-fitting package for personal computers, Los
Alamos National Laboratory report LA-11082-MS, Rev.1(March 1988).
[5] R. R. Betts, S. Mordechai, D. J. Pullen, B. Rosner, and W. Scholz, Nucl. Phys. A230
(1974) 235.
[6] M. Schrader, H. Reiss, G. Rosner, and H. V. Klapdor, Nucl. Phys. A263 (1976) 193.
[7] G. Rotbard, M. Vergnes, J. Vernotte, G. Berrier-Ronsin, J. Kalifa, and R. Tamisier, Nucl.
Phys. A401 (1983) 41.
[8] M. Poncet, Thesis, Univ. of Nice, (1971) 114 p.
[9] C. M. Lederer and V. S. Shirley, Eds., Table of Isotopes, 7th edition (John Wiley & Sons,
Inc., New York, 1978).
5