Download Measurement of the half-life of

Measurement of the half-life of
1
48
Cr
H. Kikunaga,1 N. Takahashi,1 and A. Shinohara1
Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
It is well known that decay rate of radioactive nuclides is usually independent on external conditions such
as chemical structures of sample materials. However, there are some exceptions in the electron capture decay
and the internal conversion processes [1]. In the case of electron capture decays, the decay rates depend on the
density of atomic electrons at the nucleus. When the electron density at the nucleus is perturbed by chemical
and physical conditions, the change of the decay rate can be expected. Experimental study for the 51 Cr isotope
have been reported that the difference of decay constant between two chemical forms of Na2 CrO4 and CrCl3 is
λ(Na2 CrO4 )-λ(Na2 CrO4 ) = (5.3±2.1)×10−4 λ(Na2 CrO4 ) [2]. We plan to compare the results with 51 Cr (T1/2
= 27.7 d) to that with another isotope 48 Cr. In this work, 48 Cr isotopes are produced in the nat Ti(α, xn)
reactions and measured its half-life in Ti metal.
A nat Ti-metal foil with 20-µm thickness was used as target materials. The irradiation was carried out with
the AVF cyclotron at Research Center for Nuclear Physics, Osaka University. The cyclotron was operated at an
α-particle energy of 40 MeV. The Ti target was placed in close contact with an aluminium degrader to adjust
the alpha-particle energy to 30 MeV in the middle of target. After the 30 min irradiation, the Ti sample was
set in front of a high-purity (HP) Ge detector. The γ rays from the Ti sample were measured for a real time of
1 h at one data point. The procedures were repeated over 150 h (T/T1/2 > 7). A 60 Co and 137 Cs sources was
attached to near the HP-Ge detector as a reference source to correct for influential factors for determination of
half-life such as pile-up effect.
A γ-ray spectrum measured after 26.7 h from the end of bombardment is shown in Fig. 1. The γ peaks of
47
Sc (T1/2 = 3.35 d), 48 Sc (T1/2 = 43.67 h), 48 V (T1/2 = 15.97 d), 48 Cr, 51 Cr, 60 Co, and 137 Cs are observed
in the spectrum. The γ peaks of 48 Cr at 112 keV and 308 keV are clearly separated from other peaks. In
other words, no serious background exists in the γ-ray spectrum for determination of the half-life of 48 Cr. The
half-life of 48 Cr in Ti metal is determined to be (21.17±0.01) h with a reference source method, which include
only a statistical error. The half-life obtained is 1.8% shorter than previous value (21.56±0.03) h [3]. Further
work is underway to determine the half-life of 48 Cr with various chemical forms.
109
Meas. time: 2670 s
(26.7 h after EOB)
60
48
Co
48
V
60
137
Co
48
48
Cs
48
E
+
Cr
51
105
V, Sc
V, Sc
48
Cr
48
Sc
106
47
Counts / channel
107
Cr
108
104
103
102
101
100
0
500
1000
1500
Energy / keV
Figure 1: A γ-ray spectrum measured after 26.7 h from the end of bombardment.
References
[1] Emery, G. T., Ann. Rev. Nucl. Sci. 22, 165 (1972)
[2] Kakiuchi, S. and Mukoyama, T., Bull. Inst. Chem. Res., Kyoto Univ., 59, 27 (1981)
[3] Tse, C. W. et al., Phys. Rev. C 10, 838 (1974)