Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 38 (2005)

36(LNS Experiment : #2427, #2442, #2457, #2474, #2489, #2499, #2512, #2524)

Life-time measurement of 7Be in beryllium metal
T. Ohtsuki1, Y. Yuki1, M. Muto1, J. Kasagi1, K. Ohno2
12

Laboratory of Nuclear Science, Tohoku University, Mikamine, Taihaku, Sendai 982-0826

Department of Physics, Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501

The decay rate of 7Be (nucleus of electron-capture decay) was measured in Be metal. The half life of
7

Be in Be metal (Be metal (7Be)) is found to be 53.12 ± 0.05 days. We have found that the decay rate of

7

Be in Be metal is almost corresponding to that in graphite host, Lithium ﬂuoride host etc. reported

within the errors.

§1. Introduction
As ﬁrst suggested by Segré et al. [1-4], electron-capture (EC) decay rates depend on the density ofatomic electrons within the nucleus. Environment factors such as chemical form, pressure, etc. mayalter the electron contact densities at nucleus, and thus, affect the electron-capture decay rates. Here,the nucleus 7Be is a good candidate in which to look for such variations in environmental factors
because of its simplest electronic structure, 1s22s2, in the EC decay nucleus. The 7Be decays directly to
the 3/2 － ground state of 7Li with a branching of 89.6 %, and goes to the ﬁrst excited state in 7Li (1/2 － at478 keV) with that of 10.4 %, which decays by γ emission to the ground state [5-7]. In recent research,
there have been several reports of variations as a function of the host metals [8-12] and chemical forms[13-15] and pressure [16-17]. Although, a precise measurement may be still needed to obtain theabsolute decay rate in the different circumstances [18-21].
Because of the uniform lattice structure (hcp) including 7Be in Be metal, the electron contact density
on 7Be nucleus should be essentially surveyed. In the present study, we have measured the half-life of7

Be in Be metal by using a standard clock time.

§2. Experimental procedure
Be metal (hcp lattice structure) of 10 mm (in diameter) × 0.3 mm (in thickness) was utilized toproduce 7Be uniformly in the metal. After being washed with HCl solution, the Be metal was sealed in aquartz tube (vacuum packing) of 12 mm in diameter as a target. The irradiation with a bremsstrahlung(50 MeV electrons) was carried out at the Electron Linear Accelerator, Laboratory of Nuclear Science,Tohoku University. The sample in a quartz tube was set in the middle of a sweep magnet placed on theaxis of the electron beam. A platinum converter in 2 mm thickness was set in front of the sweep magnet
to generate a bremsstrahlung. Then, the sample was irradiated only by the bremsstrahlung (allelectrons were ruled out by the magnetic field). Therefore, the damage to a lattice of Be metal wasconfined to the minimum. The experimental setup for irradiation is shown in Fig.1. The 7Be can be37produced uniformly by the 9Be(γ, 2n) 7Be reaction in the Be metal. After irradiation, the sample wasbaked in an electric oven of vacuum packing at 1100 ºC (a melting point of Be metal: 1278 ºC) for 1 hourto recover the lattice defect even if the lattice defect occurs by the (γ, 2n) reaction. Finally, the sample
was washed again with HCl solution to clean up the surface.

Fig.1. Setup for irradiation of Be metal.
The two samples of Be metal(7Be) and 7Be@C60 for reference of the sample were placed in an
automated sample changer, which horizontally moved the samples in front of a γ-ray detector. This
allowed the decay rates of the two samples to be measured in a comparable way. The system is shownin Fig.2. The activities of the 7Be, the 478 keV γ-rays emanating from 7Be, was measured with a highpurity germanium (HPGe) detector (ΔEFWHM is 1.8 keV and 50 % relative efficiency) coupled to a2048-channel pulse-height analyzer. Due to the excellent energy resolution of the HPGe detector, a goodFig.2. Experimental setup.

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signal-to-noise ratio was obtained. The background was reduced by a lead shield. Therefore, thebackground peaks do not impair the determination of the half-life of 7Be in the present experiment. The
radioactivities of 7Be could be uniquely detected by means of its characteristic γ-rays, and any othersources were ruled out. We measured 330 points with durations of Td ~ 6 hours. Total measuring time
was now 170 days that was over three half-lives of 7Be. The start time for each run was taken from atime standard signal distributed via a long-wave radio center in Japan. Therefore, the uncertainty intime measurements can be neglected.

§3. Results and discussion
A typical γ-ray spectrum obtained in the sample of Be metal (7Be) is shown in Fig.3. The expected
γ line at Eγ = 478 keV and a natural background line at Eγ = 1461 keV can be seen as two giant peaks.
No peaks were seen at around Eγ = 478 keV when the 7Be sources was absent. In Fig.4, the exponential
decay curve of the 7Be activities for sample of Be metal (7Be) is shown as a function of the time (days).
The decay curve obtained in the present measurement was ﬁtted including the statistical errors by a
Minuit program distributed from the CERN Program Library. The statistical error is dominating theuncertainty in each data point in Fig.4. The uncertainty of our measurement is given by the uncertaintyof the slope of the straight line ﬁtted to the logarithm of the counts (i.e. counts per second) of the decayspectrum. The result for the sample Be metal (7Be) is T1/2 = 53.12 ± 0.05 days. The dead time in the dataacquisition system is evaluated to be about 8 ~ 9 sec to the each running time. Therefore, theuncertainty due to the dead time is estimated to be almost 0.04 % and this value is smaller than theﬁtting errors of the half-life of 7Be. The counting rates of the natural background, which is the 1461
keV γ-rays emanating from 40K, is also shown in Fig.4. The data for 40K obtained was also ﬁtted withFig.3. Typical γ-ray spectrum of the 7Be in the sample of Be metal.

39
the same procedures. It was found that the ﬁtted line is corresponding to a horizontal one. (It should be
noted that we have also measured the half-life of 7Be in the sample of 7Be@C60. The result have beenpresented in another paper [22])The half-life obtained in the sample of Be metal ( 7Be), T 1/2 = 53.17 ± 0.05 days, is almost
corresponding to the data, LiF (7Be) and graphite etc., which is reported by Jaeger et al. and Norman etal. [8, 11], in which, the start time for each run were taken from the time standard signal distributed
publicly. Further, the half-life of 7Be in several host materials (Graphite, Boron nitride etc.) has beensummarized by Notrman et al. [11]. The value (T1/2) is almost with in 53.1 ~ 53.3 days as shown in Table 1.
Therefore, we found that the variation (T1/2) of 7Be for Be metal (7Be) almost corresponds to the data
presented so far.

Fig.4.

Exponential decay line of 7Be in the sample of Be metal (7Be). Backgroundradioactivities of 40K are also shown in the ﬁgure.
Table 1. Half-life of 7Be in Be Metal as determined with a leastsquared ﬁt. Half-lives previously measured are also shown
as a comparison.
Host material

T1/2

Ref. No.

Beryllium
Lithium ﬂuoride
Graphite
Boron niterideTantalum
GoldAluminum

53.12 ± 0.05
53.12 ± 0.07
53.107 ± 0.022
53.174 ± 0.037
53.195 ± 0.052
53.311 ± 0.042
53.17 ± 0.02This work8)
11)
11)
11)
11)
15)

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§4. Conclusion
We have measured the half-life of 7Be which is produced in Be metal using a HPGe detector taking
into account a standard time. We found that the half-life of 7Be in Be metal was T1/2 = 53.12 ± 0.05 days.

Acknowledgments
The authors are grateful to the technical staff of the Laboratory of the Nuclear Science, Tohoku
University for beam-handling.

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7

Be in different charge states which are planned at GSI.

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T. Ohtsuki, H. Yuki, M. Muto, J. Kasagi, and K. Ohno: Phys. Rev. Lett. 93 (2004) 112501.