Photon Factory Activity Report 2004 #22 Part B (2005)
Atomic and Molecular Science

NW2A/2004G020

Observation of nuclear resonance on the first excited level of Osmium-187
11Shunji KISHIMOTO , Rie HARUKI *1
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan

Experiment
A silicon avalanche diode (Si-AD, Hamamatsu
SPL0601) was used to detect internal-conversion
electrons emitted from excited nuclei. The device was 3mm in diameter and had a depletion layer 30µm thick.
The surface layer of the Si-AD was made of a thin Si3N4,
25nm thick. A focused monochromatic X-ray beam wasHV
defined to 1× 0.6 mm by slits. We used an osmium target
that was made of metal powder on adhesive carbon tape.
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The powder of Os was enriched to 99.4%. The Si-AD
was installed in a vacuum chamber for the NEET
experiment and was located 2.5mm above the target. A
fast amplifier was used for outputs from the Si-AD. Inorder to measure X-ray absorption spectra and to monitor
intensity of the incident X-rays, photodiodes (silicon PINPD, 30µm thick) of transmission type were used. One ofthe PD was located in front of the vacuum chamber andthe other was installed behind the sample in the chamber.

Table 1: Measured energies of the absorption edges
Abs.
Experiment
Referenceedges
(eV)
(eV)
Zn K
9658.64±0.03 9660.755±0.030
Er L1
9756.72±0.04
9757.8±1.1
Ta L3
9875.59±0.06
9876.7±1.2

Events emitted from decaying excited nuclei wereselected by time spectroscopy. Counts of the delayed
events were recorded by gating with a signal width
produced from a RF signal from the accelerator. Since
the prompt pulses of the amplifier outputs were huge andtheir tail part was overshot to plus level, a dead time morethan 20 ns masked signals emitted from the nuclei. We
found nuclear resonance by scanning energy of the-1
incident X-rays although the event rate was low, 0.3 s
due to the long dead time. Figure 1 shows a peak of thedelayed events measured around the resonant energy.
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We found that the first excited level of Os existed in(9776.9±1.2) eV. This value has a discrepancy of about
30eV from that written in the literature [1]. We are now
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in preparation of observing NEET on Os, considering
this value for nuclear resonance.
10080

Counts/250s

Introduction
It is known that the first excited level of Osmium-187
exists at a low energy of 9.746 keV, and the half life is2.38 ns [1]. The nucleus is expected that Nuclear
Excitation by Electron Transition (NEET) occurs in Lshell ionization [2]. The most probable NEET is between
-1-1
-1
-1
the L1 (2S1/2 , 12.968keV) →M1 (3S1/2 , 3.049keV) atomic
hole transition (9.919keV) and the nuclear transition
between the ground and 9.8-keV excited level (M1+E2,1/2− →3/2−). The energy difference between thesetransitions is only 173 eV. We have tried to observe
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NEET on Os using synchrotron X-rays at beamline ARNW2A. Here, we report on observation of nuclear
resonance at 9.8-keV level. The energy of the firstexcited level was decided from the X-ray absorption
edges.

O s -1 8 7R esonance(9 7 7 6 .9 + /-1 .2 )e V6040(2 .7 + /-0 .1 )e V

200-1 0-505Fig.1: Peak for the nuclear resonance ofResults
In order to determine the resonant energy, we need anabsolute energy scale for the incident X-rays. We tried tomeasure X-ray absorption edges near 9.8-keV level. The
X-ray absorption spectra near Zn-K, Er-L1 and Ta-L3
edges were measured by scanning energy of themonochromator. The incident beam intensity and thatbehind the sample were measured as PDF and PDB,respectively. Table 1 shows the edge energies determinedby fitting the derivative of ln (PDF/PDB) with Lorentzianand the data taken from a reference [3]. An energy scale
was decided by calibration using those data.

10∆ E (e V )187

OsReferences[1] R.B. Firestone, Nucl. Data. Sheets 62,159(1991).
[2] E.V.Tkalya, Pys.Rev.C68, 064611(2003).
[3] NIST Physical Reference Data:
http://physics.nist.gov/PhysRefData/contents.html*Present address: Research and Development Center forHigher education, Kyushu Univ., Ropponmatsu, Fukuoka
810-8560.
shunji.kishimoto@kek.jp

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