Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 51 (2018)

Evaluation of radioactivity in semiconductor samples
by Kr-ion beam irradiation
T. Kambara∗1 and A. Yoshida∗1
On the basis of a fee-based facility-sharing program,
RIKEN provides heavy-ion beams from RIKEN Ring
Cyclotron (RRC) to private companies for the irradiation of semiconductor devices to be used in satellites. By using the accelerated heavy ions, the clientssimulate the single-event eﬀects (SEEs) of the devices
caused by heavy-element components of cosmic rays.
The samples are irradiated at the E5A beamline1) in
the atmosphere, and the linear energy transfer (LET)of the beam can be adjusted with an energy-degrader
system. During the irradiation, the samples becomeradioactive through two processes: (1) nuclear reactions in the sample induced by the beam and (2) theinjection of secondary nuclides produced in upstream
materials. The secondary nuclides contaminate thebeam and aﬀect the LET distribution, while the nuclear reactions in the sample may contribute to SEEs.
To assess the beam contamination and to understandthe irradiation eﬀects in industrial utilization, we studied the radionuclides in the irradiated samples by employing a radiochemical method.
In March 2017, during beam preparation and characterization for a client’s beam utilization, we irradiatedtest samples with a 84 Kr beam under the same condition as the client’s irradiations. A 70-MeV/nucleon
84
Kr beam from the RRC passed through a 50-µmthick Au foil as a scatterer, a Kapton window separating vacuum and the atmosphere, an ionization chamber, a 0.1-mm-thick plastic scintillator, an energy degrader consisting of a 586-µm-thick aluminum layer,
and an approximately 305-mm-thick atmosphere, following which it impinged a test sample of Si wafers(100-mm diameter and 0.5-mm thickness) or acrylic
plates (75 mm × 80 mm, 1-mm thickness), where two
identical plates were stacked for each material. The Si
test sample was selected to simulate the clients’ sample of semiconductor devices, and the acrylic resin wasused to distinguish diﬀerent activation processes. The
yield of the secondary nuclides from the upstream materials should be independent of the sample materials,
whereas that of the reaction products in the samples
should be dependent. The number of the ions was approximately 4.5 × 1010 for the Si wafers and 5.5 × 1010
for the acrylic plates during irradiation for 10 min. According to SRIM calculation,2) the ions impinged thesample at 25 MeV/nucleon, which corresponded to an
LET of approximately 19 MeV/(mg/cm2 ) in Si, andthe primary 84 Kr ions stopped in the ﬁrst plate of thestack. In the following, we report the analyses of theﬁrst plate, which had most of the radioactivity.
∗1RIKEN Nishina Center- 186 -We measured the gamma rays from the irradiatedsamples with Ge detectors; for the Si wafer, we performed measurements 9 times from 7 min to 91 days
after the irradiation, and for the acrylic plate, we performed measurements 9 times from 10 min to 106 days.
We analyzed the observed gamma-ray peaks according
to the transition energies, lifetimes, and branching ratios, and we identiﬁed 61 radionuclides from 24 Na to104
Ag in the Si wafer and 49 nuclides from 24 Na to93m
Mo in the acrylic plate. Subsequently, we extrapolated the decay curves of radioactivity to the end of
the irradiation time to obtain the production rates of
the nuclides and deduced their production probabilities normalized to one incident 84 Kr ion.
Figure 1 shows some of the obtained nuclideproduction probabilities. Nuclides with atomic number Z ≤ 36 have similar production probabilities between the Si and acrylic samples, except for a few
small-Z nuclides, which indicates that these consist
mainly of secondary nuclides produced in the upstream
materials. The production probabilities of nuclides
with 37 ≤ Z ≤ 42 are similar or higher in the acrylic
sample compared with the Si sample; in particular,
those of 83 Rb and 84 Rb are three to four times higher.
Nuclides with Z > 42 are found only in the Si sample,
which indicates that these nuclides are only producedby nuclear reactions in the Si sample.

Fig. 1. Production probabilities of nuclides from 24 Na to98
Rh obtained by the gamma-ray measurements.

References
1) T. Kambara, A. Yoshida, RIKEN Accel. Prog. Rep. 48,
239 (2015).
2) J. F. Ziegler, http://www.srim.org.