1

Facility for Heavy-Ion Irradiation of
Semiconductors at RIKEN RI-Beam Factory
T. Kambara and A. Yoshida, RIKEN Nishina CenterPublished in 2018 IEEE Radiation Effects Data Workshop(REDW) Workshop Record
DOI: 10.1109/NSREC.2018.8584278
Publisher: IEEE
https://ieeexplore.ieee.org/document/8584278
Abstract-- RIKEN RI-Beam Factory (RIBF) provides fast Kr
and Ar ions to private companies in Japan for the SEE
evaluations of space-use semiconductors. The samples can be
irradiated in the atmosphere. We present the irradiation facility
and measurements of the beam characteristics and discuss
radiochemical analyses and simulations concerning the beam
impurity nuclides produced in the upstream materials.
I.

MeV/u Kr ions are in practical use while beams of Xe andAu ions are being tested. The clients irradiate their samples in
the atmosphere, which enables easy access and quick
exchange of the samples without vacuum feedthrough flanges
for electrical connections. With a beam-energy degrader, theclients can select linear energy transfer (LET) in Si between
2.2 and 41 MeV/(mg/cm2). Fig. 1 shows the relation between
the LET and range in Si for beams available at RIBF andother facilities.

INTRODUCTION

Beam Factory (RIBF) of RIKEN Nishina Center
RI(RNC)[1] is a heavy-ion accelerator facility locatednear Tokyo, Japan. RIBF features high intensity beams ofions of all elements with energy up to 345 MeV/u andvarious radioactive isotope beams (RI beams) produced byfragment separators. The beams from RIBF are mainly used
for experiments of nuclear physics, materials sciences,chemistry, and radiation biology. In 2014, RNC started toprovide fast heavy ions from the RIKEN Ring Cyclotron(RRC) housed in RIBF to private companies in Japan which
evaluate single-event effects (SEEs) of space-use
semiconductors. In section 2, we present the ion beams andirradiation facility for semiconductor irradiation. In section 3,
we describe the procedures and results of the beamcharacteristics measurements. Then we discuss beamimpurity nuclides produced in the upstream materials;
radiochemical measurements of their production probabilities
in section 4 and ion-transport simulations in section 5.

Fig. 1. LET and ion range relation of heavy ions in silicon at the client’s
sample position. Thick lines show the available range at RIBF.

Fig. 2 shows the beamline. At about 4 m upstream of theirradiation place, the beam is deflected by two wobbler
magnets at 60-Hz frequency vertically and horizontally sothat its beam-spot center traces a circle. Then, the beam
passes through a Au scatterer foil where multiple scattering
diffuses the beam spot to form a uniform dose distribution at
the irradiation place.

II. BEAM AND FACILITY
The RRC with an AVF cyclotron as injector sends aheavy-ion beam to a beam line dedicated to material
irradiations. At present, beams of 95 MeV/u Ar and 70
Manuscript received July 9, 2018.
T. Kambara is with RIKEN Nishina Center, Wako, Saitama 351-0198
Japan (telephone: +81-48-467-9507, e-mail: kambara @ribf.riken.jp).
A. Yoshida is with RIKEN Nishina Center, Wako, Saitama 351-0198
Japan (telephone: +81-48-467-9423, e-mail: ayoshida @ribf.riken.jp)
978-1-5386-5541-2/18/$31.00 ©2018 IEEE

Fig. 2. Setup of the beamline for material irradiations.

2Fig. 3 shows the irradiation place in the atmosphere at theend of the beamline. The beam passes through a vacuumseparation polyimide (Kapton) window with a diameter of 6
cm and thickness of 75 m. A transmission-type ionizationchamber (IC1) and a plastic scintillator behind the window
measure the beam flux. A 5-cm diameter window of the IC1determines the beam size downstream. An energy degrader
has eleven Al foils with the thicknesses ranging from 5.5 to2000 m and any combination of the foils can be inserted tothe beam by a remote-control pneumatic system so that theLET can be set almost continuously. Solid curves in Fig. 1
show the available LET and ion range of the Ar and Kr ionsin silicon at the clients’ sample position. The LET of Ar ionson a silicon surface is selectable between 2.2 and 18.7
MeV/(mg/cm2) and that of Kr ions between 11.7 and 41
MeV/(mg/cm2).

about 0.8 % FWHM of the mean LET at 12 MeV/(mg/cm2)
and 5.6 % FWHM at 37 MeV/(mg/cm2). According to thebeam scanning, the flux is uniform within 1.8 % over adiameter of about 35 mm. Another measurement with a
GAFCHROMIC-film irradiation shows that the local
deviation of the dose is within ±5%.

Fig. 4. Dependence of the ionization chamber IC2 output current on the
degrader thickness. Dots are measured values fitted by SRIM calculations

Clients can select a beam flux between 10 and 107 ions/cm2/s.
The flux is continuously monitored during irradiation. The
scintillator is used for the flux measurements below 7  104
ions/cm2/s, and IC1 is used above it. The IC1 output current
is calibrated by the ion count rate of the scintillator.
During an irradiation beam time, the beam flux and LET
are controlled by the clients from outside of the radiation
controlled area via LAN
IV. RADIOCHEMICAL ANALYSES OF IMPURITY NUCLIDES IN
Fig. 3. Schematic diagram of the irradiation place at the end of thebeamline where the beam is extracted to the atmosphere.
III.

BEAM CHARACTERISTICS MEASUREMENTS

The characteristics of the beam are measured before eachbeam time for the clients. At the client’s sample irradiation
place, a shallow-type ionization chamber (IC2: 14-mm
diameter, 2-mm thick) and two sets of ΔE+E Si-detector
stacks are mounted on a motor-driven linear slider.
The IC2 output current is measured at different degrader
thickness and the dependence is fitted by calculations of
SRIM-2013 [2]. An example for a Kr beam is shown in Fig.
4. Fig. 4(a) shows the IC2 current as a function of thedegrader thickness up to 900 m. It also shows the best-fit
result of SRIM-2013 calculation for the energy loss of theions in IC2 and the measurements are normalized to thecalculation. Fig. 4(b) shows the Bragg-peak region of Fig.
4(a) with the best-fit SRIM-2013 calculation and those withAl thickness altered by ±10 m. It shows that with the bestfit result to the Bragg-peak measurement, the beam energy at
the sample is determined within ±5 m accuracy of the range
in Al. The actual degrader thicknesses for the LET desired by
the clients are calculated with the SRIM-2013 code.
The Si detectors are used to measure the energy spectrumof the beam and to scan the beam for the beam-flux
distribution. The measured beam-energy distribution isconverted to LET distribution at different degrader
thicknesses. The width of the LET obtained for Kr beam is

THE BEAM

The high-energy beam from the RRC passes through thewindow, the beam-flux detectors, the energy degrader andthe atmosphere before reaching the client’s sample. Nuclear
reactions in these upstream materials convert a part of thebeam nuclei to different nuclides. These nuclides may
contaminate the beam and affect the LET distribution when
they reach the client’s sample.
We have studied the beam contamination employing aradiochemical method, where the primary Kr beam andimpurity beams (secondary beams) were stopped in samples
and the amount of radionuclides in them were measured by
gamma-ray analyses. As this measurement is very sensitive,
we can identify the radionuclides and obtain the absolutevalue of those production probabilities. Moreover, thismeasurement is insensitive to stable nuclides including the
primary Kr ions and we can analyze very small amount of
impurity radionuclides without being disturbed bybackground from the primary beam. However, thismeasurement is applicable only to radionuclides that haveproper lifetimes and emit gamma-rays with proper energies.
So, we compared the measurement results with an iontransport simulation to evaluate its validity and discuss theimpurity-beam as a whole on the basis of the simulation.
We irradiated test samples with an 84Kr beam in the same
condition as the client’s irradiations. A 70-MeV/u 84Kr beamfrom the RRC passed through a 50-m-thick Au-foil scatterer,
the 75-m-thick Kapton window, a 15-m-thick Al foil in the

3
IC1, 0.1-mm thick plastic scintillator, mylar foils of thedetectors with the total thickness of 72-m, the degrader witha 586-m-thick Al and about 305-mm thick atmosphere, andimpinged a test sample of Si wafer (100-mm diameter and
0.5-mm thick) or acrylic plate (75 mm  80 mm, 1-mm
thick). The test sample of Si was used to simulate the client’s
sample of semiconductor devices and acrylic resin to
distinguish the beam-impurity nuclides from the reaction
products in the sample. The number of the ions was about
4.5  1010 for the Si wafers and 5.5  1010 for the acrylic plates
during 10 minutes of irradiation. According to the SRIM
calculation, the energy of the 84Kr ions hitting the sample was
25 MeV/u which corresponded to the LET of about 19
MeV/(mg/cm2) in Si, and the ions stopped in the test samples.
We measured the gamma rays from the irradiated samples
with Ge detectors; for the Si wafer, 9 times during the timespan of 7 minutes to 91 days after the irradiation and for theacrylic plate, 9 times from 10 minutes to 106 days. We
analyzed the observed gamma-ray peaks according to thetransition energies, lifetimes and branching ratios, andidentified about 61 nuclear species from 24Na to 104Ag in the
Si wafer and 49 species from 24Na to 93mMo in the acrylic
plate, all with a half-life longer than 8 min. Then we
extrapolated the decay curves of the radioactivity to the endof the irradiation to obtain the production rates of thenuclides and deduced their production probabilities
normalized to one incident 84Kr ion.
Some of the obtained nuclide-production probabilities in
the Si and acrylic plates are shown in Fig. 5.

Fig. 5. Production probabilities of nuclides from 24Na to 98Rh obtained by
the radiochemical analyses are compared between the test samples of Si
wafer and acrylic plates.

Comparing the probabilities between the Si and acrylic, we
can group the nuclides by the atomic number Z. The
probabilities for the nuclides up to the primary Kr (Z ≤ 36)
are very similar between the two materials except for a few
small-Z nuclides, whereas the probabilities for 37 ≤ Z ≤ 42
are higher in the acrylic than the Si and nuclides with Z > 42

are found only in the Si. It indicates that the low-Z nuclides
consist mainly of nuclides produced in the upstream materials,
those with Z > 42 are only produced by nuclear reactions in
the Si sample, and those with 37 ≤ Z ≤ 42 are produced by
the both processes.
V. SIMULATION OF IMPURITY-NUCLIDES PRODUCTION
We simulated the production and transport of the nuclides
in the irradiations with Particle and Heavy Ion Transport
code System (PHITS) [3] which traced each resultant nucleus
from its production to stopping. First, we assumed the same
setup as the test-sample irradiations with the degrader
thickness of 586 m, calculated the probabilities thatreaction-product nuclides are found in the sample andcompared the results with the gamma-ray measurements.
PHITS predicts that the impurity beam consists mainly of
nuclides with 20 < Z ≤ 40, which are produced from theprimary Kr ions by high-velocity collisions with static nuclei.
It also predicts that the nuclear reactions in the sample
produce nuclides with Z up to 47. The results for a Si-wafer
sample are shown in Fig. 6 where (a) is the probabilities forthe impurity-beam nuclides produced in the upstream
material and stopped in the sample, (b) is those for the
nuclides produced by nuclear reactions in the sample and (c)is a sum of them. Fig. 6(c) also shows the results of theradiochemical analyses in circles. The overall features of themeasurements look well reproduced by PHITS. It validates
the use of PHITS to simulate our beam and irradiation.

4energies than the primary Kr beam and their intensity
increases up to the degrader thickness of 586 m. At
degrader thickness of 837 m, the total intensity of the
impurity beam decreases but a significant part of it is slow
and heavy nuclides.
Without degrader, a small fraction of the impurity beam
have LET higher than that of the primary Kr ions, as shownin Fig. 8. When the degrader thickness increases, the energy
distribution of the impurity beam shifts to lower energy andmore of the impurity beam have LET higher than that of the
primary Kr ions. At the thickness of 586 m, the impuritybeam energy distributes between zero and the primary Kr
peak, and the LET of most of the impurity beam exceeds thatof the primary Kr ions. At the thickness of 837 m where theprimary Kr ions is close to the Bragg peak, most of the
impurity beam is stopped before the sample and the amountof the impurity beam is small. However, the slow and high-Z
nuclides shown in Fig. 7(b) give rise to LET as high as 50
MeVcm2/mg as shown in Fig. 8(b). Probably, these very
high-LET impurity nuclides originate in fusion-like reactionsin the degrader. Similar reactions may produce 94Tc and 98Rh
in Si as shown in Fig. 5.

Fig. 6. PHITS results for probabilities of nuclides stopped in the Si-wafer
sample when the degrader is 586-m thick. Z is the atomic number and N is
the number of neutrons of the nuclide and the probability is shown in the box
by gray scale. (a) is for the impurity-beam nuclides, (b) is for the nuclides
produced by the reactions in the sample and (c) is the sum of the twocompared with the radiochemical measurements shown in the circles.

We now concentrate on the impurity beam such as shownin Fig. 6(a). We calculated the energy and LET distributionof nuclides of Z between 22 and 45 (from Ti to Rh) in the
primary and impurity beam after passing the degrader withdifferent thicknesses of 0, 300, 586, and 837 m. Fig. 7
shows the energy distribution and Fig. 8 the LET distributionin Si where (a) of each figure is for the nuclides up to theprimary Kr ions (Z ≤ 36) and (b) for heavier nuclides (Z >36). Each peak in Fig. 7a and Fig. 8a correspond to theprimary Kr beam.
The energy of the primary Kr ions in Fig. 7(a) decreases atincreasing intervals as the degrader thickness increases. Eachof the energy distribution has a continuum below the peak,which corresponds to the impurity beam. When impuritybeam nuclides pass through materials, they lose energy
quicker, and reach the Bragg peak and stop earlier, than the
primary Kr ions. The heavier nuclides in Fig. 7(b) have lower

Fig. 7. PHITS results for energy distributions of the impurity-beam
nuclides for degrader thicknesses of 0, 300, 586 and 837 m. (a) is for the
nuclides with Z ≤ 36 and (b) for Z > 36.

5high-LET impurity nuclides in the beam account for about
1 % of the Kr-beam at maximum.
VII. ACKNOWLEDGMENT
The authors are grateful to RNC’s Nuclear Chemistry
Research Team for providing their Ge detectors for the
gamma-ray measurements. This experiment was performed at
RI Beam Factory operated by RIKEN Nishina Center and
CNS, University of Tokyo.
VIII. REFERENCES
[1]
[2]
[3]
Fig. 8. PHITS results for LET distributions of the impurity-beam nuclides
for degrader thicknesses of 0, 300, 586 and 837 m. (a) is for the nuclides
with Z ≤ 36 and (b) for Z > 36.

The number of impurity nuclides which have higher LET
than the primary 84Kr ions is added up and the ratio of thesum to the number of the primary 84Kr is plotted in Fig. 9 as afunction of the degrader thickness. This high-LET impurity
ratio has a maximum value of about 1.2 % at about 600 mof the degrader thickness where the degrader is thick enoughto produce and decelerate the impurity beams but the LET ofthe 84Kr ions is still low.

Fig. 9. The calculated ratio of the high-LET impurity nuclides in a Kr
beam at the entrance of the client’s sample is shown as a function of thedegrader thickness.

VI. SUMMARY
The material irradiation facility of RIKEN RI Beam
Factory is described. Semiconductor companies use Kr and
Ar beams for SEE simulation with a uniform dose
distribution and the LET selectable from 2.2 to 41
MeV/(mg/cm2). Samples can be irradiated in air and theclients can control the LET and beam flux from outside of theradiation-controlled area.
Production probabilities of impurity-beam RI nuclides
were measured by radiochemical analyses of irradiated test
samples. Heavy-ion transport simulations reproduced theoverall features of the measured probabilities, and show that

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