Prog. Theor. Exp. Phys. 2017, 021D01 (10 pages)
DOI: 10.1093/ptep/ptw187
Letter
Spallation reaction study for the long-lived fission
product 107 Pd
He Wang1,∗ , Hideaki Otsu1 , Hiroyoshi Sakurai1 , DeukSoon Ahn1 , Masayuki Aikawa2 ,
Takashi Ando3 , Shouhei Araki4,1 , Sidong Chen1 , Nobuyuki Chiga1 , Pieter Doornenbal1 ,
Naoki Fukuda1 , Tadaaki Isobe1 , Shunsuke Kawakami5,1 , Shoichiro Kawase4,6 , Tadahiro Kin4 ,
Yosuke Kondo7 , Shunpei Koyama3 , Shigeru Kubono1 , Yukie Maeda5 , Ayano Makinaga8,9 ,
Masafumi Matsushita6 , Teiichiro Matsuzaki1 , Shin’ichiro Michimasa6 , Satoru Momiyama3 ,
Shunsuke Nagamine3 , Takashi Nakamura7 , Keita Nakano4,1 , Megumi Niikura3 ,
Tomoyuki Ozaki7 , Atsumi Saito7 , Takeshi Saito3 , Yoshiaki Shiga10,1 , Mizuki Shikata7 , Yohei
Shimizu1 , Susumu Shimoura6 , Toshiyuki Sumikama1 , Pär-Anders Söderström1 , Hiroshi
Suzuki1 , Hiroyuki Takeda1 , Satoshi Takeuchi1 , Ryo Taniuchi3 , Yasuhiro Togano7 , Junichi
Tsubota7 , Meiko Uesaka1 , Yasushi Watanabe1 , Yukinobu Watanabe4 , Kathrin Wimmer3,6,1 ,
Tatsuya Yamamoto5,1 , and Koichi Yoshida1
1
RIKEN Nishina Center, Wako, Saitama 351-0198, Japan
Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
3
Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
4
Department of Advanced Energy Engineering Science, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
5
Department of Applied Physics, University of Miyazaki, Miyazaki 889-2192, Japan
6
Center for Nuclear Study, University of Tokyo, RIKEN campus, Wako, Saitama 351-0198, Japan
7
Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
8
JEin Institute for Fundamental Science, NPO Einstein, Kyoto 606-8317, Japan
9
Graduate School of Medicine, Hokkaido University, Sapporo 060-8648, Japan
10
Department of Physics, Rikkyo University, Toshima, Tokyo 172-8501, Japan
∗
E-mail: [email protected]
2
Received August 9, 2016; Revised December 9, 2016; Accepted December 9, 2016; Published Online February 4, 2017
...................................................................................................................
Spallation reactions for the long-lived fission product 107 Pd have been studied for the purpose
of nuclear waste transmutation. The cross sections on the proton- and deuteron-induced spallation were obtained at 196 and 118 MeV/nucleon in inverse kinematics at the RIKEN Radioactive
Isotope Beam Factory. Both the target and energy dependences of cross sections have been investigated systematically. It was found that the proton-induced cross sections at 196 MeV/nucleon
are close to those for deuteron obtained at 118 MeV/nucleon for the light-mass products. The
experimental data are compared with the SPACS semi-empirical parameterization and the PHITS
calculations including both the intranuclear cascade and evaporation processes. Our data give a
design goal of proton/deuteron flux for the transmutation of 107 Pd using the spallation reaction. In
addition, it is found that the spallation reaction at 118 MeV/nucleon may have an advantage over
the 107 Pd transmutation because of the low production of other long-lived radioactive isotopes.
...................................................................................................................
Subject Index
D23, D50
1. Introduction The role of nuclear power has been emphasized in recent years, because nuclear
power plays as one of the cornerstones of energy production and is thought to be a potential candidate for environmental sustainability in economic activity. Nevertheless, nuclear safety and the
© The Author(s) 2017. Published by Oxford University Press on behalf of the Physical Society of Japan.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/),
which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
PTEP 2017, 021D01
He Wang et al.
management of nuclear waste from spent nuclear fuel have received much attention. In recent years,
a substantial worldwide research and development activity has been devoted to partitioning and transmutation technology for reduction in high-level radioactive waste (HLW) in terms of magnitude and
duration [1], as well as for resource recycling from spent nuclear fuel [2].
Fission products in HLW contain useful materials, such as rare metals with variable elements, that
could be chemically separated for industrial use. One of the promising metals is palladium. However,
the palladium metal recovered from the waste has a radioactive isotope, 107 Pd, which is a typical
long-lived fission product (LLFP) with a half-life of 6.5 × 106 years [3]. The abundance of 107 Pd in
the fission palladium is 17% by weight. Removal of 107 Pd isotopes from the Pd metal is necessary for
industrial use and is achieved by isotope separation or even–odd isotope separation techniques. After
the separation process, the 107 Pd or the odd Pd isotopes would be obtained as final nuclear waste.
Because of the long half-life and the small β decay energy, the radiotoxicity of 107 Pd is relatively
low compared with those of other LLFPs. In considering a possible mechanism for further reduction
in the radioactivity of 107 Pd, the related nuclear reaction data are very scarce. The thermal neutron
capture reaction was once measured [4], but no other reaction data have been obtained.
Spallation reaction could be one possible transmutation reaction for 107 Pd. Use of proton and/or
deuteron as primary beams gives the highest luminosity for spallation reaction, compared with those
of other beam species. Proton-/deuteron-induced spallation reactions of heavy stable nuclei at high
reaction energies have been investigated at GSI [5–7]. These studies are useful for the design of
the target system of an accelerator-driven system. According to a recent experimental work for
the spallation reaction with 137 Cs and 90 Sr at 190 MeV/nucleon [8], both proton- and deuteroninduced reactions are found to be promising for the transmutation of fission products because the
total spallation cross sections are much larger than that for the neutron-capture reaction. It was
also found that the deuteron-induced reaction has a different isotopic distribution of cross sections
from the proton-induced one. The work suggested that the differences may be caused by the isospin
effects and the excitation-energy difference for pre-fragments [8]. A study of energy dependence
for both proton- and deuteron-induced reactions would be highly desirable to give a clear picture of
such effects and also to optimize beam species and energy for an accelerator-driven transmutation
system.
The present letter reports on the results from the spallation reaction of long-lived fission product
107 Pd. The isotopic distribution cross sections on proton and deuteron were obtained at both 196
and 118 MeV/nucleon using the inverse kinematics technique. This reaction study enables us to
provide high-quality data for a possible solution to the LLFP transmutation. Reactions at around
100 MeV/nucleon were chosen to ensure a large difference in cross section at different energies
because both the proton–proton and proton–neutron interaction cross sections are expected to increase
significantly when the reaction energy decreases from 200 MeV/nucleon to 100 MeV/nucleon; and
to compare the production cross sections on deuteron at 100 MeV/nucleon and those on proton
at 200 MeV/nucleon because these two reaction systems are expected to have a similar energy
deposition in the central collision.
2. Experiment The experiment was performed at the RIKEN Radioactive Isotope Beam Factory,
operated by RIKEN Nishina Center and the Center for Nuclear Study, University of Tokyo. The
secondary beams were produced by in-flight fission of a 238 U beam at 345 MeV/nucleon incident
on a 1 mm thick beryllium target in the first stage of the BigRIPS separator [9]. Two settings
were applied in BigRIPS to make the 107 Pd beams at 196 and 118 MeV/nucleon in front of the
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Table 1. List of targets used in the present work.
Target
Thickness (mg/cm2 )
Setting
CH2
CD2
C
C
179.2
218.2
226.0
317.2
196 and 118 MeV/nucleon
196 and 118 MeV/nucleon
118 MeV/nucleon
196 MeV/nucleon
secondary targets, respectively. The particles in the secondary beams were identified event by event
by measuring the time of flight (TOF), the magnetic rigidity (Bρ), and the energy loss (E) as
described in Refs. [10,11] with similar experimental setups. The average intensities of the 107 Pd
beams were 3.2×102 and 1.1×103 particles per second (pps), with purities of 16% and 25% in the
196 and 118 MeV/nucleon settings, respectively.
To induce the secondary reactions, polyethylene (CH2 ), deuterated polyethylene (CD2 ) [12], and
natural carbon targets were used. The thicknesses of targets are summarized in Table 1. Uncertainties
of the target thicknesses were less than 2% determined by weight measurement. In order to measure
the background contribution, data were taken additionally by using the target holder with no target
material inserted (empty target).
The reaction residues were analyzed by the ZeroDegree spectrometer [9] with its full momentum
acceptance (6%) and full angular acceptances in both the horizontal (90 mrad) and vertical (60 mrad)
directions. In order to cover a broad range of fragments, several different Bρ settings were applied
in the ZeroDegree spectrometer: −9%, −6%, −3%, 0%, and +3% relative to the Bρ value of the
secondary beam. In order to ensure the full acceptances in both angle and momentum in each Bρ
setting, the momentum acceptance was limited to be p/p ≤ ±2.5% in the analysis as described in
Ref. [8].
In a similar way to BigRIPS, the particle identification was achieved using the TOF-Bρ-E
method. The atomic number Z and the mass-to-charge ratio A/Q were deduced from the TOF-E
and Bρ-TOF correlations, respectively. The resolutions in Z were 0.47 [given here as full widths
at half maximum (FWHM)] for both settings and the respective A/Q resolutions were 5.2×10−3
(FWHM) and 5.8×10−3 (FWHM) for the 196 and 118 MeV/nucleon settings, respectively.
Examples of the particle identification plots for reaction products produced from the 107 Pd beam at
both energies and detected by the ZeroDegree spectrometer are shown in Fig. 1. Both the examples
were taken by using the CD2 target with the −6% ZeroDegree setting. For the reaction residues
produced at 196 MeV/nucleon, the fully stripped (Q = Z) ions were more than 94% for the Pd
isotopes. The fractions of ions in the fully stripped, hydrogen-like (Q = Z − 1), and heliumlike (Q = Z − 2) charge states were 71%, 26%, and 3%, respectively, for the Pd isotopes at
118 MeV/nucleon.
3. Results The isotopic distributions of cross sections for the different elements produced in the
107 Pd
+ p and 107 Pd + d reactions at 196 and 118 MeV/nucleon in the present work are displayed
in Figs. 2 and 3, respectively. The proton- and deuteron-induced cross sections (σp and σd ) were
deduced from the measurements using the CH2 and CD2 targets, respectively, after the subtraction
of measured contributions from carbon (using data from the C target run) and beam-line materials
(using data from the empty-target run).
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Atomic number Z
(a)
(b)
48
48
46
46
44
44
42
42
40
40
2.15 2.20 2.25 2.30
2.15 2.20 2.25 2.30
Mass-to-charge ratio A/Q
Fig. 1. Particle identification plots for reaction residues produced from 107 Pd and detected by the ZeroDegree
spectrometer. Panel (a) displays a two-dimensional plot of Z versus A/Q for the reaction residues produced
at 196 MeV/nucleon and panel (b) for those produced at 118 MeV/nucleon. The red circles indicate the 102 Pd
products in both panels to guide the eye.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Fig. 2. Isotopic distributions of the measured cross sections for 40 ≤ Z ≤ 47 elements produced by 107 Pd
on proton (circle) and deuteron (square) at 196 MeV/nucleon. The solid and dot-dashed lines represent the
PHITS [14] calculations on proton and deuteron, respectively. The dashed lines indicate the SPACS [15]
calculations on proton.
The errors in the cross sections include both the statistical and systematic uncertainties. The former
is the main source as presented in Figs. 2–4 and the propagation of statistical uncertainties was
included for the subtraction processes mentioned above. The systematic uncertainties were from
the contributions of the target thicknesses (less than 2%) and the charge-state distribution in the
ZeroDegree spectrometer. The last was simulated by the LISE++ code [13]. The simulated results
agreed within 2% and 5% with the experimental data at 196 and 118 MeV/nucleon, respectively.
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(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3. Same as Fig. 2, but for 42 ≤ Z ≤ 47 elements produced by 107 Pd at 118 MeV/nucleon.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 4. Isotopic distributions of the measured cross sections for 42 ≤ Z ≤ 47 elements produced by 107 Pd
on proton and deuteron at different reaction energies. The open symbols represent the results obtained at the
reaction energy of 196 MeV/nucleon and the crosses at 118 MeV/nucleon.
4. Discussion The silver isotopes are produced in a process in which the nuclear charge of the
projectile is increased (Z = +1). At both energies, σp is found to be larger than σd in this reaction,
as shown in Figs. 2(a) and 3(a). At a high reaction energy, a similar trend was observed in the Cs
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isotopes produced from 136 Xe [7]. According to Ref. [16], the Ag isotopes are probably produced
by a quasi-free (p, n) charge-exchange scattering followed by an evaporation of neutrons. For the
isotopes close to the projectile, such as palladium and rhodium, both σd and σp keep almost constant
over a wide range of mass numbers, and then slightly decrease towards the neutron-deficient side of
the isotopic chain. In the fragmentation reaction, this behavior is related to the so-called “peripheral"
reactions [17]. In such a picture, the projectile deposits little energy and thus the evaporation of only
a few neutrons is allowed, resulting in a constant cross-section distribution over a wide range [5,7,17]
for the heavy isotopes close to the projectile in mass. It is found that σd and σp have similar values
for the peripheral reactions. For other elements, the isotopic distributions exhibit bell shapes, as
displayed in Fig. 2(d)–(h) and Fig. 3(d)–(f). It has been suggested that the position of the maximum
is determined by the competition between the evaporation of protons and neutrons [5]. In addition, it
is found that the difference between σd and σp appears in these channels and becomes larger towards
the neutron-deficient side.
Theoretically, spallation reactions could be described as a two-step process. In the first stage, the
pre-fragments are formed during the nucleon–nucleon collision inside the nucleus. In the second
stage, the pre-fragments deexcite by the evaporation of light particles. These two processes can be
described by the intranuclear cascade and evaporation models, respectively. For quantitative comparisons, calculations including these two models are performed by using the particle and heavy
ion transport code system (PHITS) [14]. In the PHITS calculations, the cascade and evaporation
processes were simulated by using the intranuclear cascade model of Liège (INCL4.6) [18] and the
generalized evaporation model (GEM) [19], respectively. In the INCL model [18,20], the stopping
time, namely the time at which the cascade model is terminated and connected to the evaporation
process, is determined self-consistently and its equation is given in Sect. II.D in Ref. [20]. In the
final state of cascade, the pre-fragment is recognized based on the conservation laws of mass and
charge. The excitation energy of the pre-fragment is calculated based on the conservation law of
energy by taking into account the kinetic energy of ejectiles, the total energy of pions, the recoil
energy of the pre-fragments and the separation energy. The detailed description on the equation to
the determination of the excitation energy can be found in Section II.C in Ref. [20]. The PHITS
calculations reproduce the experimental systematic trends well for both reaction energies as shown
in both Figs. 2 and 3. PHITS slightly overestimates cross sections on the neutron-rich sides for
some low-Z products, such as Mo, Nb, and Zr at 196 MeV/nucleon as shown in Fig. 2(f)–(h)
and Mo at 118 MeV/nucleon as displayed in Fig. 3(f). For the results at 118 MeV/nucleon,
the cross sections for the light-mass products are underestimated by PHITS as displayed in
Fig. 3(c)–(f).
Recently, a new semi-empirical parameterization SPACS [15], has been developed to suit protonand neutron-induced spallation reactions. The SPACS results are also plotted in both Figs. 2 and 3
to compare with the experimental results on proton. Calculated values by SPACS show a reasonable agreement with the proton-induced cross sections at both energies, in particular for those with
the bell-shaped distributions. An overestimation has been found for the 107 Pd(p, n)107Ag reaction
at both 196 and 118 MeV/nucleon as shown in Figs. 2(a) and 3(a). The SPACS calculation underestimates cross sections on the multi-neutron removal channels as shown in Figs. 2(b) and 3(b).
Overestimated cross sections on the neutron-rich sides are found for Ru, Tc, and Mo as shown
in Fig. 3(d)–(f). Such underestimation on the multi-neutron removal and overestimation on the
neutron-rich sides was also found in the proton-induced spallation reactions for 137 Cs and 90 Sr at
185 MeV/nucleon [8].
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(a)
(b)
Fig. 5. The σd /σp ratios for spallation products produced from the 107 Pd beam. Panel (a) shows elements with
40 ≤ Z ≤ 47 at 196 MeV/nucleon and panel (b) shows 42 ≤ Z ≤ 47 at 118 MeV/nucleon. Theoretical
calculations using PHITS (solid) [14] and the geometric model (dashed) [23] are displayed for comparison in
both panels. The atomic numbers of the products and projectiles are represented by Z and Zproj , respectively.
See text for details.
In order to investigate the energy dependence of σd and σp , the experimental data obtained at
different energies are presented together in Fig. 4. As shown in Fig. 4(a), both the proton- and
deuteron-induced cross sections for the Ag isotopes at 118 MeV/nucleon are almost 2 times larger
than those for 196 MeV/nucleon. As reported in Refs. [16,21,22], the production cross section
for the Z = +1 reaction increases as the reaction energy decreases from 1 GeV/nucleon to
200 MeV/nucleon. Our data indicate that the cross section for the Z = +1 reaction continues to increase down to an energy of 100 MeV/nucleon. The production cross sections for the
Pd isotopes at 118 MeV/nucleon are generally larger than those at 196 MeV/nucleon, in particular for the deuteron-induced cross section of 106 Pd as displayed in Fig. 4(b). As 106 Pd is produced
by one-neutron removal, the large σd value at 118 MeV/nucleon might be understood as a consequence of the increase of nucleon–nucleon interaction cross section from 200 MeV/nucleon
to 100 MeV/nucleon. For the Rh isotopes, both σd and σp show a relatively small difference between the values at the two reaction energies as shown in Fig. 4(c). From panels (d)
to (f), both the σd and σp values obtained at 118 MeV/nucleon become smaller than those
obtained at 196 MeV/nucleon. In addition, for the light-mass products, the difference between
σd and σp at 118 MeV/nucleon is larger than that at 196 MeV/nucleon within one isotopic
chain. These light-mass products are produced mainly via the central collision, where the cross
section depends on the energy deposited. In the reaction at 196 MeV/nucleon, a higher excitation energy can be gained by the pre-fragments, resulting in a larger evaporation of nucleons.
Thus, the production of light-mass residual nuclei is higher at 196 MeV/nucleon than that at
118 MeV/nucleon.
It is worth noting that the production cross sections induced by 107 Pd on proton at 196 MeV/nucleon
and on deuteron at 118 MeV/nucleon are similar to each other for the light products, as shown in
panels (e) and (f). As mentioned above, the production of these light products depends on the energy
deposited. Because deuteron has two nucleons, the deuteron-induced reaction at 118 MeV/nucleon
dissipates similar energy to the proton-induced one at 196 MeV/nucleon, resulting in a similar production. It was also found that proton-induced cross sections at 1 GeV/nucleon are similar to the
deuteron-induced ones at 500 MeV/nucleon for the light-mass products produced in the spallation
of 136 Xe [7].
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To demonstrate the difference between σd and σp , their ratio σd /σp is plotted in Fig. 5 as a function
of mass number. At both energies, the σd /σp ratio increases towards the neutron-deficient side
within an isotopic chain. Such a σd /σp increase could be attributed to a high energy deposition in the
deuteron-induced reactions [8]. It is noted that the σd /σp ratio increases faster towards the light-mass
products in the 118 MeV/nucleon results than those for 196 MeV/nucleon.
Calculations using the PHITS and geometric models [23] are displayed in Fig. 5 for comparison.
A constant σd /σp ratio around 1.1 is given by the geometric calculation for different isotopes. The
experimental results for the Z = Zproj and Z = Zproj − 1 products, which are produced in the
peripheral reactions, are similar to the geometric calculations. For the light-mass products, the ratios
are much larger than the geometric prediction. The PHITS calculations show good agreement with
the σd /σp systematics for both energies.
To evaluate the potential of spallation reaction on the transmutation of 107 Pd, the total cross
sections on proton and deuteron have been deduced by integrating the distributions in Figs. 2 and 3.
The respective integral cross sections for the proton and deuteron were determined to be 1016(15) mb
and 1135(14) mb at 196 MeV/nucleon, and 906(9) mb and 1085(8) mb at 118 MeV/nucleon. These
values are about one order of magnitude lower than the thermal neutron-capture cross section of
9.16 b [4].
Our experimental data give a design goal of proton/deuteron flux for the transmutation on 107 Pd
using the spallation reaction. Considering the cross-section difference between the spallation and
neutron-capture reactions mentioned above, a similar transmutation effect could be expected when
the proton/deuteron flux is one order of magnitude higher than that for neutron. At present, the
highest neutron flux is about 1015 s−1 cm−2 [24]. The available proton beam intensities at the highpower proton accelerators are 1.4 × 1016 pps at PSI (1.3 MW at 590 MeV) [25], 7.4 × 1015 pps
at SNS (1.1 MW at 940 MeV) [26], and 2.1 × 1015 pps at J-PARC (1 MW at 3 GeV) [27]. The
proton/deuteron flux depends on the technical design for the accelerator transmutation system, such
as the target system and the beam profile. When the proton/deuteron flux reaches 1016 s−1 cm−2 , the
spallation reaction could be an option for the transmutation of 107 Pd.
The number of radioactive nuclei with long half-lives created in the spallation reaction is important
to evaluate the reduction of radiotoxicities after the reaction. Among the reaction products, three longlived fission products 93 Zr, 94 Nb, and 99Tc are the main components of radioactivity following the
spallation of 107 Pd. Their production cross sections on proton are around 0.2 (extrapolation from the
systematics), 1, and 6 mb at 196 MeV/nucleon, respectively. Since the total reaction cross section of
107 Pd is around 1 barn, the spallation reaction will lead to a considerable reduction in the number of
LLFP elements. However, the respective half-lives of 93 Zr, 94 Nb, and 99Tc are 1.5 × 106 , 2.0 × 104 ,
and 2.1 × 105 years, which are shorter than for 107 Pd. It is, therefore, necessary to reduce their
production.
To reduce the production of the other long-lived radioactive nuclei, the spallation reaction induced
by protons at low reaction energy might have an advantage. Indeed, as shown in Fig. 4, the production
cross section for 99Tc at 118 MeV/nucleon is 40% smaller than at 196 MeV/nucleon. The difference
in the isotopic cross sections is larger than that in the total cross sections (13%). In other words,
the ratio between the production cross sections of 93 Zr, 94 Nb, and 99Tc and the total one becomes
smaller when the reaction energy decreases. Therefore, the spallation reaction at 118 MeV/nucleon
might be suitable for the transmutation of 107 Pd because of the low production of the other long-lived
radioactive nuclei. Concerning the transmutation cost, the production ratio of the other long-lived
nuclei has to be discussed in terms of the total kinetic energies of the proton/deuteron beams. The
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production ratio is 0.73% in the proton-induced reaction at 196 MeV, while the ratio is 0.68% for the
deuteron-induced reaction at 118 MeV/nucleon, namely at the total kinetic energy of 236 MeV. As
a larger production ratio is expected when the energy increases, our data suggest that the ratio with
the deuteron-induced reaction is lower than that for proton, even at the higher total kinetic energy.
It appears that the deuteron-induced reaction might be more promising to reduce the transmutation
cost while providing a small production ratio of the other LLFP nuclei. In order to further investigate
production of other long-lived nuclei, the performance of a study for both proton- and deuteroninduced reactions at lower energies is to be encouraged.
5. Summary In summary, spallation cross sections have been measured for the long-lived fission
product 107 Pd on proton and deuteron at 196 and 118 MeV/nucleon in inverse kinematics. The
experimental results are generally reproduced by the PHITS code including both the intranuclear
cascade and evaporation processes. The SPACS parameterization also shows good agreement with
the results of the proton-induced reactions at both energies. It was found that the production cross
sections for the light products are very similar between the reactions at 118 MeV/nucleon on deuteron
and the reactions at 196 MeV/nucleon on proton. Our data provide a design goal of proton/deuteron
flux for the transmutation of 107 Pd via the spallation reactions. In addition, it is found that the
productions of other long-lived radioactive nuclei can be reduced by choosing the low reaction
energy of 118 MeV/nucleon. Cross-section measurements at lower reaction energies are desired
for a comprehensive understanding of the energy dependence as well as for the application of
transmutation.
Acknowledgements
We express our gratitude to the accelerator staff of the RIKEN Nishina Center for providing the U beam. H.W.
acknowledges financial support from the Foreign Postdoctoral Researcher program of RIKEN. This work
was supported by the ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office,
Government of Japan).
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