Residual Nuclides Induced in Cu Target by a 250MeV Proton Beam

CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501
Residual Nuclides Induced in Cu Target by a 250 MeV Proton Beam
*
ZHANG Hong-Bin(张宏斌)1 , ZHANG Xue-Ying(张雪荧)1** , MA Fei(马飞)1 , JU Yong-Qin(鞠永芹)1 ,
GE Hong-Lin(葛红林)1 , CHEN Liang(陈亮)1 , ZHANG Yan-Bin(张艳斌)1 , WEI Ji-Fang(魏计房)2 ,
LI Yan-Yan(李严严)1,3 , LUO Peng(骆鹏)1 , WANG Jian-Guo(王建国)1 , WAN Bo(万波)1,3 ,
XU Xiao-Wei(许晓伟)1,3 , ZHOU Bin(周斌)4
1
2
Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000
North China Sea Environmental Monitoring Center, State Oceanic Administration, Qingdao 266033
3
University of Chinese Academy of Sciences, Beijing 100049
4
Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049
(Received 28 October 2014)
Residual nuclide production is studied experimentally by bombarding a Cu target with a 250 MeV proton beam.
The data are measured by the off-line 𝛾-spectroscopy method. Six nuclides are identified and their cross sections
are determined. The corresponding calculated results by the MCNPX and GEANT4 codes are compared with
the experimental data to check the validity of the codes. A comparison shows that the MCNPX simulation has a
better agreement with the experiment. The energy dependence of residual nuclide production is studied with the
aid of MCNPX simulation, and it is found that the mass yields for the nuclides in the light mass region increase
significantly with the proton energy.
PACS: 25.40.Sc, 25.70.Mn, 24.10.Lx
DOI: 10.1088/0256-307X/32/4/042501
High-energy and high-current proton accelerators
can be used in many fields, such as the environmental
sciences, medicine, and nuclear waste transmutation
and energy amplification.[1−3] Generally, uncontrolled
particle loss in an accelerator must be kept to a very
low level, while there is still no way to completely prevent the activation of the accelerator components. To
avoid suffering radiation damage, it is necessary to
estimate the radioactive products induced in the accelerator components by a high-energy proton beam.
A common approach to predict nuclide production
is based on a simulation with Monte-Carlo transport
codes, such as MCNPX,[4] PHITS,[5] GEANT4[6] and
FLUKA.[7] Several experimental studies on the production of radioactive nuclides in target irradiated by
protons have been carried out.[8−10] However, it is still
necessary to accumulate more experimental data to
check the applicability of all kinds of codes. Recently,
such accelerator facilities have been started or planned
to be built in China, including the China spallation
neutron source (CSNS) and the China initiative accelerator driven sub-critical system (CIADS). For the
safety and shielding of these facilities, we must have
good predictions for the activation production that
comes from the accelerator components, especially for
the copper that is a main element of accelerator components. It was thus decided to measure the activation
yields by irradiating a Cu target with a high energy
proton beam. In the present work the cross section
data for a Cu target are obtained by an off-line 𝛾spectrometry method, and they are compared with
the results simulated by MCNPX2.7.0 and GEANT4
codes.
Our irradiation experiment was performed at the
heavy ion research facility and cooling storage ring
(HIRFL-CSR)[11] in Lanzhou, China. Protons with
energy of 250 MeV were used to bombard a thin Cu
plate target. The proton beam spot was less than
20 mm in diameter at the target position. A schematic
view of the experimental setup is shown in Fig. 1. The
diameter and thickness of the Cu target were 10 cm
and 50 µm, respectively. A natural Pb cylinder with
100 mm diameter and 100 mm length was used as the
beam dump behind the Cu target. The primary protons were stopped in the Pb cylinder after passing
through the Cu target. An ionization chamber was
placed in front of the Cu target to monitor the beam
intensity. The high-purity Al foil, which was 100 mm
in diameter and 800 µm in thickness, was placed in
front of the ionization chamber to record the total
number of protons. The total number of protons in
the process of irradiation was given by activation analysis via the yield of the reaction 27 Al(p, 3p1n)24 Na.
The cross section has been reported to be 10.6 mb.[12]
The irradiation was performed for about 24 h to make
the beam fluence enough. A combination of the activity analysis of Al foil and the calibration of ionization
chamber enabled us to obtain the absolute beam current. The average beam intensity was 2.8 × 107 pps.
After the end of the irradiation experiment, the
𝛾 activities of the Cu target and Al foil were immediately measured with a high purity germanium (HPGe)
detector. The relative efficiency of the HPGe detector
was about 65% and energy resolution was 1.90 keV
at 1.33 MeV. The absolute efficiency was calibrated
with the standard sources 60 Co, 133 Ba, 137 Cs, and
152
Eu. The irradiated Cu sample was measured several times to detect the nuclides with different halflives. The gamma spectra, obtained in three measurements: 8.1 d, 56 d and 133 d after the end of irradia-
* Supported by the National Natural Science Foundation of China under Grant Nos 11305229, 11105186, 91226107 and 91026009,
and the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDA03030300.
** Corresponding author. Email: [email protected]
© 2015 Chinese Physical Society and IOP Publishing Ltd
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CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501
tion, are shown in Fig. 2.
Analysis of the gamma spectra and identification
of radionuclides were performed by the GAMMA-W
code,[13,14] which was applied to calculate the net 𝛾peak areas via an unfolding algorithm by using a leastsquare fit. Six radioactive nuclides were clearly recognized by different characteristic gamma rays from
measured gamma spectra, see Table 1. All of the
types of products are denoted as independent 𝐼, cumulative positron 𝐶 + or cumulative electron 𝐶 − .[1]
As listed in Table 2, the half-lives of these identified
nuclides vary from a few days to a few hundreds of
days, and no nuclide with short (or very long) halflife was found. There is also no light nuclide, such
as 7 Be (𝑇1/2 ≈ 53.12 D), to be found in the experiment, and all of the mass numbers of the measured nuclides are large and are not far from Cu (𝐴 ≈ 64 amu).
This means that the nuclides are produced dominantly
by the primary protons and then by the secondary
particles.[15]
Ionization chamber
Beam dump
Cu target
Al Foil
Proton beam
Fig. 1. A schematic view of the experimental setup.
103
(a)
102
0
10
103
tcount=2 h
tdecay=8.1d
(b)
57
Co
102
101
100
103
Cross sections (mb)
Number of counts
101
tcount=49h
tdecay=56d
(c)
102
101
1000
tcount=48h
tdecay=133d
2000
4000
6000
8000
54
Co
56
Mn
Co
51
Cr
52
Mn
Present work (250 MeV)
MCNPX (250 MeV)
GEANT4 (250 MeV)
Ref. [16] (200 MeV)
50
Channel number
58
51
52
53
54
55
56
57
58
59
Mass numbers
Fig. 2. Examples of gamma spectra at different times
after the end of irradiation.
Fig. 3. Comparison of the measured and calculated cross
sections of residual nuclides.
Table 1. Features of production and decay of measured radionuclides produced in Cu.
Residual nuclides
51 Cr
Type of yield
𝐶+
Half-life (d)
27.702
52 Mn
𝐼
5.591
54 Mn
𝐼
312.12
56 Co
𝐶+
77.27
57 Co
𝐶+
271.79
58 Co
𝐼
70.82
The peak level of the gamma decay spectrum is
associated with the decay of the nuclide during the
time of irradiation, cooling, and measurement, thus
the total counts 𝐶 of gamma-ray peak area could be
expressed as
Gamma-ray energy (keV)
320.08
744.23
935.54
1434.07
834.84
846.76
1238.27
122.06
136.47
810.77
For each nuclide, the yield 𝑁 (0) (i.e., number of
activated nuclei) at the end of irradiation could be
determined according to the relation
∫︁
𝑡irr
𝑁 (0) =
𝐶 = (𝑁 (𝑡d ) − 𝑁 (𝑡d + 𝑡c ))𝜖𝛾 𝐼𝛾
= 𝑁 (0)𝑒−𝜆𝑡d (1 − 𝑒−𝜆𝑡c )𝜖𝛾 𝐼𝛾 ,
Branching ratio(%)
9.86
90.60
94.50
100
99.98
99.93
66.07
85.60
10.68
99.45
(Φ𝜎𝑁𝑡 𝐷𝑒−𝜆𝑡 )d𝑡
𝑡0
(1)
where 𝑡d is cooling time (i.e., the time from the end of
irradiation to the beginning of the measurement), 𝑡c is
the time of measurement, 𝜖𝛾 is the detector efficiency,
𝐼𝛾 is the intensity of the 𝛾 transition per decay, and
𝜆 = ln 2/𝑇1/2 is the disintegration constant.
=
Φ𝜎𝑁𝑡 𝐷
(1 − 𝑒−𝜆𝑡irr ),
𝜆
(2)
where the parameter Φ is the average beam intensity,
𝑁𝑡 is the atomic density of the Cu foil, 𝐷 is the thickness of the Cu foil, and 𝑡irr is the irradiation time.
Combining Eqs. (1) and (2), the cross section of
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CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501
radioactive nuclide can be derived as follows:
𝜎=
𝐶𝜆𝑒𝜆𝑡d
.
𝐼𝛾 𝜖𝛾 Φ𝑁𝑡 𝐷(1 − 𝑒−𝜆𝑡c )(1 − 𝑒−𝜆𝑡irr )
(3)
According to Eq. (3), the cross sections of the six
radioactive nuclides are determined, and the specific
results are given in the experimental column of Table
2. Errors which include the systematic and statistic
uncertainties are in the range 8.5–20.5%. The similar work by Titarenko et al.[16] at a proton-energy
of 200 MeV is reasonably in agreement with our work
with the exception of 57 Co and 58 Co, which deviates
from ours by nearly 50%, see Fig. 3.
Table 2. Cross sections of residual nuclides from experiment and simulation by MCNPX and GEANT4.
Residual nuclides
Half-life
47 Ca
4.536 day
330 day
1.4×1017 year
−a
27.702 day
−
−
5.591 day
3.74×106 year
312.12 day
−
−
2.73year
−
−
−
77.27 day
271.79 day
70.82 day
−
5.271 year
−
7.6×104 year
−
−
−
101.1 year
−
3.333 hour
9.74 minute
12.7 hour
49 V
50 V
50 Cr
51 Cr
52 Cr
53 Cr
52 Mn
53 Mn
54 Mn
55 Mn
54 Fe
55 Fe
56 Fe
57 Fe
58 Fe
56 Co
57 Co
58 Co
59 Co
60 Co
58 Ni
59 Ni
60 Ni
61 Ni
62 Ni
63 Ni
64 Ni
61 Cu
62 Cu
64 Cu
a Here
Experiment
12.43±2.55
4.00±0.57
15.80±2.50
10.09±1.37
24.25±2.11
27.30±2.57
Cross sections (mb)
MCNPX
1.63±0.20
3.94±0.31
3.57±0.29
5.29±0.35
8.24±0.44
12.73±0.55
3.45±0.29
5.53±0.36
15.18±0.59
17.60±0.64
7.61±0.42
10.44±0.50
24.59±0.76
30.92±0.85
11.05±0.51
5.08±0.35
11.00±0.51
30.90±0.85
33.80±0.89
26.60±0.79
10.26±0.47
15.28±0.60
36.16±0.93
64.18±1.23
34.18±0.90
39.07±0.96
11.91±0.36
10.24±0.49
28.82±0.82
66.81±1.26
28.81±0.76
GEANT4
3.17±0.27
7.56±0.42
7.11±0.41
7.94±0.43
15.12±0.60
19.06±0.67
4.98±0.34
12.00±0.53
27.28±0.80
21.44±0.71
8.53±0.45
19.56±0.70
33.54±0.89
29.72±0.84
10.28±0.49
5.03±0.34
20.95±0.70
41.60±0.99
35.76±0.92
20.93±0.70
6.07±0.38
20.50±0.70
37.98±0.95
56.83±1.16
35.05±0.91
38.13±0.95
6.17±0.38
7.35±0.42
20.50±0.70
53.24±1.12
21.00±0.70
−− stands for stable nuclides.
The Monte Carlo simulations for the production of residual products in the Cu target were performed by the MCNPX code 2.7.0[4] and GEANT4
code.[6,17] In the MCNPX simulation, the Bertini,[18]
pre-equilibrium and RAL[19] models were used in
the intra-nuclear cascade (INC) stage, pre-equilibrium
stage, and in the process of evaporation, respectively. The binary cascade model[17] was chosen in
the GEANT4 simulation.
In Table 2, we list the major nuclides with large
cross sections (𝜎 > 1 mb) calculated by MCNPX
and GEANT4 codes. The calculation only considers
the direct production of the certain nuclide from the
proton-induced reaction. For the experimental results,
52
Mn, 54 Mn and 58 Co are independent nuclides which
are directly produced in the reaction. The nuclides
of 51 Cr, 56 Co and 57 Co, except for the direct proton
induced production, may also come from the decay
of 51 Mn, 56 Ni and 57 Ni, respectively. However, these
mother nuclides are not observed in Table 2, which
means that their calculated cross sections are less than
1 mb. Therefore, we conclude that the decay contribution from mother nuclides could be negligible. Except
for the nuclides which were identified in experiment,
the rest of the nuclides can be divided into three categories: stable nuclides, short-lived radionuclides, and
long-lived ones. They are all unable or very difficult
to be detected, and this is why only six radionuclides
were measured when there was not enough beam intensity in our experiment. A comparison between the
measured and calculated cross sections of residual nuclides is shown in Fig. 3. It is obvious that the crosssection values simulated by MCNPX are more consistent with the experimental data, where the maximum
difference is less than 35%. For the GEANT4 simulation, however, the calculated results are greater than
the experimental data, especially for 52 Mn production. It is obviously found that the MCNPX simulation could give better predictions on residual nuclide
production than GEANT4 code. We also compare the
simulated cross sections as functions of mass number
and charge number between MCNPX with GEANT4,
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CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501
Cross sections (mb)
Cross sections (mb)
as shown in Fig. 4. It can be found that the values
calculated by GEANT4 are higher overall than the results by MCNPX, while they both have similar trends
of cross sections with the increasing mass number or
charge number.
102
101
100
10-1
10-235
(a)
MCNPX
GEANT4
40
102
101
100
10-1
10-2 18
45
50
55
60
Mass numbers
65
(b)
MCNPX
GEANT4
20
22
24
Charge numbers
26
28
30
Cross sections (mb)
Cross sections (mb)
Fig. 4. Production distribution of the calculated products
as functions of (a) mass number and (b) charge number.
102
101
100
10-1
10-2 20
(a)
102
101
100
10-1
10-2
30
40
50
1200 MeV
800 MeV
400 MeV
250 MeV
60
Mass numbers
References
70
(b)
10
15
20
Charge numbers
25
the lighter nuclides in that the yields increase strongly
with the projectile energy. This means that we must
evaluate the contribution of the production of light
nuclides as the energy of the proton beam increases.
All of the above calculations have considered the
contribution from the thick Pb cylinder (i.e., beam
dump) and lab setup. The residual nuclide production, regardless of the interference of the thick Pb
cylinder, is also calculated. The results show that the
contribution of secondary particle from beam dump
could be negligible.
In summary, we have studied the production of
residual nuclides via the reaction of 250 MeV protons bombarding Cu foil. Six radioactive nuclides
are identified in our experiment by using the off-line
𝛾-spectrometry method. The experimental data are
compared with Monte Carlo simulations performed by
the MCNPX and GEANT4 codes, and the comparison suggests that MCNPX could give good predictions
on the productions of the residual nuclides. We also
calculate the cross sections of unmeasured nuclides to
obtain the mass-yield distribution. Finally, we study
the energy dependence of residual nuclide production
in a Cu target by using the MCNPX code. The simulation shows that the light nuclides would play a more
important role as the energy of proton beam increases.
We gratefully acknowledge the support and assistance of the accelerator operation staff at HIRFLCSR.
1200 MeV
800 MeV
400 MeV
250 MeV
30
Fig. 5. Comparison of the mass-yield distribution trends
in different proton energies.
Considering the good agreement between the measured data with the calculated results from MCNPX2.7.0, we try to study the energy dependence of
proton-induced nuclides of the Cu target on the basis
of the MCNPX simulation. Figure 5 shows the massyield distributions for calculated residual products induced in the Cu target by protons with energies of 250,
400, 800, and 1200 MeV, respectively. For each energy
point, as shown in Fig. 5, the yields of the residual
products increase with the mass number. However,
the distributions become flatter and the yields of light
fragments are enhanced with the increase of the proton energy. Furthermore, for the products in the mass
region with 𝑍 ≥ 25 and 𝐴 ≥ 50, the cross sections are
almost equal and do not depend strongly on the proton energy. In contrast, a clear trend is observed for
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