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Experimental study and simulation of the residual activity induced by high-energy argon ions in copper
Nuclear Instruments and Methods in Physics Research B 268 (2010) 573–580
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Experimental study and simulation of the residual activity induced by high-energy argon ions in copper I. Strašík a,b,*, E. Mustafin a, T. Seidl a, M. Pavlovicˇ b a b
GSI Darmstadt, Planckstrasse 1, D-64291 Darmstadt, Germany FEI STU, Ilkovicˇova 3, SK-812 19 Bratislava, Slovak Republic
a r t i c l e
i n f o
Article history: Received 26 May 2009 Received in revised form 20 November 2009 Available online 11 December 2009 Keywords: Residual activity FAIR FLUKA Gamma-ray spectroscopy
a b s t r a c t The paper presents new experimental results and FLUKA-simulations of residual activation induced by high-energy argon ions in copper. It follows the previous residual activation studies performed at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt with uranium ions as a preparatory work for constructing the FAIR facility. Copper samples were irradiated by 1 GeV/u and 500 MeV/u 40Ar ions and investigated by gamma-ray spectroscopy. The samples were irradiated in the stacked-foil geometry. The isotopes with dominating contribution to the total residual activity were identified and their partial activities were quantified. Depth-profiling of the partial residual activities of all identified isotopes was performed by measurements of individual target foils. The experimental results were compared with simulations by the FLUKA-code. A satisfactory agreement between the experiment and the simulations was observed. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Activation of accelerator components due to beam losses is an important issue for existing (LHC/CERN, RHIC/BNL, and SNS/ORNL) and planned (FAIR/GSI) high-energy hadron facilities. Residual activity induced by lost beam-particles may become a main source of exposure to personnel and a serious access-restriction for ‘‘hands-on” maintenance [1,2]. In the frame of the FAIR project (Facility for Antiproton and Ion Research) [3,4], extensive experimental studies [5–7] and computer simulations [8–11] of the residual activity induced by high-energy heavy ions in copper and stainless steel are in progress at GSI Darmstadt. Copper and stainless steel have been chosen as the representatives of the most common materials for accelerator structures. The computer simulations were performed for several ion species [8,9], but the experiments have been so far focused mostly on uranium ions [5,6]. Whereas the computer simulations can be in principle run for any ion species, activation experiments are much more demanding from the both points of view: beam availability as well as analysis of the irradiated samples. It is practically impossible to carry out irradiation experiments for all ions of interest at various energies. The computer simulations are the only tool to provide this information. Various simulation codes like FLUKA [12,13], GEANT4 [14,15], SHIELD [16], MCNPX [17], PHITS [18], * Corresponding author. Address: GSI Darmstadt, Planckstrasse 1, D-64291 Darmstadt, Germany. E-mail address: [email protected] (I. Strašík). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.12.004
MARS [19] etc., are available. However, the computer simulations must be verified by experimental data and more check-points are necessary to validate the codes. That is why the irradiation experiments with uranium ions have recently been completed by another experiment with 1 GeV/u and 500 MeV/u argon ions. The irradiated targets were analysed by gamma-ray spectroscopy and depth-profiling of the partial residual activities of all identified isotopes was performed by measuring the activities of individual target foils. The obtained experimental data were compared with FLUKA-simulations. The activation process is very complex. This is true especially for activation induced by heavy-ion beams. The radioactive nuclides are produced by nuclear reactions induced by primary ions (projectiles) as well as by secondary particles, mostly neutrons and protons, generated by interaction of the primary beam with the target material. Among the secondary particles, neutrons are most difficult to be taken into account because their contribution is not well localized and extends far beyond the region where the primary particles stop. On top of that, the projectiles are fragmented into many radioactive projectile fragments that remain implanted in the target. However, their contribution to the total residual activity is negligible for high-energy projectiles [5,6]. Generally, the residual activity depends on the amount, energy and mass of the lost particles as well as on the target composition. A summary of the radio-nuclides identified in common materials for accelerator structures irradiated by high-energy charged particles is presented in Ref. [20]. Understanding of the activation process provides fundamental information that can be used in two
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ways: (1) to specify the tolerable beam losses in the machine and (2) to optimize the choice of construction materials. Both measures are important with respect to the reduction of personnel exposure. As far as the tolerable beam losses are concerned, the losses of 1 W/ m are presently accepted for high-energy proton machines as a threshold for ‘‘hands-on” maintenance [21]. Tolerances for heavyion accelerators can then be specified by scaling the 1 W/m criterion for proton machines [8,9,22,23]. However, the scaling factors have to be obtained again by computer simulations, which brings back the necessity to validate the simulation codes and to collect experimental data for various ion species at various energies.
Table 2 Geometrical configuration of the copper target T2 irradiated by 500 MeV/u argon beam – foil number and its thickness. The target consisted of 15 foils. The overall target thickness was 46.63 mm. The foil thickness was measured by an electronic micrometer in several foil points. Average values are given in the table. The 0.5 mm foils were used for gamma-spectroscopy measurements of the depth profiles of residual activity. The foils thicker than 0.5 mm defined the distance between the sampling-points of the depth profile. Foil Foil Foil Foil Foil Foil
number thickness (mm) number thickness (mm) number thickness (mm)
1 0.50 8 0.50 15 0.50
2 7.02 9 0.50
3 0.50 10 7.03
4 7.02 11 0.50
5 0.50 12 7.04
6 6.99 13 0.50
7 0.50 14 7.03
2. Experiment and method 2.1. Targets and irradiation conditions Three targets assigned as T0, T1 and T2 were used in the experiment for different purposes: target T0 for assessment of the timeevolution of the total residual activity, and targets T1 and T2 for depth-profiling of the residual activity. The target T0 was irradiated by 1 GeV/u argon beam. The targets T1 and T2 were irradiated by 1 GeV/u and 500 MeV/u argon beam, respectively. The target material was 99.9% natural copper (q = 8.96 g/cm3 at 20 °C). The target T0 was a single circular foil 1.00 mm thick, 50 mm in diameter. The targets T1 and T2 were cylinders assembled from many individual foils of various thicknesses, 50 mm in diameter (see Tables 1 and 2 for the target T1 and T2, respectively). The thinner foils were placed in the region of the range of primary ions that was estimated by ATIMA version 1.2 [24], SRIM2008 [25] and FLUKA version 2008.3.6 [12,13]. The calculated values of the range are collected in Table 3. Further details concerning the range estimation can be found in Ref. [26]. The overall thickness of the targets was about two times the estimated range of the primary ions. Argon beam from the SIS-18 synchrotron at the GSI Darmstadt was used to irradiate the targets. The targets were irradiated in the fast-extraction regime with cycle-duration of 3 s. The beam spotsize was about 2 cm in horizontal plane and 2 cm in vertical plane (checked visually on a scintillation screen before irradiation and measured by a profile-meter). The beam profile was approximately Gaussian according to the profile-meter. The beam intensity was monitored by a current transformer [27]. Application software recorded the irradiation history and summed-up the overall charge delivered to the target, which was recalculated to the total number of ions. The target T0 was irradiated by 1.10 1012 argon ions. The targets T1 and T2 were irradiated by 1.12 1012 and 2.16 1012 argon ions, respectively. 2.2. Measurement of the gamma spectra After irradiation, the gamma spectra of individual targets foils were measured. Target T0 (1 mm thick foil) was measured repeat-
Table 1 Geometrical configuration of the copper target T1 irradiated by 1 GeV/u argon beam – foil number and its thickness. The target consisted of 21 foils. The overall target thickness was 126.40 mm. The foil thickness was measured by an electronic micrometer in several foil points. Average values are given in the table. The 0.5 mm foils were used for gamma-spectroscopy measurements of the depth profiles of residual activity. The foils thicker than 0.5 mm defined the distance between the sampling-points of the depth profile. Foil Foil Foil Foil Foil Foil
number thickness (mm) number thickness (mm) number thickness (mm)
1 0.50 8 14.99 15 0.50
2 15.00 9 0.50 16 14.98
3 0.50 10 0.50 17 0.50
4 15.00 11 0.50 18 14.98
5 0.50 12 0.50 19 0.50
6 14.99 13 0.50 20 14.97
7 0.50 14 14.99 21 0.50
edly 24 times. The first measurement started 40 min and the last measurement finished 4 h and 30 min after the end of irradiation. The live-time of each measurement lasted 300 s and was followed by a 5-min break before the next measurement started. The detector faced the back-side of the target, i.e., the side that was not faced to the primary beam. The target-to-detector distance was 10 cm. The total number of counts in the corresponding channel of the spectrometer was recorded at each measurement. Target T1 was measured two times: 6–26 and 34–48 days after the end of irradiation. The measurement time varied from 5 h up to 70 h per foil. Target T2 was measured also two times: 13–28 and 38–51 days after the end of irradiation. The measurement time varied from 5 h up to 44 h per foil. The target-to-detector distance was 7 cm for both targets. The gamma spectra from the targets T1 and T2 were used for depth-profiling of the partial residual activities of all identified isotopes. The gamma-ray spectroscopy measurements were carried out with a Canberra HPGe-GC3518 detector and Ortec GEM-25P4 detector coupled to Silena multi-channel analysers (8192-channels). Both detectors were powered by an Ortec HV-supply. This set-up allowed measuring gamma spectra from 30 keV up to 3 MeV. For the energy and efficiency calibration, a standard set of calibration sources was used: 22Na, 60Co, 137Cs, 152Eu, 210Pb and 241 Am. Data-acquisition and handling of spectra were controlled by the WINGAM software package. The spectra were analysed by the Genie2000 software package. 3. Time-evolution of the total residual activity The purpose of the target T0 irradiation was to validate the FLUKA-simulation of the time-evolution of the total residual activity after the end of irradiation. The measurement started as early as the target activity dropped below a tolerable level (40 min after the end of irradiation) in order to take into account the contribution from short-lived isotopes. Because the gamma spectra measured very shortly after the end of irradiation are extremely complex [5,6,28], the isotope identification and determination of their partial activities was not considered. Instead of that, the average count-rate registered by the detector was taken from the gamma spectrum. The average count-rate was obtained as the total number of counts divided by the measurement time. Subsequently, the count-rate was converted into the photon fluence rate using the energy and efficiency calibration curves of the detection chain. This simplified approach overestimates the photon fluence rate, because the Compton signal cannot be properly treated. Exact conversion could only be based on the detector response operator method [29]. The irradiation conditions of the target T0 were simulated by FLUKA. The simulation was performed with 106 particles and 20 cycles. For the comparison with the experiment, only the photons in the energy range from 30 keV to 3 MeV passing through a circu-
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I. Strašík et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 573–580 Table 3 The range including the range straggling of 1 GeV/u and 500 MeV/u ATIMA
1 GeV/u 500 MeV/u
40
Ar18+ ions in copper calculated by different computer codes. SRIM
FLUKA
Range (mm)
Straggling (mm)
Range (mm)
Straggling (mm)
Range (mm)
Straggling (mm)
63.35 22.91
0.10 0.04
63.93 22.98
2.21 0.83
65.27 23.51
0.41 0.15
Fig. 1. Comparison of the time-evolution of the photon fluence rate based on the gamma-spectroscopic measurements and FLUKA-simulation together with the residual activity simulated by FLUKA.
lar area 7 cm in diameter at the distance of 10 cm from the target were considered. This corresponds to the energy range of the detection chain and measurement geometry. The photon fluence rates based on the count-rate measurement and on the FLUKAsimulation as a function of time are shown in Fig. 1 together with the simulated residual activity. As expected, the photon fluence rate based on the count-rate measurement is higher compared to the FLUKA-simulation, but the discrepancy is not dramatic. This qualitative agreement indicates that the FLUKA-simulation is reasonably close to reality. 4. Residual activities 4.1. Isotope identification and activity measurements Results of isotope identification and their partial activities are summarized in Tables 4 and 5 for the target irradiated by 1 GeV/ u and 500 MeV/u argon beam, respectively. Activities are normalized per one incident ion. The isotope identification was based on the energy and abundance of the gamma lines, half-life of the isotopes as well as on the experience from the previous experiments [5,6]. The isotope characteristics were taken from the WWW Table of Radioactive Isotopes [30]. In problematic cases of isotope identification, supporting information was gained from the depth profiles of residual activity [6]. For all identified isotopes, the activity for each foil was obtained from the peak-net-areas (PNA) calculated by Genie2000 including the standard uncertainty of the PNA. The measured activity was then extrapolated backwards in time to the end of the irradiation using the characteristic decay constant of a given isotope and converted into the activity per unit length. Because the measured foils were thin enough, no correction for self-absorption was necessary and the activity per unit length could be obtained as the activity of the foil divided by its thickness. Finally, the activity of each isotope, i.e., its partial activity with respect to the total target activity cor-
responding to the sum of all isotopes, was obtained by numerical integration of its depth profile using a trapezoidal method. Generally, the activation products have several energy lines in the spectra. Not all of them are included in Tables 4 and 5. The most pronounced energy lines or lines without an interference with other isotopes were chosen for activity determination. Determination of the activity of short-lived isotopes was not possible because their activity decreased below minimum detectable activity (MDA, the Curie’s concept with confidence level at 95%) before the measurement of all discs was completed. For this reason, their depth profiles could not be obtained in full and the corresponding activities are not indicated in the Tables although the presence of the isotopes was identified. Determination of the activities of 22 Na from the spectra measured 6–26 days and 13–28 days after the end of irradiation was not reliable. A large number of peaks belonging to short-lived isotopes with energy-separation below the resolution of the spectroscopy apparatus created an ‘‘apparent” background that was much higher than the true background at the measurement place [5,6]. The activity of some isotopes of interest may get below the minimum detectable activity in this case. The apparent background decreases with time as the short-lived isotopes decay. That is why the determination of the 22Na-activity became possible from the spectra measured 34–48 and 38–51 days after the end of irradiation. In opposite to this, some isotopes that could be quantified in the early-measured spectra, could not be quantified in the later measured spectra, because they decayed below the MDA level. 4.2. Depth profiles of the partial residual activities Partial activities of individual isotopes were calculated by integration of their depth profiles. Figs. 2 and 3 show typical depth profiles of the residual activity for the target-activation products, namely 7Be, 46Sc, 51Cr, 54Mn, 56Co and 60Co identified in the spectra of the targets irradiated by 1 GeV/u and 500 MeV/u argon ions, respectively. The profiles were obtained from the spectra measured 6–26 days (1 GeV/u) and 13–28 days (500 MeV/u) after the end of irradiation. It can be seen that the profiles extend deeply beyond the range of primary ions and decrease modestly with depth. This is mainly due to large amount of secondary neutrons, protons and light fragments that have the range much longer compared to the range of the primary ions. Depth profiles of 7Be are almost identical for both energies. The profiles of heavier isotopes depend on the primary beam energy. They start from the same activity in the close-to-surface target region, but in the range-region, the 1 GeV/u argon beam induces two to three times higher activity per one incident ion compared to the 500 MeV/u beam. This indicates the important role of the secondary particles in the activation process. In most cases, the profiles obtained from the spectra measured in the first (earlier) and the second (later) set of measurements are well consistent, which indicates that no significant contribution from an isotope with different half-life is present. In the case of 60 Co, there is an interference with 56Co (1173 keV gamma line) and with 52Mn (1333 keV gamma line). The contribution of the interfering isotopes can be taken into account by calculation of their contribution to the net-peak-area corresponding to their
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Table 4 Identified isotopes and their activities in the copper target irradiated by 1 GeV/u argon beam. A1, A2 – activity, u1, u2 – relative combined standard uncertainty comprising uncertainty of the beam intensity and uncertainty of the partial activity. Activities are extrapolated to the end of irradiation. Subscript ‘‘1” is related to the spectra measured 6– 26 days after the end of irradiation, subscript ‘‘2” is related to the spectra measured 34–48 days after the end of irradiation. The Curie’s concept with confidence level at 95% implemented in Genie2000 was applied for MDA. Isotope
Half-live
Energy (keV)
A1 (Bq/ion)
u1 (%)
A2 (Bq/ion)
u2 (%)
7
Be 22 Na 43 K
53.12 d 7 2.6019 y 4 22.3 h 1
8.90E-09 1.07E-10
2.32 8.19
3.927 h 8 58.6 h 1 83.79 d 4
8.72E-09 Below MDA Below MDA Below MDA Below MDA Below MDA 5.37E-09 5.42E-09 7.14E-08
1.74
44
477.595 1274.53 2 372.760 617.490 1157.031 271.13 889.277 3 1120.545 4 159.377 12 944.104 7 983.517 5 1312.096 6 320.0824 4 744.233 13 935.538 11 1246.278 15 1333.649 17 1434.068 14 834.848 3 1099.251 4 1291.596 7 477.2 2 931.3 2 1408.4 2 846.771 5 1037.840 6 1238.282 7 1771.351 16 2034.755 13 122.0614 4 136.4743 5 810.775 9 1173.237 4 1332.501 5 1377.63 3 1115.546 4
0.81 0.82 2.42
5.50E-09 5.26E-09 Below MDA
0.93 0.97
4.97E-08 4.94E-08 7.77E-08 1.33E-07 1.31E-07
0.70 0.71 0.75 0.74 0.74
4.94E-08 4.96E-08 7.66E-08 Below MDA Below MDA
0.80 0.82 0.89
7.68E-09 7.29E-09 7.34E-09 Below MDA Below MDA Below MDA 1.40E-08
0.75 0.92 0.97
7.67E-09 7.47E-09 7.46E-09
0.80 1.17 1.30
0.72
1.40E-08
0.77
1.32E-08
0.76
1.35E-08
0.85
1.31E-08 1.34E-08 7.42E-08 1.09E-09 1.16E-09 Below MDA 6.53E-10
1.90 1.97 0.68 1.36 0.02
1.39E-08 1.39E-08 7.52E-08 1.05E-09 1.10E-09
1.91 1.99 0.69 1.54 1.58
3.29
5.43E-10
4.63
Sc Sc Sc
44m 46
47
Sc V
3.3492 d 6 15.9735 d 25
48
51
Cr Mn
27.7025 d 24 5.591 d 3
52
54
Mn Fe
312.3 d 4 44.503 d 6
59
55
17.53 h 3
56
77.27 d 3
Co
Co
57
271.79 d 9
58
70.86 d 7 5.2714 y 5
Co
Co Co
60
57
Ni Zn
35.60 h 6 244.26 d 26
65
activity obtained from other gamma lines of 56Co and 52Mn. This contribution can then be subtracted from the total net-peak-area to get the net-peak-area belonging to 60Co. In Figs. 2 and 3, the 60 Co-profiles are presented without the contribution of 56Co and 52 Mn.
The standard uncertainty of the total number of ions delivered to the target, u(I), is calculated using the formula:
4.3. Uncertainty assessment
where u(ni) is the standard uncertainty of the number of ions in the ith pulse. The relative standard uncertainty of the total number of ions delivered to the target is then expressed as
The uncertainties were analysed according to the ISO Guide to the Expression of Uncertainty in Measurement (GUM) [31]. The relative combined standard uncertainty of the data presented in Tables 4 and 5 comprises two components: (1) uncertainty of the beam intensity and (2) uncertainty of the partial activity. 4.3.1. Uncertainty of the beam intensity The relative standard uncertainty of the number of ions per beam pulse measured by the current transformer is assumed to be less than 3%. The total number of the pulses during irradiation was 79 and 159 in case of 1 GeV/u beam and 500 MeV/u beam, respectively. The total number of ions delivered to the target was obtained as a sum of the number of ions in individual pulses:
I¼
N X
ni ;
ð1Þ
i¼1
where I is the total number of ions delivered to the target, ni is the number of ions in the ith pulse and N is the total number of pulses.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N 2 u N uX @I uX uðIÞ ¼ t uðni Þ2 ¼ t uðni Þ2 ; @n i i¼1 i¼1
uðIÞ½% ¼
uðIÞ 100%: I
ð2Þ
ð3Þ
The relative standard uncertainty of the total number of ions delivered to the target is 0.35% for 1 GeV/u beam and 0.24% for 500 MeV/u beam. 4.3.2. Uncertainty of the partial activity There are several major sources of the uncertainty of the partial activity, u(A). The sources listed below were included in our uncertainty analysis. 4.3.2.1. Uncertainty of the net-peak-area (NPA). The uncertainty of the net-peak-area depends on the statistics in the peak (i.e., on the count-rate). The relative standard uncertainty of the net-peak-area ranged from 0.24% for high count-rate to 37.75% for very low count-rate. The relative standard uncertainty of the net-peak-area
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Table 5 Identified isotopes and their activities in the copper target irradiated by 500 MeV/u argon beam. A1, A2 – activity, u1, u2 – relative combined standard uncertainty comprising uncertainty of the beam intensity and uncertainty of the partial activity. Activities are extrapolated to the end of irradiation. Subscript ‘‘1” is related to the spectra measured 13– 28 days after the end of irradiation, subscript ‘‘2” is related to the spectra measured 38–51 days after the end of irradiation. The Curie’s concept with confidence level at 95% implemented in Genie2000 was applied for MDA. Isotope
Half-live
Energy (keV)
A1 (Bq/ion)
u1 (%)
A2 (Bq/ion)
u2 (%)
7
Be 22 Na 44 Sc 44m Sc 46 Sc
53.12 d 7 2.6019 y 4 3.927 h 8 58.6 h 1 83.79 d 4
2.92E-09 3.05E-11
1.57 6.72
0.96 1.01 3.40
7.52E-10 7.20E-10 Below MDA
1.06 1.12
3.3492 d 6 15.9735 d 25
2.98E-09 Below MDA Below MDA Below MDA 7.33E-10 7.22E-10 1.03E-08
1.37
47
477.595 1274.53 2 1157.031 271.13 889.277 3 1120.545 4 159.377 12 944.104 7 983.517 5 1312.096 6 320.0824 4 744.233 13 935.538 11 1246.278 15 1333.649 17 1434.068 14 834.848 3 1099.251 4 1291.596 7 846.771 5 1037.840 6 1238.282 7 1771.351 16 2034.755 13 122.0614 4 136.4743 5 810.775 9 1173.237 4 1332.501 5 1115.546 4
6.96E-09 6.90E-09 1.16E-08 2.14E-08 2.09E-08
0.75 0.77 0.83 0.82 0.83
7.04E-09 6.90E-09 1.17E-08 Below MDA Below MDA
0.89 0.92 0.99
1.25E-09 1.24E-09 1.22E-09 2.48E-09
0.81 1.06 1.14 0.76
1.27E-09 1.24E-09 1.19E-09 2.47E-09
0.84 1.27 1.43 0.80
2.34E-09
0.82
2.41E-09
0.89
2.39E-09 2.43E-09 1.35E-08 2.03E-10 2.06E-10 1.90E-10
2.13 2.20 0.70 1.49 1.28 2.77
2.47E-09 2.46E-09 1.35E-08 2.04E-10 2.09E-10 1.77E-10
2.14 2.21 0.71 1.57 1.63 3.14
Sc V
48
51
Cr Mn
52
54
Mn Fe
59
56
Co
27.7025 d 24 5.591 d 3
312.3 d 4 44.503 d 6 77.27 d 3
57
271.79 d 9
58
70.86 d 7 5.2714 y 5
Co
60
Co Co
65
Zn
244.26 d 26
Fig. 2. Activity depth profiles of 7Be, 46Sc, 51Cr, 54Mn, 56Co, 60Co identified in the spectra of the copper target irradiated by 1 GeV/u argon beam measured 6–26 days after the end of irradiation.
is indicated directly by Genie2000. Its quantification is described in details in Genie2000 Customization Tools Manual. 4.3.2.2. Uncertainty of the efficiency calibration. The uncertainty of the efficiency calibration includes: (1) the uncertainty of the netpeak-area used to determine the efficiency, (2) uncertainty of fitting of the efficiency curve and (3) the uncertainty of the calibration source activities. The relative standard uncertainty of the net-peak-area used to determine the efficiency was less than 0.57%. The relative standard uncertainty of the fitting of the efficiency curve is 1% for gamma-ray energies above 200 keV and 7% for gamma-ray energies below 200 keV. There is a lack of calibration data-points below 200 keV, which increases the uncertainty
Fig. 3. Activity depth profiles of 7Be, 46Sc, 51Cr, 54Mn, 56Co, 60Co identified in the spectra of the copper target irradiated by 500 MeV/u argon beam measured 13– 28 days after the end of irradiation.
in this low-energy region. The above values are also indicated by Genie2000. The activities of the calibration sources have a certified relative standard uncertainty less than 2%. 4.3.2.3. Uncertainty of the half-lives. Uncertainties of the half-lives are indicated in Tables 4 and 5 (in italics). The relative standard uncertainty of the half-lives for all pertinent isotopes was less than 0.13%. 4.3.2.4. Uncertainty of the gamma-photon abundances. The relative standard uncertainty of the gamma-photon abundances for all pertinent energies was less than 0.75%.
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4.3.2.5. Uncertainty in the integration of the depth profiles. The partial activity for each isotope was obtained by integration of the corresponding depth profile using a trapezoidal method. The integration was performed numerically as a sum of contributing activities Pi:
A¼
N X
Pi ;
ð4Þ
i¼1
Pi ¼
d ðAi þ Aiþ1 Þ; 2
ð5Þ
where A is the partial residual activity of the isotope, Ai is the activity per unit length in the ‘‘i” point of the depth profile and d is the distance between the ‘‘i” and ‘‘i + 1” points of the depth profile. We assume that uncertainty of the distance d is considerably lower than the uncertainty of the activity. For this reason the expression for the calculation of the uncertainty of the activity Pi can be simplified as
d uðPi Þ ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uðAi Þ2 þ uðAiþ1 Þ2 ;
ð6Þ
where u(Ai) is the standard uncertainty of the activity per unit length in the ‘‘i” point of the depth profile. u(Ai) includes all the above listed sources of the uncertainty. Uncertainty of the foil thickness was neglected. The standard uncertainty of the partial activity is then obtained as
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX uðAÞ ¼ t uðPi Þ2 ;
Fig. 4. Ratio AE/AS for the isotopes identified in the spectra of the copper target irradiated by 1 GeV/u 40Ar beam. AE is the experimentally measured activity and AS is the activity calculated by FLUKA.
chains), it is difficult to be taken into account in measured data. The comparison-results for partial residual activities of individual isotopes are presented in Figs. 4 and 5 for 1 GeV/u and 500 MeV/ u, respectively. The discrepancies vary from factor of 0.67 to 1.53 for the 1 GeV/u experiment and from 0.71 to 1.70 for the 500 MeV/u experiment. In general, the activity-ratios are similar for both energies except for 7Be. In this case, the activity-ratio is about 0.96 at 1 GeV/u and almost 1.60 at 500 MeV/u.
ð7Þ
i¼1
and the relative standard uncertainty as
uðAÞ½% ¼
uðAÞ 100%: A
ð8Þ
The same procedure has been applied in the previous experiments with uranium beam [6]. 4.3.3. Combined standard uncertainty The standard uncertainty of the activity per one incident ion is represented by the combined standard uncertainty of two (uncorrelated) components: (1) uncertainty of the beam intensity and (2) uncertainty of the partial activity. Resulting values of the relative combined standard uncertainties are listed in Tables 4 and 5.
Fig. 5. Ratio AE/AS for the isotopes identified in the spectra of the copper target irradiated by 500 MeV/u 40Ar beam. AE is the experimentally measured activity and AS is the activity calculated by FLUKA.
5. Validation of the FLUKA-code The Monte Carlo code FLUKA was used to simulate the experiment. The beam profile used in simulations was assumed to be a Gaussian distribution with 1r = 0.5 cm. The simulations were performed with 105 particles and 20 cycles. The calculated and measured activities were compared in the same time-points: (1) at the beginning of the first (earlier) spectroscopy measurements and (2) at the beginning of the second (later) set of sample measurements. This means that the comparison has been done altogether in four time-points (two for each energy). For the 1 GeV/u beam, the comparison time-points are 6 days and 34 days after the end of irradiation. For the 500 MeV/u beam, the comparison time-points are 13 and 38 days after the end of irradiation. Choosing the comparison time-points following some cooling-time after the end of irradiation eliminates a possible contribution from short-lived isotopes. This is due to the fact that some isotopes of interest may be produced additionally even after the end of irradiation being daughter products of the decaying short-lived isotopes [32]. While this contribution can be treated correctly in FLUKAsimulations (FLUKA does take into account complete decay-
Fig. 6. Comparison of the measured and simulated depth profiles of the partial residual activity of 54Mn induced by 1 GeV/u argon beam.
I. Strašík et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 573–580
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Fig. 7. Comparison of the measured and simulated depth profiles of the partial residual activity of 56Co induced by 500 MeV/u argon beam.
In addition to the activity-comparisons, the depth profiles were also simulated and compared with the measured ones. As an example, the comparison of the measured and simulated depth profile of 54 Mn induced by 1 GeV/u argon beam is presented in Fig. 6. The activities of individual target foils were extrapolated to the beginning of the first-foil measurement, i.e., 6 days after the end of irradiation. Similarly, Fig. 7 shows the comparison of the measured and simulated depth profiles of the residual activity of 56Co induced by 500 MeV/u argon beam. The activities of individual target foils were extrapolated again to the beginning of the first-foil measurements of this target, i.e., 13 days after the end of irradiation. 6. Discussion In the previous experiments performed with uranium beam [5,6], two types of isotopes could clearly be distinguished: (1) products of target activation and (2) projectile fragments. The depth profiles of projectile fragments show no signal upstream of the range of primary ions. The profiles start at the range and occupy a region beyond the range from about few mm up to few cm thick. In contrast to that, the profiles of the target-activation products start at the sample surface and extend deeply beyond the range of primary ions. Unlike the uranium experiment, the present experiment with argon beam shows no typical depth profiles corresponding exclusively to projectile fragments. In the process of the interaction of high-energy argon ions with target material, only fragments with mass number lower than 40 can be produced. However, these low mass-number isotopes can also be produced by target activation, which makes it more difficult to distinguish between the target-activation products and the projectile fragmentation products. Among the identified isotopes, this concerns 7Be and 22Na. Their depth profiles differ from typical target-activation profiles and exhibit a discontinuity in the region of the range of primary ions, which may be partially due to formation of projectile fragmentation. An interesting result is that similar discontinuity in the rangeregion has been observed also for some isotopes with mass number higher than 40 that cannot be projectile fragments. This effect is decreasing with increasing beam energy. For 1 GeV/u argon beam, it is significantly reduced and almost absent (see Fig. 8). Although such an observation might be just an effect of insufficient depthresolution of measured depth profiles in the range-region, it seems not to be the case because it is present also in the FLUKA-simulations where continuous depth-scale is available. Generally, the isotopes are produced by several mechanisms including activation by
Fig. 8. Comparison of the measured and simulated depth profiles of the partial residual activity of 56Co induced by 500 MeV/u argon beam and 1 GeV/u argon beam.
primary ions, secondary particles (mainly protons and neutrons) and fragmentation. The observed depth profiles suggest that significant contribution from primary ions is present upstream to the range especially at the 500 MeV/u argon beam. This contribution is missing downstream to the range. Because of similar energies and target geometries, the partial activities per one incident ion induced by 1 GeV/u argon beam can be directly compared with partial activities induced by 950 MeV/u uranium beam [6]. This comparison shows that the partial activities per one incident ion induced by uranium beam are from 2.9 to 4.2 times higher than the activities induced by argon beam. However, if the activity is normalized to the unit beampower of 1 W delivered to the target, this normalized activity of a particular isotope is from 1.7 to 2.5 times higher for argon beam compared to the uranium beam. This finding is in a good agreement with the scaling-law for induced activity as a function of the primary ion mass as predicted by simulation codes FLUKA and SHIELD [8,9]. The decrease of the normalized activity can be explained by the fact that the heavier ions are mostly stopped by Coulomb interaction with the target electrons (electronic stopping) and have a lower probability of interaction with target nuclei [22]. Comparison of the experimental data with FLUKA-simulations can be interpreted as a very good agreement. It allows us to extend the simulated data over the range that can be practically covered by experiments. For 1 GeV/u argon beam, FLUKA underestimates and overestimates the partial residual activities rather uniformly. In case of 500 MeV/u argon beam, FLUKA mostly underestimates
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the partial residual activities. In the case of uranium beam [8,9,30], the agreement is better for the products of target activation and worse for the projectile fragments. The simulated data underestimate the experimental values, too. This is true mainly for projectile fragments. However, the contribution of the projectile fragments to the total residual activity is much less compared to the targetactivation products. The agreement between the experimental and the simulated total activities (all isotopes together) is better for argon beam than for uranium beam. This can be caused by enormous complexity of the activation processes induced by very heavy ions. Nevertheless, the FLUKA simulation code was validated as a suitable tool for investigation of the ion-induced activation of materials used for construction of accelerator components. 7. Conclusions Partial residual activities and their depth profiles in copper targets irradiated by 1 GeV/u and 500 MeV/u 40Ar beam were measured using a gamma-ray spectroscopy analysis of the stackedfoils targets. The isotopes that dominate the residual activity from few days to several weeks after the end of irradiation were identified and their partial activities were quantified by integration of their depth profiles. All activation products are present upstream of the range of primary ions as well as beyond the range. The activation beyond the range is caused by neutrons, protons and lighter fragments. The isotope-inventory and the profiles’ shape are similar for both beam energies. However, the residual activity per unit length induced by 1 GeV/u argon beam is about two to three times higher compared to the 500 MeV/u beam in the range-region. This can be explained by higher activation efficiency of light fragments compared to the original primary ions. The light fragments created in the sub-surface region of the target during slowing-down the 1 GeV/u beam to the 500 MeV/u beam activate the range-region more than the primary ions in the case of 500 MeV/u incident beam energy. Experimental results were compared with FLUKA-simulations. The comparisons confirmed that FLUKA is a valid code for the activation studies also for heavy ions. The FLUKA-simulations can provide reasonably precise information on partial activities of the induced isotopes. The given approach can be extended to other accelerator materials, beam species and beam energies, hence extending the knowledge beyond experimental possibilities of any accelerator facility. Acknowledgement This work was partially supported by project VEGA 1/0129/09.
References [1] E. Mauro, M. Silari, Nucl. Instrum. Methods Phys. Res. A 605 (2009) 249. [2] J. Alonso, in: Proceedings of the Seventh ICFA Mini-Workshop on High Intensity High Brightness Hadron Beams, Wisconsin, USA, 13–15 September 1999, p. 51. [3] P. Spiller, G. Franchetti, Nucl. Instrum. Methods Phys. Res. A 561 (2006) 305. [4] H. Stöcker, in: Proceedings of the 11th European Particle Accelerator Conference EPAC08, Genoa, Italy, 23–28 June 2008, p. 1517. [5] A. Fertman, E. Mustafin, R. Hinca, et al., Nucl. Instrum. Methods Phys. Res. B 260 (2007) 579. [6] I. Strasik, E. Mustafin, A. Fertman, et al., Nucl. Instrum. Methods Phys. Res. B 266 (2008) 3443. [7] A. Fertman, I. Bakhmetjev, V. Batyaev, et al., Laser Part. Beams 20 (2002) 511. [8] I. Strasik, E. Kozlova, E. Mustafin, et al., Nucl. Technol. 168 (2009) 643. [9] I. Strasik, E. Mustafin, E. Kozlova, et al., in: Proceedings of the 11th European Particle Accelerator Conference EPAC08, Genoa, Italy, 23–28 June 2008, p. 3551. [10] E. Kozlova, H. Weick, B. Achenbach, et al., Nucl. Instrum. Methods Phys. Res. B 266 (2008) 4275. [11] H. Iwase, H. Weick, T. Radon, et al., Nucl. Instrum. Methods Phys. Res. A 562 (2006) 972. [12] G. Battistoni, S. Muraro, P.R. Sala, et al., Proceedings of the Hadronic Shower Simulation Workshop 2006, Fermilab, USA, 6–8 September 2006, AIP Conference Proceeding, vol. 896, 2007, p. 31. [13] A. Fasso, A. Ferrari, J. Ranft, et al., FLUKA: A Multi-Particle Transport Code, CERN-2005-10 (2005), INFN/TC_05/11, SLAC-R-773. [14] S. Agostinelli, J. Allison, K. Amako, et al., Nucl. Instrum. Methods Phys. Res. A 506 (2003) 250. [15] J. Allison, K. Amako, J. Apostolakis, et al., IEEE Trans. Nucl. Sci. 53 (2006) 270. [16] A.V. Dementyev, N.M. Sobolevsky, Radiat. Meas. 30 (1999) 553. [17]. [25] . [31] International Organization for Standardization (ISO), Guide to the Expression of Uncertainty in Measurement, Geneva, Switzerland, 1995, ISBN 92-6710188-9. [32] E. Kozlova, I. Strasik, A. Fertman, et al., Nucl. Technol. 168 (2009) 747.
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Experimental study and simulation of the residual activity induced by high-energy argon ions in copper I. Strašík a,b,*, E. Mustafin a, T. Seidl a, M. Pavlovicˇ b a b
GSI Darmstadt, Planckstrasse 1, D-64291 Darmstadt, Germany FEI STU, Ilkovicˇova 3, SK-812 19 Bratislava, Slovak Republic
a r t i c l e
i n f o
Article history: Received 26 May 2009 Received in revised form 20 November 2009 Available online 11 December 2009 Keywords: Residual activity FAIR FLUKA Gamma-ray spectroscopy
a b s t r a c t The paper presents new experimental results and FLUKA-simulations of residual activation induced by high-energy argon ions in copper. It follows the previous residual activation studies performed at GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt with uranium ions as a preparatory work for constructing the FAIR facility. Copper samples were irradiated by 1 GeV/u and 500 MeV/u 40Ar ions and investigated by gamma-ray spectroscopy. The samples were irradiated in the stacked-foil geometry. The isotopes with dominating contribution to the total residual activity were identified and their partial activities were quantified. Depth-profiling of the partial residual activities of all identified isotopes was performed by measurements of individual target foils. The experimental results were compared with simulations by the FLUKA-code. A satisfactory agreement between the experiment and the simulations was observed. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Activation of accelerator components due to beam losses is an important issue for existing (LHC/CERN, RHIC/BNL, and SNS/ORNL) and planned (FAIR/GSI) high-energy hadron facilities. Residual activity induced by lost beam-particles may become a main source of exposure to personnel and a serious access-restriction for ‘‘hands-on” maintenance [1,2]. In the frame of the FAIR project (Facility for Antiproton and Ion Research) [3,4], extensive experimental studies [5–7] and computer simulations [8–11] of the residual activity induced by high-energy heavy ions in copper and stainless steel are in progress at GSI Darmstadt. Copper and stainless steel have been chosen as the representatives of the most common materials for accelerator structures. The computer simulations were performed for several ion species [8,9], but the experiments have been so far focused mostly on uranium ions [5,6]. Whereas the computer simulations can be in principle run for any ion species, activation experiments are much more demanding from the both points of view: beam availability as well as analysis of the irradiated samples. It is practically impossible to carry out irradiation experiments for all ions of interest at various energies. The computer simulations are the only tool to provide this information. Various simulation codes like FLUKA [12,13], GEANT4 [14,15], SHIELD [16], MCNPX [17], PHITS [18], * Corresponding author. Address: GSI Darmstadt, Planckstrasse 1, D-64291 Darmstadt, Germany. E-mail address: [email protected] (I. Strašík). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.12.004
MARS [19] etc., are available. However, the computer simulations must be verified by experimental data and more check-points are necessary to validate the codes. That is why the irradiation experiments with uranium ions have recently been completed by another experiment with 1 GeV/u and 500 MeV/u argon ions. The irradiated targets were analysed by gamma-ray spectroscopy and depth-profiling of the partial residual activities of all identified isotopes was performed by measuring the activities of individual target foils. The obtained experimental data were compared with FLUKA-simulations. The activation process is very complex. This is true especially for activation induced by heavy-ion beams. The radioactive nuclides are produced by nuclear reactions induced by primary ions (projectiles) as well as by secondary particles, mostly neutrons and protons, generated by interaction of the primary beam with the target material. Among the secondary particles, neutrons are most difficult to be taken into account because their contribution is not well localized and extends far beyond the region where the primary particles stop. On top of that, the projectiles are fragmented into many radioactive projectile fragments that remain implanted in the target. However, their contribution to the total residual activity is negligible for high-energy projectiles [5,6]. Generally, the residual activity depends on the amount, energy and mass of the lost particles as well as on the target composition. A summary of the radio-nuclides identified in common materials for accelerator structures irradiated by high-energy charged particles is presented in Ref. [20]. Understanding of the activation process provides fundamental information that can be used in two
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ways: (1) to specify the tolerable beam losses in the machine and (2) to optimize the choice of construction materials. Both measures are important with respect to the reduction of personnel exposure. As far as the tolerable beam losses are concerned, the losses of 1 W/ m are presently accepted for high-energy proton machines as a threshold for ‘‘hands-on” maintenance [21]. Tolerances for heavyion accelerators can then be specified by scaling the 1 W/m criterion for proton machines [8,9,22,23]. However, the scaling factors have to be obtained again by computer simulations, which brings back the necessity to validate the simulation codes and to collect experimental data for various ion species at various energies.
Table 2 Geometrical configuration of the copper target T2 irradiated by 500 MeV/u argon beam – foil number and its thickness. The target consisted of 15 foils. The overall target thickness was 46.63 mm. The foil thickness was measured by an electronic micrometer in several foil points. Average values are given in the table. The 0.5 mm foils were used for gamma-spectroscopy measurements of the depth profiles of residual activity. The foils thicker than 0.5 mm defined the distance between the sampling-points of the depth profile. Foil Foil Foil Foil Foil Foil
number thickness (mm) number thickness (mm) number thickness (mm)
1 0.50 8 0.50 15 0.50
2 7.02 9 0.50
3 0.50 10 7.03
4 7.02 11 0.50
5 0.50 12 7.04
6 6.99 13 0.50
7 0.50 14 7.03
2. Experiment and method 2.1. Targets and irradiation conditions Three targets assigned as T0, T1 and T2 were used in the experiment for different purposes: target T0 for assessment of the timeevolution of the total residual activity, and targets T1 and T2 for depth-profiling of the residual activity. The target T0 was irradiated by 1 GeV/u argon beam. The targets T1 and T2 were irradiated by 1 GeV/u and 500 MeV/u argon beam, respectively. The target material was 99.9% natural copper (q = 8.96 g/cm3 at 20 °C). The target T0 was a single circular foil 1.00 mm thick, 50 mm in diameter. The targets T1 and T2 were cylinders assembled from many individual foils of various thicknesses, 50 mm in diameter (see Tables 1 and 2 for the target T1 and T2, respectively). The thinner foils were placed in the region of the range of primary ions that was estimated by ATIMA version 1.2 [24], SRIM2008 [25] and FLUKA version 2008.3.6 [12,13]. The calculated values of the range are collected in Table 3. Further details concerning the range estimation can be found in Ref. [26]. The overall thickness of the targets was about two times the estimated range of the primary ions. Argon beam from the SIS-18 synchrotron at the GSI Darmstadt was used to irradiate the targets. The targets were irradiated in the fast-extraction regime with cycle-duration of 3 s. The beam spotsize was about 2 cm in horizontal plane and 2 cm in vertical plane (checked visually on a scintillation screen before irradiation and measured by a profile-meter). The beam profile was approximately Gaussian according to the profile-meter. The beam intensity was monitored by a current transformer [27]. Application software recorded the irradiation history and summed-up the overall charge delivered to the target, which was recalculated to the total number of ions. The target T0 was irradiated by 1.10 1012 argon ions. The targets T1 and T2 were irradiated by 1.12 1012 and 2.16 1012 argon ions, respectively. 2.2. Measurement of the gamma spectra After irradiation, the gamma spectra of individual targets foils were measured. Target T0 (1 mm thick foil) was measured repeat-
Table 1 Geometrical configuration of the copper target T1 irradiated by 1 GeV/u argon beam – foil number and its thickness. The target consisted of 21 foils. The overall target thickness was 126.40 mm. The foil thickness was measured by an electronic micrometer in several foil points. Average values are given in the table. The 0.5 mm foils were used for gamma-spectroscopy measurements of the depth profiles of residual activity. The foils thicker than 0.5 mm defined the distance between the sampling-points of the depth profile. Foil Foil Foil Foil Foil Foil
number thickness (mm) number thickness (mm) number thickness (mm)
1 0.50 8 14.99 15 0.50
2 15.00 9 0.50 16 14.98
3 0.50 10 0.50 17 0.50
4 15.00 11 0.50 18 14.98
5 0.50 12 0.50 19 0.50
6 14.99 13 0.50 20 14.97
7 0.50 14 14.99 21 0.50
edly 24 times. The first measurement started 40 min and the last measurement finished 4 h and 30 min after the end of irradiation. The live-time of each measurement lasted 300 s and was followed by a 5-min break before the next measurement started. The detector faced the back-side of the target, i.e., the side that was not faced to the primary beam. The target-to-detector distance was 10 cm. The total number of counts in the corresponding channel of the spectrometer was recorded at each measurement. Target T1 was measured two times: 6–26 and 34–48 days after the end of irradiation. The measurement time varied from 5 h up to 70 h per foil. Target T2 was measured also two times: 13–28 and 38–51 days after the end of irradiation. The measurement time varied from 5 h up to 44 h per foil. The target-to-detector distance was 7 cm for both targets. The gamma spectra from the targets T1 and T2 were used for depth-profiling of the partial residual activities of all identified isotopes. The gamma-ray spectroscopy measurements were carried out with a Canberra HPGe-GC3518 detector and Ortec GEM-25P4 detector coupled to Silena multi-channel analysers (8192-channels). Both detectors were powered by an Ortec HV-supply. This set-up allowed measuring gamma spectra from 30 keV up to 3 MeV. For the energy and efficiency calibration, a standard set of calibration sources was used: 22Na, 60Co, 137Cs, 152Eu, 210Pb and 241 Am. Data-acquisition and handling of spectra were controlled by the WINGAM software package. The spectra were analysed by the Genie2000 software package. 3. Time-evolution of the total residual activity The purpose of the target T0 irradiation was to validate the FLUKA-simulation of the time-evolution of the total residual activity after the end of irradiation. The measurement started as early as the target activity dropped below a tolerable level (40 min after the end of irradiation) in order to take into account the contribution from short-lived isotopes. Because the gamma spectra measured very shortly after the end of irradiation are extremely complex [5,6,28], the isotope identification and determination of their partial activities was not considered. Instead of that, the average count-rate registered by the detector was taken from the gamma spectrum. The average count-rate was obtained as the total number of counts divided by the measurement time. Subsequently, the count-rate was converted into the photon fluence rate using the energy and efficiency calibration curves of the detection chain. This simplified approach overestimates the photon fluence rate, because the Compton signal cannot be properly treated. Exact conversion could only be based on the detector response operator method [29]. The irradiation conditions of the target T0 were simulated by FLUKA. The simulation was performed with 106 particles and 20 cycles. For the comparison with the experiment, only the photons in the energy range from 30 keV to 3 MeV passing through a circu-
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I. Strašík et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 573–580 Table 3 The range including the range straggling of 1 GeV/u and 500 MeV/u ATIMA
1 GeV/u 500 MeV/u
40
Ar18+ ions in copper calculated by different computer codes. SRIM
FLUKA
Range (mm)
Straggling (mm)
Range (mm)
Straggling (mm)
Range (mm)
Straggling (mm)
63.35 22.91
0.10 0.04
63.93 22.98
2.21 0.83
65.27 23.51
0.41 0.15
Fig. 1. Comparison of the time-evolution of the photon fluence rate based on the gamma-spectroscopic measurements and FLUKA-simulation together with the residual activity simulated by FLUKA.
lar area 7 cm in diameter at the distance of 10 cm from the target were considered. This corresponds to the energy range of the detection chain and measurement geometry. The photon fluence rates based on the count-rate measurement and on the FLUKAsimulation as a function of time are shown in Fig. 1 together with the simulated residual activity. As expected, the photon fluence rate based on the count-rate measurement is higher compared to the FLUKA-simulation, but the discrepancy is not dramatic. This qualitative agreement indicates that the FLUKA-simulation is reasonably close to reality. 4. Residual activities 4.1. Isotope identification and activity measurements Results of isotope identification and their partial activities are summarized in Tables 4 and 5 for the target irradiated by 1 GeV/ u and 500 MeV/u argon beam, respectively. Activities are normalized per one incident ion. The isotope identification was based on the energy and abundance of the gamma lines, half-life of the isotopes as well as on the experience from the previous experiments [5,6]. The isotope characteristics were taken from the WWW Table of Radioactive Isotopes [30]. In problematic cases of isotope identification, supporting information was gained from the depth profiles of residual activity [6]. For all identified isotopes, the activity for each foil was obtained from the peak-net-areas (PNA) calculated by Genie2000 including the standard uncertainty of the PNA. The measured activity was then extrapolated backwards in time to the end of the irradiation using the characteristic decay constant of a given isotope and converted into the activity per unit length. Because the measured foils were thin enough, no correction for self-absorption was necessary and the activity per unit length could be obtained as the activity of the foil divided by its thickness. Finally, the activity of each isotope, i.e., its partial activity with respect to the total target activity cor-
responding to the sum of all isotopes, was obtained by numerical integration of its depth profile using a trapezoidal method. Generally, the activation products have several energy lines in the spectra. Not all of them are included in Tables 4 and 5. The most pronounced energy lines or lines without an interference with other isotopes were chosen for activity determination. Determination of the activity of short-lived isotopes was not possible because their activity decreased below minimum detectable activity (MDA, the Curie’s concept with confidence level at 95%) before the measurement of all discs was completed. For this reason, their depth profiles could not be obtained in full and the corresponding activities are not indicated in the Tables although the presence of the isotopes was identified. Determination of the activities of 22 Na from the spectra measured 6–26 days and 13–28 days after the end of irradiation was not reliable. A large number of peaks belonging to short-lived isotopes with energy-separation below the resolution of the spectroscopy apparatus created an ‘‘apparent” background that was much higher than the true background at the measurement place [5,6]. The activity of some isotopes of interest may get below the minimum detectable activity in this case. The apparent background decreases with time as the short-lived isotopes decay. That is why the determination of the 22Na-activity became possible from the spectra measured 34–48 and 38–51 days after the end of irradiation. In opposite to this, some isotopes that could be quantified in the early-measured spectra, could not be quantified in the later measured spectra, because they decayed below the MDA level. 4.2. Depth profiles of the partial residual activities Partial activities of individual isotopes were calculated by integration of their depth profiles. Figs. 2 and 3 show typical depth profiles of the residual activity for the target-activation products, namely 7Be, 46Sc, 51Cr, 54Mn, 56Co and 60Co identified in the spectra of the targets irradiated by 1 GeV/u and 500 MeV/u argon ions, respectively. The profiles were obtained from the spectra measured 6–26 days (1 GeV/u) and 13–28 days (500 MeV/u) after the end of irradiation. It can be seen that the profiles extend deeply beyond the range of primary ions and decrease modestly with depth. This is mainly due to large amount of secondary neutrons, protons and light fragments that have the range much longer compared to the range of the primary ions. Depth profiles of 7Be are almost identical for both energies. The profiles of heavier isotopes depend on the primary beam energy. They start from the same activity in the close-to-surface target region, but in the range-region, the 1 GeV/u argon beam induces two to three times higher activity per one incident ion compared to the 500 MeV/u beam. This indicates the important role of the secondary particles in the activation process. In most cases, the profiles obtained from the spectra measured in the first (earlier) and the second (later) set of measurements are well consistent, which indicates that no significant contribution from an isotope with different half-life is present. In the case of 60 Co, there is an interference with 56Co (1173 keV gamma line) and with 52Mn (1333 keV gamma line). The contribution of the interfering isotopes can be taken into account by calculation of their contribution to the net-peak-area corresponding to their
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Table 4 Identified isotopes and their activities in the copper target irradiated by 1 GeV/u argon beam. A1, A2 – activity, u1, u2 – relative combined standard uncertainty comprising uncertainty of the beam intensity and uncertainty of the partial activity. Activities are extrapolated to the end of irradiation. Subscript ‘‘1” is related to the spectra measured 6– 26 days after the end of irradiation, subscript ‘‘2” is related to the spectra measured 34–48 days after the end of irradiation. The Curie’s concept with confidence level at 95% implemented in Genie2000 was applied for MDA. Isotope
Half-live
Energy (keV)
A1 (Bq/ion)
u1 (%)
A2 (Bq/ion)
u2 (%)
7
Be 22 Na 43 K
53.12 d 7 2.6019 y 4 22.3 h 1
8.90E-09 1.07E-10
2.32 8.19
3.927 h 8 58.6 h 1 83.79 d 4
8.72E-09 Below MDA Below MDA Below MDA Below MDA Below MDA 5.37E-09 5.42E-09 7.14E-08
1.74
44
477.595 1274.53 2 372.760 617.490 1157.031 271.13 889.277 3 1120.545 4 159.377 12 944.104 7 983.517 5 1312.096 6 320.0824 4 744.233 13 935.538 11 1246.278 15 1333.649 17 1434.068 14 834.848 3 1099.251 4 1291.596 7 477.2 2 931.3 2 1408.4 2 846.771 5 1037.840 6 1238.282 7 1771.351 16 2034.755 13 122.0614 4 136.4743 5 810.775 9 1173.237 4 1332.501 5 1377.63 3 1115.546 4
0.81 0.82 2.42
5.50E-09 5.26E-09 Below MDA
0.93 0.97
4.97E-08 4.94E-08 7.77E-08 1.33E-07 1.31E-07
0.70 0.71 0.75 0.74 0.74
4.94E-08 4.96E-08 7.66E-08 Below MDA Below MDA
0.80 0.82 0.89
7.68E-09 7.29E-09 7.34E-09 Below MDA Below MDA Below MDA 1.40E-08
0.75 0.92 0.97
7.67E-09 7.47E-09 7.46E-09
0.80 1.17 1.30
0.72
1.40E-08
0.77
1.32E-08
0.76
1.35E-08
0.85
1.31E-08 1.34E-08 7.42E-08 1.09E-09 1.16E-09 Below MDA 6.53E-10
1.90 1.97 0.68 1.36 0.02
1.39E-08 1.39E-08 7.52E-08 1.05E-09 1.10E-09
1.91 1.99 0.69 1.54 1.58
3.29
5.43E-10
4.63
Sc Sc Sc
44m 46
47
Sc V
3.3492 d 6 15.9735 d 25
48
51
Cr Mn
27.7025 d 24 5.591 d 3
52
54
Mn Fe
312.3 d 4 44.503 d 6
59
55
17.53 h 3
56
77.27 d 3
Co
Co
57
271.79 d 9
58
70.86 d 7 5.2714 y 5
Co
Co Co
60
57
Ni Zn
35.60 h 6 244.26 d 26
65
activity obtained from other gamma lines of 56Co and 52Mn. This contribution can then be subtracted from the total net-peak-area to get the net-peak-area belonging to 60Co. In Figs. 2 and 3, the 60 Co-profiles are presented without the contribution of 56Co and 52 Mn.
The standard uncertainty of the total number of ions delivered to the target, u(I), is calculated using the formula:
4.3. Uncertainty assessment
where u(ni) is the standard uncertainty of the number of ions in the ith pulse. The relative standard uncertainty of the total number of ions delivered to the target is then expressed as
The uncertainties were analysed according to the ISO Guide to the Expression of Uncertainty in Measurement (GUM) [31]. The relative combined standard uncertainty of the data presented in Tables 4 and 5 comprises two components: (1) uncertainty of the beam intensity and (2) uncertainty of the partial activity. 4.3.1. Uncertainty of the beam intensity The relative standard uncertainty of the number of ions per beam pulse measured by the current transformer is assumed to be less than 3%. The total number of the pulses during irradiation was 79 and 159 in case of 1 GeV/u beam and 500 MeV/u beam, respectively. The total number of ions delivered to the target was obtained as a sum of the number of ions in individual pulses:
I¼
N X
ni ;
ð1Þ
i¼1
where I is the total number of ions delivered to the target, ni is the number of ions in the ith pulse and N is the total number of pulses.
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N 2 u N uX @I uX uðIÞ ¼ t uðni Þ2 ¼ t uðni Þ2 ; @n i i¼1 i¼1
uðIÞ½% ¼
uðIÞ 100%: I
ð2Þ
ð3Þ
The relative standard uncertainty of the total number of ions delivered to the target is 0.35% for 1 GeV/u beam and 0.24% for 500 MeV/u beam. 4.3.2. Uncertainty of the partial activity There are several major sources of the uncertainty of the partial activity, u(A). The sources listed below were included in our uncertainty analysis. 4.3.2.1. Uncertainty of the net-peak-area (NPA). The uncertainty of the net-peak-area depends on the statistics in the peak (i.e., on the count-rate). The relative standard uncertainty of the net-peak-area ranged from 0.24% for high count-rate to 37.75% for very low count-rate. The relative standard uncertainty of the net-peak-area
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Table 5 Identified isotopes and their activities in the copper target irradiated by 500 MeV/u argon beam. A1, A2 – activity, u1, u2 – relative combined standard uncertainty comprising uncertainty of the beam intensity and uncertainty of the partial activity. Activities are extrapolated to the end of irradiation. Subscript ‘‘1” is related to the spectra measured 13– 28 days after the end of irradiation, subscript ‘‘2” is related to the spectra measured 38–51 days after the end of irradiation. The Curie’s concept with confidence level at 95% implemented in Genie2000 was applied for MDA. Isotope
Half-live
Energy (keV)
A1 (Bq/ion)
u1 (%)
A2 (Bq/ion)
u2 (%)
7
Be 22 Na 44 Sc 44m Sc 46 Sc
53.12 d 7 2.6019 y 4 3.927 h 8 58.6 h 1 83.79 d 4
2.92E-09 3.05E-11
1.57 6.72
0.96 1.01 3.40
7.52E-10 7.20E-10 Below MDA
1.06 1.12
3.3492 d 6 15.9735 d 25
2.98E-09 Below MDA Below MDA Below MDA 7.33E-10 7.22E-10 1.03E-08
1.37
47
477.595 1274.53 2 1157.031 271.13 889.277 3 1120.545 4 159.377 12 944.104 7 983.517 5 1312.096 6 320.0824 4 744.233 13 935.538 11 1246.278 15 1333.649 17 1434.068 14 834.848 3 1099.251 4 1291.596 7 846.771 5 1037.840 6 1238.282 7 1771.351 16 2034.755 13 122.0614 4 136.4743 5 810.775 9 1173.237 4 1332.501 5 1115.546 4
6.96E-09 6.90E-09 1.16E-08 2.14E-08 2.09E-08
0.75 0.77 0.83 0.82 0.83
7.04E-09 6.90E-09 1.17E-08 Below MDA Below MDA
0.89 0.92 0.99
1.25E-09 1.24E-09 1.22E-09 2.48E-09
0.81 1.06 1.14 0.76
1.27E-09 1.24E-09 1.19E-09 2.47E-09
0.84 1.27 1.43 0.80
2.34E-09
0.82
2.41E-09
0.89
2.39E-09 2.43E-09 1.35E-08 2.03E-10 2.06E-10 1.90E-10
2.13 2.20 0.70 1.49 1.28 2.77
2.47E-09 2.46E-09 1.35E-08 2.04E-10 2.09E-10 1.77E-10
2.14 2.21 0.71 1.57 1.63 3.14
Sc V
48
51
Cr Mn
52
54
Mn Fe
59
56
Co
27.7025 d 24 5.591 d 3
312.3 d 4 44.503 d 6 77.27 d 3
57
271.79 d 9
58
70.86 d 7 5.2714 y 5
Co
60
Co Co
65
Zn
244.26 d 26
Fig. 2. Activity depth profiles of 7Be, 46Sc, 51Cr, 54Mn, 56Co, 60Co identified in the spectra of the copper target irradiated by 1 GeV/u argon beam measured 6–26 days after the end of irradiation.
is indicated directly by Genie2000. Its quantification is described in details in Genie2000 Customization Tools Manual. 4.3.2.2. Uncertainty of the efficiency calibration. The uncertainty of the efficiency calibration includes: (1) the uncertainty of the netpeak-area used to determine the efficiency, (2) uncertainty of fitting of the efficiency curve and (3) the uncertainty of the calibration source activities. The relative standard uncertainty of the net-peak-area used to determine the efficiency was less than 0.57%. The relative standard uncertainty of the fitting of the efficiency curve is 1% for gamma-ray energies above 200 keV and 7% for gamma-ray energies below 200 keV. There is a lack of calibration data-points below 200 keV, which increases the uncertainty
Fig. 3. Activity depth profiles of 7Be, 46Sc, 51Cr, 54Mn, 56Co, 60Co identified in the spectra of the copper target irradiated by 500 MeV/u argon beam measured 13– 28 days after the end of irradiation.
in this low-energy region. The above values are also indicated by Genie2000. The activities of the calibration sources have a certified relative standard uncertainty less than 2%. 4.3.2.3. Uncertainty of the half-lives. Uncertainties of the half-lives are indicated in Tables 4 and 5 (in italics). The relative standard uncertainty of the half-lives for all pertinent isotopes was less than 0.13%. 4.3.2.4. Uncertainty of the gamma-photon abundances. The relative standard uncertainty of the gamma-photon abundances for all pertinent energies was less than 0.75%.
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4.3.2.5. Uncertainty in the integration of the depth profiles. The partial activity for each isotope was obtained by integration of the corresponding depth profile using a trapezoidal method. The integration was performed numerically as a sum of contributing activities Pi:
A¼
N X
Pi ;
ð4Þ
i¼1
Pi ¼
d ðAi þ Aiþ1 Þ; 2
ð5Þ
where A is the partial residual activity of the isotope, Ai is the activity per unit length in the ‘‘i” point of the depth profile and d is the distance between the ‘‘i” and ‘‘i + 1” points of the depth profile. We assume that uncertainty of the distance d is considerably lower than the uncertainty of the activity. For this reason the expression for the calculation of the uncertainty of the activity Pi can be simplified as
d uðPi Þ ¼ 2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uðAi Þ2 þ uðAiþ1 Þ2 ;
ð6Þ
where u(Ai) is the standard uncertainty of the activity per unit length in the ‘‘i” point of the depth profile. u(Ai) includes all the above listed sources of the uncertainty. Uncertainty of the foil thickness was neglected. The standard uncertainty of the partial activity is then obtained as
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX uðAÞ ¼ t uðPi Þ2 ;
Fig. 4. Ratio AE/AS for the isotopes identified in the spectra of the copper target irradiated by 1 GeV/u 40Ar beam. AE is the experimentally measured activity and AS is the activity calculated by FLUKA.
chains), it is difficult to be taken into account in measured data. The comparison-results for partial residual activities of individual isotopes are presented in Figs. 4 and 5 for 1 GeV/u and 500 MeV/ u, respectively. The discrepancies vary from factor of 0.67 to 1.53 for the 1 GeV/u experiment and from 0.71 to 1.70 for the 500 MeV/u experiment. In general, the activity-ratios are similar for both energies except for 7Be. In this case, the activity-ratio is about 0.96 at 1 GeV/u and almost 1.60 at 500 MeV/u.
ð7Þ
i¼1
and the relative standard uncertainty as
uðAÞ½% ¼
uðAÞ 100%: A
ð8Þ
The same procedure has been applied in the previous experiments with uranium beam [6]. 4.3.3. Combined standard uncertainty The standard uncertainty of the activity per one incident ion is represented by the combined standard uncertainty of two (uncorrelated) components: (1) uncertainty of the beam intensity and (2) uncertainty of the partial activity. Resulting values of the relative combined standard uncertainties are listed in Tables 4 and 5.
Fig. 5. Ratio AE/AS for the isotopes identified in the spectra of the copper target irradiated by 500 MeV/u 40Ar beam. AE is the experimentally measured activity and AS is the activity calculated by FLUKA.
5. Validation of the FLUKA-code The Monte Carlo code FLUKA was used to simulate the experiment. The beam profile used in simulations was assumed to be a Gaussian distribution with 1r = 0.5 cm. The simulations were performed with 105 particles and 20 cycles. The calculated and measured activities were compared in the same time-points: (1) at the beginning of the first (earlier) spectroscopy measurements and (2) at the beginning of the second (later) set of sample measurements. This means that the comparison has been done altogether in four time-points (two for each energy). For the 1 GeV/u beam, the comparison time-points are 6 days and 34 days after the end of irradiation. For the 500 MeV/u beam, the comparison time-points are 13 and 38 days after the end of irradiation. Choosing the comparison time-points following some cooling-time after the end of irradiation eliminates a possible contribution from short-lived isotopes. This is due to the fact that some isotopes of interest may be produced additionally even after the end of irradiation being daughter products of the decaying short-lived isotopes [32]. While this contribution can be treated correctly in FLUKAsimulations (FLUKA does take into account complete decay-
Fig. 6. Comparison of the measured and simulated depth profiles of the partial residual activity of 54Mn induced by 1 GeV/u argon beam.
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Fig. 7. Comparison of the measured and simulated depth profiles of the partial residual activity of 56Co induced by 500 MeV/u argon beam.
In addition to the activity-comparisons, the depth profiles were also simulated and compared with the measured ones. As an example, the comparison of the measured and simulated depth profile of 54 Mn induced by 1 GeV/u argon beam is presented in Fig. 6. The activities of individual target foils were extrapolated to the beginning of the first-foil measurement, i.e., 6 days after the end of irradiation. Similarly, Fig. 7 shows the comparison of the measured and simulated depth profiles of the residual activity of 56Co induced by 500 MeV/u argon beam. The activities of individual target foils were extrapolated again to the beginning of the first-foil measurements of this target, i.e., 13 days after the end of irradiation. 6. Discussion In the previous experiments performed with uranium beam [5,6], two types of isotopes could clearly be distinguished: (1) products of target activation and (2) projectile fragments. The depth profiles of projectile fragments show no signal upstream of the range of primary ions. The profiles start at the range and occupy a region beyond the range from about few mm up to few cm thick. In contrast to that, the profiles of the target-activation products start at the sample surface and extend deeply beyond the range of primary ions. Unlike the uranium experiment, the present experiment with argon beam shows no typical depth profiles corresponding exclusively to projectile fragments. In the process of the interaction of high-energy argon ions with target material, only fragments with mass number lower than 40 can be produced. However, these low mass-number isotopes can also be produced by target activation, which makes it more difficult to distinguish between the target-activation products and the projectile fragmentation products. Among the identified isotopes, this concerns 7Be and 22Na. Their depth profiles differ from typical target-activation profiles and exhibit a discontinuity in the region of the range of primary ions, which may be partially due to formation of projectile fragmentation. An interesting result is that similar discontinuity in the rangeregion has been observed also for some isotopes with mass number higher than 40 that cannot be projectile fragments. This effect is decreasing with increasing beam energy. For 1 GeV/u argon beam, it is significantly reduced and almost absent (see Fig. 8). Although such an observation might be just an effect of insufficient depthresolution of measured depth profiles in the range-region, it seems not to be the case because it is present also in the FLUKA-simulations where continuous depth-scale is available. Generally, the isotopes are produced by several mechanisms including activation by
Fig. 8. Comparison of the measured and simulated depth profiles of the partial residual activity of 56Co induced by 500 MeV/u argon beam and 1 GeV/u argon beam.
primary ions, secondary particles (mainly protons and neutrons) and fragmentation. The observed depth profiles suggest that significant contribution from primary ions is present upstream to the range especially at the 500 MeV/u argon beam. This contribution is missing downstream to the range. Because of similar energies and target geometries, the partial activities per one incident ion induced by 1 GeV/u argon beam can be directly compared with partial activities induced by 950 MeV/u uranium beam [6]. This comparison shows that the partial activities per one incident ion induced by uranium beam are from 2.9 to 4.2 times higher than the activities induced by argon beam. However, if the activity is normalized to the unit beampower of 1 W delivered to the target, this normalized activity of a particular isotope is from 1.7 to 2.5 times higher for argon beam compared to the uranium beam. This finding is in a good agreement with the scaling-law for induced activity as a function of the primary ion mass as predicted by simulation codes FLUKA and SHIELD [8,9]. The decrease of the normalized activity can be explained by the fact that the heavier ions are mostly stopped by Coulomb interaction with the target electrons (electronic stopping) and have a lower probability of interaction with target nuclei [22]. Comparison of the experimental data with FLUKA-simulations can be interpreted as a very good agreement. It allows us to extend the simulated data over the range that can be practically covered by experiments. For 1 GeV/u argon beam, FLUKA underestimates and overestimates the partial residual activities rather uniformly. In case of 500 MeV/u argon beam, FLUKA mostly underestimates
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the partial residual activities. In the case of uranium beam [8,9,30], the agreement is better for the products of target activation and worse for the projectile fragments. The simulated data underestimate the experimental values, too. This is true mainly for projectile fragments. However, the contribution of the projectile fragments to the total residual activity is much less compared to the targetactivation products. The agreement between the experimental and the simulated total activities (all isotopes together) is better for argon beam than for uranium beam. This can be caused by enormous complexity of the activation processes induced by very heavy ions. Nevertheless, the FLUKA simulation code was validated as a suitable tool for investigation of the ion-induced activation of materials used for construction of accelerator components. 7. Conclusions Partial residual activities and their depth profiles in copper targets irradiated by 1 GeV/u and 500 MeV/u 40Ar beam were measured using a gamma-ray spectroscopy analysis of the stackedfoils targets. The isotopes that dominate the residual activity from few days to several weeks after the end of irradiation were identified and their partial activities were quantified by integration of their depth profiles. All activation products are present upstream of the range of primary ions as well as beyond the range. The activation beyond the range is caused by neutrons, protons and lighter fragments. The isotope-inventory and the profiles’ shape are similar for both beam energies. However, the residual activity per unit length induced by 1 GeV/u argon beam is about two to three times higher compared to the 500 MeV/u beam in the range-region. This can be explained by higher activation efficiency of light fragments compared to the original primary ions. The light fragments created in the sub-surface region of the target during slowing-down the 1 GeV/u beam to the 500 MeV/u beam activate the range-region more than the primary ions in the case of 500 MeV/u incident beam energy. Experimental results were compared with FLUKA-simulations. The comparisons confirmed that FLUKA is a valid code for the activation studies also for heavy ions. The FLUKA-simulations can provide reasonably precise information on partial activities of the induced isotopes. The given approach can be extended to other accelerator materials, beam species and beam energies, hence extending the knowledge beyond experimental possibilities of any accelerator facility. Acknowledgement This work was partially supported by project VEGA 1/0129/09.
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