Production of residual nuclei by proton-induced reactions on C, N, O, Mg, AI and Si

Nuclear Instruments and Methods in Physics Research B 82 (1993) 9-31 North-Holland

Beam interactions with Materials & Atoms

Production of residual nuclei by proton-induced reactions on C, N, O, Mg, AI and Si R. Bodemann, H.-J. Lange, I. Leya and R. Michel Zentraleinrichtung fiir Strahlenschutz, Universith't Hannover, Am Kleinen Felde 30, D-3000 Hannover 1, Germany

T. Schiekel, R. R6sel and U. Herpers Abteilung Nuklearchemie, Universitiit zu Ki31n, Ziilpicher Sir. 47, D-5000 KiSln-1, Germany

H.J. Hofmann t, B. Dittrich, M. Suter and W. W61fli lnstitut fiir Mittelenergiephysik, ETH HSnggerberg, CH-8093 Ziirich, Switzerland

B. Holmqvist The Studsvik Neutron Research Laboratory, University of Uppsala, S-611 82 Studsvik, Sweden

H. Cond6 Department of Neutron Research, University of Uppsala, P.O.B. 535, S-751 21 Uppsala, Sweden

P. Malmborg The Svedberg Laboratory, University of Uppsala, P.O.B. 533, S-751 21 Uppsala, Sweden Received 23 December 1992 and in revised form 1 March 1993

Cross sections for the production of residual nuclides by p-induced reactions are the basic nuclear quantities for an accurate modelling of the interaction of solar cosmic protons with matter. In a series of irradiation experiments at the cyclotron of the Svedberg Laboratory/University of Uppsala sixteen different target elements were investigated for proton energies up to 100 MeV in order to determine such thin-target excitation functions. Residual nuclides were measured by gamma-spectrometry and, in the case of 1°Be and 26A1,by accelerator mass spectrometry. Here, we report results for the light target elements C, N, O, Mg, AI, and Si including also new cross sections for the production of l°Be and 26A1from aluminum for p-energies between 100 MeV and 200 MeV. The latter were derived from targets irradiated earlier at the IPN Orsay. The new experimental data are compared with earlier work and analyzed in the framework of the hybrid model of preequilibrium reactions investigating several options with respect to the choice of nuclear masses including also corrections for shell effects of nuclear masses and level densities.

1. Introduction Inclusive data on nuclear reactions, as e.g. integral cross sections for the production of residual nuclides, are of fundamental importance for many fields of basic and applied sciences, such as astro- and cosmophysics,

Correspondence to: R. Michel, Zentraleinrichtung fiir Strahlenschutz, Universifiit Hannover, Am Kleinen Felde 30, D-W-3000 Hannover 1, Germany. 1 Present address: Leybold AG, CH8050 Ziirich, Switzerland.

cosmic ray physics and space technology. They are essential for design and operation of accelerators, for radiation protection in space and on earth, and for optimization of radionuclide production in nuclear medicine. For the interpretation of solar cosmic ray (SCR) produced cosmogenic nuclides in terrestrial and extraterrestrial matter the knowledge of integral excitation functions for the production of residual nuclides by proton-induced reactions is a necessary prerequisite. Cosmogenic nuclides are stable or radioactive natural tracers, which allow geo- and cosmochemical pro-

0168-583X/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

10

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

cesses to be investigated and the cosmic ray exposure history of lunar samples, meteorites and cosmic dust to be described. They provide the only tools by which the ancient solar activity can be monitored on time scales extending up to billions of years. SCR produced cosmogenic nuclides have been investigated intensively in lunar surface material, see refs. [1,2] for reviews. In meteorites SCR interactions are relevant in small meteoroids and in near-surface samples which by chance survive ablation during atmospheric transit. Though SCR effects in meteorites were predicted as early as 1982 [3], they were unambiguously discovered only recently in the meteorite Salem [4,5]. SCR particles are emitted from the sun during short-term events, the solar flares. They consist mainly of protons, 4He abundances varying from flare to flare but being less than 10%. The frequency of flare events is strongly related to sun spot frequency. Most SCR particles are emitted in just a few flares around solar maximum [6]. The spectra of SCR particles can be described by exponentially decreasing rigidity spectra for individual flare events as well as for long-term averaged spectra. Characteristic rigidities of individual flare events range from 30 to 150 MV. The relevant energies for the SCR production of cosmogenic nuclides extend up to 200 MeV. Due to the relatively low energies of SCR particles, their interactions are restricted to the outmost surface (depth < 15 g cm-2) of the irradiated material and the production of secondary particles, which can initiate nuclear reactions, can generally be neglected. Therefore, the spectra of SCR particles can be calculated as a function of depth inside the irradiated material by simply taking into account stopping and attenuation by inelastic processes. Theoretical production rates of cosmogenic nuclides can then be calculated by folding these spectra with integral excitation functions for the production of the respective nuclides from all contributing target elements [3,7-9]. The reliability of such model calculations depends exclusively on the quality of the excitation functions of the underlying nuclear reactions. However, there still exists a considerable lack in the experimental data with respect to the investigation of relevant target/product combinations over the whole energy range, especially for light elements with A < 30, which are the main target elements in lunar surface materials, in stony meteorite~ as well as in the earth's atmosphere. Moreover, the existing data are often unreliable and contradictory. A detailed discussion of the unsatisfactory situation and the existing data needs was given recently [2,10]. Therefore, in this work we deal with the production of radionuclides from the target elements C, N, O, Mg, Al and Si using conventional gamma-spectrometry for the investigation of 7Be, .22Na, 24Na and 28Mg as well

as accelerator mass spectrometry (AMS) for the longlived radionuclides 1°Be (7"1/2 = 1.51 Ma [11]) and 26A1 (T1/2 = 716 ka [12]). This study is part of a larger project to measure cosmochemically relevant thintarget cross section for p- and 4He-induced reactions for 18 target elements (6 < Z _< 79). It extends earlier cross section measurements of our group for protons up to 200 MeV, which up to now covered the target elements Ti, V, Mn, Fe, Co, Ni and Ba [13-19]. From the viewpoint of applications of integral production cross sections the capabilities of nuclear reaction theories to predict unkown excitation functions by a priori calculations are important. In this respect, the hybrid model of preequilibrium reactions [20] in the form of the ALICE code [21-25] has been found to be quite reliable for nucleon-induced reactions on the target elements dealt with earlier [13-19]. In order to estimate the applicability of this model for predictions of p-induced reactions on light target elements, hybrid model analyses are performed using the codes ALICE LIVERMORE 87 [24] and ALICE 900 [25]. Hybrid model analyses of p-induced reactions on light target elements gi~,e information whether the statistical Fermi-gas-approach is still valid for nuclei with A < 30 or to what extent direct reactions, individual nuclear states or internal cluster structure of the target nuclei have to be considered.

2. Experimental The cross sections were determined using the stacked-foil technique. Six different cylindrical stacks (I-VI) were irradiated at an external proton beam of the cyclotron at the Svedberg Laboratory/University of Uppsala. The target elements irradiated were chosen to be of interest for cosmophysics and nuclear physics. The experiments covered a total of eightteen different elements (6 < Z < 79), of which C, N, O, Mg, A1, and Si are dealt with here. For the elements C, Mg, Al and Si pure element foils were used with thicknesses (d) of 0.1 mm, 0.1 mm, 0.125 mm, and 0.5 mm, respectively. For nitrogen and oxygen disks of quartz (d = 1 and 3 mm) and Si3N 4ceramics (d = 1 mm) were chosen as target materials. The target foils and disks had diameters of 15.7 mm with the exception of the quartz disks, which had diameters of 15.0 mm. All targets were made of highpurity materials. The purities were 99.9% for C and Mg, 99.999% for A1, > 99.999% for Si and SiO2, and > 99.5% for SiaN 4. The stacks were designed to obtain an optimum of information for "unknown" excitation functions, to allow for detailed cross checking of the new results by comparison with data from our earlier work on target

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Fig. 1. Design of stacks III and IV as typical examples for multi-element and single-element stacks. The multi-element stack III consisted of 5 parts, the individual compositions of which is given in detail. The single-element stack IV consisted of 37 identical parts, each containing three Al-foils (d = 0.036 mm) and one Si disc (d = 1 mm). The stack-holders were made of commercial aluminum and had a total length of 5 cm.

elements ( 2 2 < Z < 2 8 ) [13-16,18] and to minimize cross contamination and recoil losses. The stacks I and II consisted of a large number of different target elements (12 < Z < 79), among others containing foils of 0.125 mm Al, 0.1 mm Mg and 0.5 mm Si. A detailed description of these stacks will be given elsewhere [26]. The stack III was made Out of several smaller stacks, each consisting of a 0.5 mm Si foil, a 3 mm quartz disk and a 3.25 mm disk of a Ba-containing glass. Included in this stack, there were also some high-purity foils of V, Nb and Au. Stack IV consisted of the target material quartz only, containing 37 disks with a thickness of 1 mm each. Stack V was made of 16 smaller stacks each consisting of three 0.5 mm Si foils and three 0.1 mm C foils. Finally, stack VI contained 32 disks of 1 mm SiaN 4 ceramics. Six groups of three 0.1 mm C foils were distributed in the final part of this stack. Exemplarily, the design of stacks II and IV, which represent the extreme design differences between all stacks used, are shown in fig. 1. For compensation of recoil losses from thin foils (d < 0.15 mm), three foils of each target element were packed together and only the one in the middle was used for the measurements. This turned out to be not necessary for foils with thicknesses of 0.5 mm and more. All six stacks contained various (between 33 and 57) sets of three 0.036 mm Al foils. The inner ones of these foils in each first set of each stack were used for flux monitoring (see below). Such sets of three 0.036 mm aluminum foils were also always inserted between different target elements in order to minimize effects of cross contamination between them. Only in cases where the differences between the atomic numbers of

11

adjacent target elements were strongly different, were single foils of one element acceptable. For such elements the production of the same isotope from both elements could be excluded and recoil losses could be directly measured and corrected for. The stacks were irradiated with proton currents of 50 n A (stacks I, III, V and V I ) a n d 100 nA (stacks II and IV) for irradiation times between 120 and 269 min. The constancy of the beam currents was controlled by intermittent Faraday-cup measurements. The fluctuations turned out to be less than 1%. The proton energies were 94.4 + 0.3 MeV and 98.9 + 0.3 MeV for stacks I - I I I and IV-VI, respectively. The proton fluxes' were monitored using the reaction 27Al(p,3p3n)22Na, see [27] and references therein for experimental data. In this work, the evaluated cross sections given by Tobailem and de Lassus St. Genies [28] as "recommended values" were chosen. The actual cross sections interpolated from ref. [28] for initial energies of 94.4 MeV and 98.9 MeV are 19.3 mb and 18.9 mb, respectively. For energies below 200 MeV these "valeurs adoptees" [28,29] are also in good agreement with recent absolute cross section measurements of Steyn et al. [30]. The deviations between these two data set s [28,30] are less than 3% on average, where the maximum deviation is less than 5% (see fig. 7b). The above proton energies are different from those of 100.5 + 0.5 M e ¥ and 103.0 + 0.5 MeV adopted earlier [31], which werebased on statements of the accelerator personnel after a reconstruction of the accelerator. In our experiments it turned out, however, that the assumption of these initial p-energies lead to contradictions with the well known excitation function for the reactionE7Al(p,3p3n)EENa when determining the respective cross sections from the Al-catcher-foils in our experiments. A f t e r a direct energy measurement via time-of-flight methods, these energies were corrected to the final values of 94.4_ 0.3 MeV and 98.9 +_0.3 MeV [32]. With these energies, our results for the reaction 27Al(p,3p3n)E2Na became fully consistent with

Table 1 Nuclear data [37,38] used for the determination of cross sections via gamma-spectrometry Nuclide

Half-life

Egamma[keV]

I [%]

7Be 22Na 24Na 2SMg

53.29 d 2.602 a 14.96 h 20.90 h

477.6 1274.5 1368.5 400.6 941.7 1342.2 1778.8

10.4 99.9 100.0 35.9 35.9 54.0 t00.0

12

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

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R. Bodemann et al. / Proton-induced reactions on meteoritic elements

the cross sections evaluated by Tobailem and de Lassus St. Genies [28,29]. The proton energies for the individual target foils were calculated by a computer program using the work of Andersen and Ziegler [33]. The results of these calculations a r e in excellent agreement with those of our earlier work [13-18], which was based on the report of Williamson et al. [34]. The uncertainties given below for the proton energies in the individual targets take into account the finite thickness of the foils as well as the energy straggling calculated according to Bohr's approximation [35]. The gradient of the proton flux inside the stacks was computationally estimated, taking into account effects of nuclear absorption and angle straggling. After the irradiations the stacks were transported to Cologne and Hannover within 24 h. There, repeated gamma-spectrometer measurements were made using several Ge(Li)- and high-purity Ge-detectors. The measurements and the evaluational procedure was the same as used earlier [13-15]. The nuclear data necessary for the calculation of cross sections (table 1) were taken from the following sources: half-lives from the Chart of Nuclides [36], energies and branching ratios of gamma-rays from the compilation of Reus and Westmeier [37]. The Q-values, used in the discussions below, were calculated on the basis of the data of Keller et al. [38]. After finishing the gamma-spectrometric investigations, l°Be and 26Al were chemically separated from the targets for AMS measurements of these long-lived radionuclides. The separation schemes used for SiO2, Mg, AI and Si were the same as described earlier [31]. For the C and Si3N 4 targets the following procedures were used. Carbon: T h e graphite targets (m = 25-30 mg) together with 1.8 mg Be carrier were dissolved in 15 ml boiling HC10 4 (72%) with a catalytical amount of CrO 3. The solution was evaporated with 65% HNO 3 and

dissolved in diluted HCI. Afterwards the Be was precipitated as Be(OH) 2 by adding ammonia solution. Nitrogen: The Si3N 4 targets (m = 400-500 mg) were digested after addition of 1.5 mg Be carrier and 3 mg AI carrier with 10 ml 48% H F and 4 ml 65% HNO 3 for 12 h in a "Parr" bomb at 150°C. The silicon was removed as SiF4 by evaporating with H F / H C I O 4. The residue was taken up with 10 m o l / l HCI and loaded on a Dowex 1 × 8 anion exchange column (diameter 1 cm, length 20 cm), from which Be and Al were eluted with 15 ml 10 mol/1 HCI. The eluate was reduced to dryness, dissolved in 1 mol/1 HC1 and loaded on a Dowex 50W X 8 cation exchange column. The Be fraction was eluted with 130 ml 1 m o l / l HCI and precipitated as Be(OH) 2 with ammonia solution. Aluminum was eluted with 50 ml 4.5 mol/1 HCI and precipitated as hydroxide, too. At the end of the separations, the hydroxides were washed with bi-distilled water, dried at 100°C and glowed at 850°C to the oxides. Then the oxides were mixed with copper powder and pressed into sample holders for the AMS measurements. l ° B e / 9 B e and 26Al/27Al ratios were measured at the PSI-ETH AMS facility in Ziirich. A description of the AMS technique is given elsewhere [39]. As in our earlier work [31], for 1°Be the standard "$433" with a l ° B e / 9 B e ratio of (9.31 + 0.23)× 10 - n [11] and for 26A1 the standard "Al9" (26Al//27Al = 1.19 × 10 -9) [40] was used.

3. Experimental results In this work we present a total of 21 detailed excitation functions describing the production by proton-induced reactions of 7Be and l°Be from carbon, nitrogen and oxygen (table 2), of 7Be, 22Na and 24Na from magnesium and aluminum (table 3), of 1°Be a n d 26A1 from Al (table 4) and of 7Be, 1°Be, 22Na, 24Na, 26Al and 28Mg from Si (tables 5-7). The cross sections

Table 3 Experimental cross sections for the proton-induced production of 7Be, 22Na and 24Na from magnesium and aluminum. For 1°Be from Mg, in addition, one cross section, 0.099 ± 0.010 mb at E = 98.0 ± 0.3 MeV, was measured. target: magnesium

target: aluminum

E [MeV]

Be-7 [mb]

Na-22 [mb]

Na-24 [mb]

E [MeV]

Be-7 [mb]

Na-22 [mb]

98.0±0.3 93.5±0.5 85.0±0.6 70.4±0.9 62.3±1.1 53.2±1.4 30.4±2.5

1.87±0.~ 2.05±0.~ 1.76±0.19 1.61±0.18 1.48±0.18 1.08±0.13

48.1± 4.9 52.0± 5.4 52.2± 5.3 60.3± 6.1 65.4± 6.8 73.0±11.0 23.8± 2.4

6.34±0.65 6.88±0.72 7.19±0.74 7.60±0.78 9.07±0.94 8.30±0.85 6.40±0.65

98.5±0.3 93.9±0.5 85.6±0.7 71.0±0.9 62.9±1.2 54.0±1.4 31.6±2.5

0.912±0.098 0.98 ±0.12 0.829±0.090 0.692±0.078 0.629±0.081 0.521±0.066 0.061±0.018

19.8±2.0 ~.8±2.3 20.9±2.1 22.8±2.3 26.0±3.0 33.3±3.4 7.24±0.74

Na-24 [mb] 12.0±1.2

13.3±1.4

14

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

Table 4 Experimental cross sections for the p-induced production of roBe and Z6Alfrom aluminum E [MeV]

Be-10 [mb]

199.5±0.4 183.6±0.6 172.3±0.9 160.6±1.1 147.6±0.8 132.0±1.0 115.0±1.1 98.8±0.4 96.6±0.4 93.8±0.4 90.9±0.5 88.0±0.5 85.0±0.5 81.9±0.6 78.7±0.5

0.244 0.2~ 0.200 0.162 0.150 0.127 0.105

-+0.0~ ±0.017 ±0.017 ±0.014 ±0.013 ±0.011 -+0.009

AI-26 [rob]

E [MeV]

Be-10 [rob]

26.9±2.2 27.8±2.1 28.2±2.2 31.5-+3.3 32.1-+2.5 37.8±2.9 39.6±3.0 45.5±3.5

~.9±0.7 68.6±0.7 66.8±0.7 64.3±0.7 60.4±0.8 57.1±0.8 52.1±0.8 47.5±0.8 39.8±0.9 35.2-+0.9 31.1-+0,9 29.9-+0,9 ~.8±1.0 18.0±1.0 16.1±1.0

0.0395 ± 0.0037 0.0361 ± 0.0042

0.0747±0.0065 48.6-+3.8 0.0571±0.0~3 50.0-+3.8 0.0571±0.0~3 53.6±4.2 0.0422-+0.0~9

cover proton energies up to 100 MeV with the exception of 1°Be and 26A1from AI, for which data up to 200 MeV are given. The data for p-energies above 100

A1-26 [mb]

66.3± 5.1 0.0179 ± 0.0023 69.1± 5.3 0.0080 ± 0.0014 78,3± 6.0 78.1± 6.2 96.7± 7.1 106.0± 8.0 135.0±10.0 140.0±10.0 181.0±13.0 124.0± 9.0 78.1± 5.6

MeV were determined from target foils which were irradiated earlier at IPN/Orsay as described in detail elsewhere [16-18].

Table 5 Experimental cross sections for the production of radionuclides from natural silicon determined from pure Si-targets E

[MeV] 97.7±0.6 97.0±0.6 96.8±0.6 94.9±0.7 94.0±0.5 92.9±0.6 92.1±0.7 89.2±0.8 86.2±0.8 83.6±0.9 83.2±0.9 80.1±0.9 76.8±1.0 73.5±1.0 70.0-+1.1 68.8-+1.3 66.5±1.1 65.5-+1.1 62.7±1.2 61.5±1,2 58.8-+1.2 57,8±1.2 54,6±1.3 51.2±1.7 50.2±,1.3 45.6±1.4 44.3±1.4

Be-7 [rob]

Be-10 [mb]

Na-22 [mb]

Na-24 [mb]

Mg-28 [mb]

AI-26 [rob]

1.02 +0.10

0.0231 + 0 . 0 0 2 1 0.0182 ± 0.0019

18.7±1.9

3.35 ±0.34

0.0269-+0.0038

33.1±3.6 32.0±3.5 ~.3±5.1

3.27 ±0.33

0.0262±0.0038

3.91 3.25 3.20 3.08

±0.41 ±0.33 ±0.33 ±0.31

0.033±0.0~33 0.0265±0.0038 0.0260±0.0038 0.0256±0.0~7

3.02 2.85 2.63 2.34 2.00

±0.31 ±0.29 ±0.27 ±0.24 ±0.20

0.0~9±0.0037 0.0~1±0.0036 0.0~8±0.0035 0.0245±0.0034 0.0247±0.0034

1.07 ±0.11 0.981±0.102 1.05 ±0.11 1.14 ±0.12 0.963±0.100 0,930±0.096 0.908±0.094 0.969±0.100 0.879±0.091 0.853±0.089 0.803±0.084 0.751±0.079 0.718±0.075 0.785±0.081 0,645±0.068 0.706±0.073 0.565±0.060 0.683±0.074 0.485±0.052

0.0193 ±0.0017 0.0178 ±0.0017

0.0092 ± 0 . 0 0 1 1 0.0079 ±0.0010 0.00394±0.00~5 0.00221±0.00034 0.00139-+0.00~5

~.7±2.1 18.4±1.9 21.3±2.1 22.7±2.3 18.9±1.9 19.1±2.0 19.7±2.0 22.5±2.3 20.2±2.1 21.0±2.1 21.8±2.2 22.5±2.3 ~.1±2.4 25.7±2.6 ~.7±2.3 26.3-+2.6 ~.9-+2.1 26.6-+2.8 16,6±1.7

1.62 ±0.17

0.0244±0.0033

1.28~±0.13 1.87 ±0.20 0.98 ±0.10

0.0242±0.0032 0.0325-+0.0040 0.0240±0.0031

0.724±0.074

0.0227±0.0029

0.567±0.058 0.384+0.039

0.0186±0.0024 0.0101±0.0015

0.000~±0.00035 0.399±0.043 0.424±0.~5 0.307±0.034 0.204±0.023

10.2±1.0 11.6±1.2 4.32±0.44 1.42±0.15

46,1±5.1 37,8±4.5 37.1±4.5

36.9±4.2 34.4±4.1 43.6±5.2 53.7±5.9 41.6-+5.0 41.3±4.5

61.2±6.7 57.9±6.4 74.1±8.2 59.1±6.5 66.0±7.3

R. BodemannetaLIProton-induced reactionson meteoriticelements

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R. Bodemann et al. / Proton-induced reactions on meteoritic elements

Table 7 Experimental cross sections for the production of 28Mg from natural silicon determined from SiO2-targets. In addition, there were two cross sections for the production of 22Na from natural silicon determined from SiO2-targets, which are not affected by thermodiffusion: 21.75:1.5 mb at 91.3+2.5 MeV and 26.1 4-1.9 mb at 61.0+3.7 MeV E [MeV]

Mg-28 [mb]

98.2+0.9 96.6+ 1.0 94.95:1.0 93.2 + 1.1 91.5-1-1.1 89.8 4- 1.2 88.1 4- 1.2 82.85:1.2 79.2 4- 1.1 77.4 4- 1.3 73.64-1.5 71.6 5:1.5 67.4 4- 1.6 63. I + 1.7

0.0292+0.0048 0.0291 +0.0044

AI-26 [mb]

41.95:5.0 0.0276 5:0.0043 32.5__.+3.6 0.0294 + 0.0045 0.0310+0.0046 0.0281 4-0.0043 38.5 5:4.6 0.0299 + 0.0044 34.9+4.2 0.0296 4-0.0043 0.0271 + 0.0039 0.0275 -t-0.0037

3. I. Errors affecting the experimental data

The errors quoted for the cross sections include the uncertainties in the determination of the number of target atoms, commonly 2%, and the reproducibility of repeated measurements of monitor foils (1.5-3.5%). For the gamma-spectrometric measurements uncertainties of the efficiency calibration of the Ge- and Ge(Li)-spectrometers of 5%, the statistical errors of the net peaks and the reproducibility of repeated measurements were taken into account, the latter to estimate the uncertainties of peak evaluation and sample positioning at the detector. In case of 1°Be and 26A1, the errors of the mass of the carrier material (0.2%), of the graduated pipette (0.2%) and the errors of the AMS measurements, i.e. statistical counting errors and reproducibility of repeated measurements (3-13%) and error of standard measurements (0.5-1%), were considered. For 26A1 from Si and SiO 2 targets errors of the AMS measurements were somewhat larger (up to 6%) and an reproducibility of the cross section determination of about 9% has to be accounted for. Except for SiaN 4, Nb and Zr all targets had well defined thicknesses. The Si3N 4 ceramics showed some roughness of the surfaces. From the results obtained for ZTAl(p,3p3n)24Na from the AI catcher foils in the Si3N 4 stack (VI) an effective thickness of 1.03 mm had to be adopted for the Si3N 4 discs, well within the results of direct measurements of their thicknesses. The Nb and Zr foils showed some differences in thickness between center and outer parts of the targets due to the manufacturing process. In this stack the energies of the individual targets were corrected for this effect by taking into account the real thicknesses of those

E [MeV] 60.8::1:1.5 58.6:1:1.7 56.3+ 1.7 51.5 5:1.8 46.45-2.0 49.0 4-1.9 46.4-t-2.0 40.44-2.2 37.1 + 2.1 33.5 4-2.4 29.8-t-2.5 25.7 4-2.7 21.0 ____3.0 .

Mg-28 [mb]

AI-26 [mb] 43.5 ::1:4.8

0.0268 +0.0037 53.5 5:5.9 0.0257 + 0.0033 0.0198 +0.0027 0.0244 4-0.0032 0.0198 4-0.0027 0.00903+0.00168

78.2 +9.4

69.9 5:8.4 63.5 + 7.6

0.00237 4-0.00066 17.3 5:2.0 5.58 + 0.67 2.84 4-0.34

parts of the foils that were irradiated by the well focussed beam. The additional uncertainty introduced by this correction was accounted for in the energy errors of targets from stack III. In case of the production of 7Be and 1°Be from SiO 2 and Si3N 4 targets the contribution of Si was corrected by using the respective cross sections measured in this work. This correction turned out to be less than 2% in all cases. Except for SiaN 4, interfering reactions from impurities of the target materials were negligible. In the Si3N4 targets, however, chemical analyses showed considerable abundances of aluminum (0.2%) and iron (0.2-0.5%) together with a number of other impurities, of which only Na (33-140 ppm) and Mg (35 ppm) are of interest here as sources of interfering reactions. Moreover, these analyses demonstrated that these impurities changed for different batches of the raw Si3N 4 material. The observed impurities do not influence the results for 7Be and 1°Be from nitrogen. Only in the case of the Si(p,4pxn)24Na cross sections obtained from Si3N 4 targets a correction had to be applied for interfering reactions from Na, Mg and A1. This correction amounted to 0.024 + 0.002 mb and was only significant for cross sections near the threshold. Interferences from secondary neutrons were estimated from reactions as 27Al(n,4He)24Na and 59Co(n,p)59Fe (Co foils were among the targets contained in stack I). These reactions have Q-values of - 3 . 1 MeV and - 0 . 8 MeV respectively. 59Fe can be produced from 59C0 by neutron-induced reactions only. The neutron-induced contribution to the production of 24Na from 27A1 can be determined from targets with p-energies below the threshold of - 3 1 . 4 MeV of the

17

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

reaction 27Al(p,3pn)24Na. From these estimates the contribution of these isotopes due to secondary neutrons was found to be more than two orders of magnitude smaller than the production by primary protons in most cases. The corrections were about 2% for the 27Al(p,3pn)24Na cross sections of table 3. There were only two reactions, namely C(p, 3pxn)mBe and N(p,4pxn)l°Be, for which significant interferences of fast secondary neutron-induced reactions were found. In these two cases the Q-values of the neutron-induced reactions are lower than those of the proton-induced ones. Therefore, the pure neutron contribution can be measured in targets with energies near to or below the lowest threshold of the p-induced production. The relevant Q-values are:

only the near-threshold data, were subtracted from the measured 1°Be production cross sections for all C and N targets. An estimate of the corrections due to secondary protons is not possible from the experimental data in the same way as for the secondary neutrons. However, the fact, that excitation functions of p-induced reactions on Fe, Co, Ni and Mn, which were derived from targets irradiated in stacks I and II [26], showed excellent transition to low-energy data determined earlier by our group from targets irradiated with 45 MeV protons [13-15,17], demonstrates that interferences from secondary protons can also be neglected in our experiments. This is also supported by estimating the contributions of secondary protons from those of secondary neutrons under the assumption that the production of secondary protons is not significantly higher than that of secondary neutrons. The production data for t°Be from O, Mg, AI and Si overlap in energy with the same data for energies below 100 MeV published earlier by our group [31]. These earlier data [31] were obtained from the same type of targets of stacks I and II of the present work. They were published before the above mentioned revision of the proton energies of the accelerator. The revision [32] had two implications. First, the proton energies of the individual target foils given in [31] had to be recalculated taking into account the new energies. Second, adjusted values due to the energy shift had to be used for the monitor cross sections of the reaction 27AJ(p,3p3n)e2Na. As a consequence our cross sections for the production of 1°Be from O, Mg, AI, Si (and Mn, Fe and Ni), which we published earlier for energies below 100 MeV [31] have to be revised. The

12C(p,3p)1°Be: Q -- - 19.9 MeV 12C(n,3He)1°Be: Q - - 19.5 MeV

13C(p,p3He)'°Be: Q = - 24.4 M e V 13C(n,4He)1°Be: O = -3.8 M e V 14N(p,2p3He)1°Be: Q == - 32.0 M e V 14N(n,p4He)l°Be: Q = - 18.7 M e V 15N(p,2p4He)i°Be: Q-- -22.2 M e V ISN(n,d4He)1°Be: Q = - 20.0 MeV. From the last C and N foilsin the stacks apparent 1°Be production cross sections of 0.012 + 0.02 m b and 0.024 + 0.02 m b were derived and attributed to neutron-induced reactions in carbon and nitrogen, respectively.These contributions, which significantly affected

Table 8 Revised experimental cross sections for the production of 1°Be from various target elements. The data in this table are recalculated according to the new initial proton energy [32] and supersede those given in former work of our group [31] Target

Ea old [MeV]

E new [MeV]

Be-10 [mb]

Target

Ea old [Me~

E Be-10 new [Me~

[mb]

O

96.7~1.0 93.4±1.7 68.1~1.6 52.6~1.8 95.0±0.2 66.0±0.7 95.5±0.2 66.7±0.8

96.6±1.0 91.3±2.5 67.4±1.6 51.5±1.8 95.0±0.3 62.3±0.8 ~.0±0.4 63.0±0.9

0.52 ±0.03 0.52 ±0.03 0.34 ±0.02 0.073±0.005 0.078±0.009 0.017±0.006 0.041±0.~3 0.015±0.~4

Si

96.9±0.7 94.6±0.5 68.2±1.2 50.0±1.4 93.1±0.5 63.5±1.0 91.7±0.6 61.7±1.1 88.4±0.6 57.2±1.1

96.8±0.7 92.9±0.6 68.8±1.3 51.2±1.7 93.1±0.5 59.4±1.0 ~.1±0.6 57.9±1.1 86.9~0.7 53.3±1.2

0.0~±0.004 0.023±0.~2 0.0~±0.~3 c 0.009±0.~3 c 0.0~±0.004 0.019±0.~3 0.021±0.011 0.~8±0.~3 0.~7~0.~2 0.~3~0.~1

Mg ~b

Mn Fe Ni

a Energies according tO ref. [31], tables 1 and 2. b There are two cross sections for the reaction 27Al(p,pn)26~l in ref. [31], which also have to be revised: 77.0+4.3 mb at 63.2 MeV and 56.2+3.1 mb at 94.0 MeV, respectively. c Probably affected by contamination.

18

R, Bodemann et al. / Proton-induced reactions on meteoritic elements

revised data are presented in table 8 and supersede those given in ref. [31].

1.2

1.0

i

,

i

,

f

i

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,

4-............. ~ -

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3.2. Thermodiffusion of Na isotopes from quartz A particular problem was encountered when comparing the cross sections for production of 22Na, 24Na, and ZSMg from silicon which were derived from the three different chemical target materials silicon, quartz and silicon-nitride. I n the c a s e of ZSMg, t h e results obtained from the three target materials agreed within the limits of experimental~ errors. Good agreement was also observed between the results for the product nuclides e2Na and :4Na from the silicon foils and the Si3N 4 targets. The production cross sections for the same .two nuclides derived from the quartz targets, however, were l o w e r b y 20-50% than those obtained from Si and Si3N4 and showed broad nonsystematic scatter. This underestimation and scatter was only observed for the quartz targets of stack IV, which had been irradiated with a beam current o f l 0 0 nA. The quartz foils of stack III, which had received a beam current o f 50 nA, gave, however, consistent results with the Si and Si3N4 targets. I t is known that N a is soluble in AI [41] and that quartz can act as a sponge for N a [42]. Thus, the discrepancies could be due to diffusive loss of Na isotopes from the quartz targets when heated during irradiation; Subsequently the Na isotopes a r e t a k e n up by the adjacent A l foils. In order fo estimate the diffusion rate, a series of heating experiments was performed, using 22Na as tracer. Two irradiated quartz disks were separately enveloped in non-irradiated aluminum foils, placed in an oven and heated to preselected constant temperatures for 4 h, a typical time for our irradiation experiments. The 22Na activities in the quartz disks were gamma-spectrometrically measured without the surrounding aluminum before and after each heating procedure. New non-irradiated Al covers were used for each heating step, The temperature was gradually increased from 150°C up to 600°C. A reasonable upper limit for the temperatures in the stacks is the melting point of aluminum (670°C), as no melting effects could be observed in the aluminum foils from the irradiation experiments. Fig. 2 shows the dependence of the 22Na activities, A22 , in the quartz targets as function of the heating temperature T given as the ratios, A22(T)/A22(20°C). For temperatures up to 4000C no significant change in the ratios was observed. Between 400°C and 600°C a strong decrease of the ratios indicated increasing loss of Na isotopes from the quartz targets by thermodiffusion. At 600°C this loss went up to more than 60%. The 22Na,activity measured by gamma-spectrometry in the surrounding aluminum foils corresponded to the estimated activity loss by diffusion.

G ~

0.8

,~

0.6

~

0.4

,z

D

O

Ii]

0.2 I I I I l l l l t

I I I I l [ l l l ~ l l l l l l l l l l l l l l

200 4O0 TEMPERATURE T [oC ]

600

Fig. 2. Loss of 22Na from two quartz targets by thermodiffusion. The quartz targets wrapped in 0.038 mm Al-foil were heated in an oven for 4 h at each temperature step. The quartz targets had received a proton dose of (3.7+0.1)× 10 ~ cm -2 s - t of 93.2 MeV and of 71.6 MeV at a beam current of 100 nA. The error bars given for two data are typical for all of the measurements

Thus, the temperatures during the irradiation of stack IV must - at least locally - have been higher than 500°C. This is in coarse agreement with a temperature estimate of 420°C for this stack which takes into account the beam current and the energy loss of the protons in the stack assuming the stack to be a black radiator. In reality one has to take into account that a focussed beam was used, whereby the effective surface of the black radiator is decreased and therefore the temperature is increased. For stack III an analogous estimate gave a temperature of 320°C, well below the critical temperature. As a consequence no experimental cross sections for the production of 22Na and 24Na from Si derived from quartz targets of stack IV are reported in this work.

3.3. Discussion and comparison with earlier work The present data (tables 2-8) give fully consistent results between the different irradiation experiments. The individual excitation functions consist of up to 60 data points; They allow for a detailed comparison with all earlier measurements: Our new cross sections for the production of 7Be and 1°Be from C, N and O were obtained with targets having natural isotopic abundances. The data are compared with results from earlier works, including also those which were reported as 7Be and 1°Be production cross section from pure target isotopes 12C (98.90% abundance in natural isotopic composition), 14N (99.63%), and 160 (99.762%). Moreover, earlier measurements are considered for proton

19

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

energies up to 200 MeV in order to describe shortcomings of the existing experimental data base. First, some general comments to earlier reports, which will not be repeated when discussing the individual reactions. The cross sections reported by Kortelling and Caretto [43] for the production of Na isotopes from Mg and Si at 100 MeV were measured relative to a monitor cross section of 15.5 mb for the reaction 27Al(p,3p3n)22Na according to Cumming [44]. After renormalization to the value of 18.8 mb given by Tobailem [29]. their data are in good agreement with our present data. Other authors used cross sections for monitoring which either were not significantly different from ours or were not stated. The data for these references were not changed. Several authors [45-52] used a branching ratio of 12-12.3% for the 478 keV

10 .2

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gamma-decay of 7Be. Their data were renormalized to a more recent value of 10.4% according to ref. [37]. Several measurements were made at an internal beam of the synchrocyclotron at IPN/Orsay [49-54]. All these measurements are discrepant to other experiments. One reason of these discrepancies might be a systematic shift too higher energies of the internal beam. For example, Brun et al. [50] found the maximum of the excitation function of the reaction 27Al(p,3p3n)22Na at 52 MeV while all other measurements gave the maximum between 42 and 45 MeV [27,28]. The cross sections reported by Bimbot and Gauvin [54] at 50 MeV give also discrepant results, which can again be explained by too high adopted energies. The same authors reported cross sections for a proton energy of 153 MeV from targets irradiated in

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Fig. 3. Excitation functions for the production of 7Be from (a) carbon, (b) nitrogen and (c) oxygen. Data from this work are given as open squares. Earlier measurements are coded as A1.£2 [52], BI71 [54], BR62 [50], DI51 [45], EP71 [57] (the shaded area gives the range of data measured by these authors), GA62 [51], GE71 [59], GR60 [47], HE76 [63], JA74 [58], JU70 [56], LA66 [61], LA73 [60], LE61 [49], RA64 [53], RO76 [55], VA63 [46], WI67 [48], YI68 [62]. Measurements by Jung et al. [56], the errors of which were larger than the actual data are omitted in this figure. Errors are plotted only if they exceed the symbol size. In the case of data from refs. [48] and [60] only some exemplary error bars are shown. Hybrid model calculations are plotted as full lines (MS) and dashed lines (EX-MS).

20

R. Bodemann et aL / Proton-induced reactions on meteoritic elements

the external beam of the Orsay facility, which all arc in reasonable agreement with other experimental data. Also our own experiments at the external beam of the Orsay accelerator [16,18] did not give any evidence of a miscalibration. An overestimation of the proton energies at the internal beam of the Orsay accelerator has also been supposed to be responsible for another earlier observed discrepancy in excitation functions for p-induced reactions on barium [19].

&El. Cross sections for the production of rBe The excitation functions for the production of 7Be from carbon, nitrogen and oxygen (fig. 3) are dominated by the production from the most abundant isotopes ]2C, 14N and 160 via the reactions 12C(p,3p3n)7Be ( Q = - 5 4 . 6 7 MeV), 14N(p,4p4n)TBe ( Q = - 6 7 . 1 MeV) and 160(p,5pSn)TBe ( Q = - 9 0 . 0 MeV). There is no indication that the low abundant

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Mg (p, 9 p x n ) B e - 7

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10 ÷0

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isotopes of C, N and O contribute significantly to the production of 7Be. Since the Q-values for the production of 7Be from C, N, O by the above reactions all are above 50 MeV, the apparent thresholds of the excitation functions demonstrate that the production of 7Be involves emission of complex 4He. Production of 7Be involving larger clusters is only possible for ]2C via 12C(p,6Li)7Be. The excitation function for production of 7Be from carbon and oxygen do not, however, show prominent maxima which could be attributed to the reactions 12C(p,pn4He)7Be, 160(p,pn24He)7Be and ]60(p, 3p3n4Hc)7Be, respectively. Both show just shallow maxima followed by nearly constant cross sections up to 200 M e V . The excitation function for 7Be from nitrogen exhibits a significant peak attributable to 14N(p,24He)TBe at about 20 MeV. Above 50 MeV the cross sections are rather constant with no specific

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10 *0 Z 0

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10-1

~ 10 -1 W if?

0 THIS 0 LI62 BI71 0 HE76 RA64

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Si (p, 11pxn) B e - 7

BI71 WI67 LA66 GR83

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Excitation functions for the production of 7Be from (a) magnesium, (b) aluminum and (c) silicon. Data from this work are given as open squares. Earlier measurements are coded as BI71 [54]* FU65 [66]* GA71 [69]* GR62 [70]* GR81 [66], GR83 [71]* HE76 [63]* LA66 [61], LI62 [641, LI64 [72]* MY73 [65]* NE63 [68], RA64 [53]* SH68 [73]* WI67 [48]. Errors are plotted only if they exceed the symbol size. Fig.

4.

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

shapes indicating dominance of the reactions 14N(p,2p2n4He)TBe or 14N(p,4p4n)7Be. There is no evidence that an increase in energy significantly opens up new reaction channels for production of 7Be up to 200 MeV. The production of 7Be from carbon (fig. 3a) has been investigated in detail by several authors [4551,53,55,56] up to 155 MeV. Above 50 MeV, our new cross sections fall well within the range of data reported by [48-51]. However, the data of Williams and Fulmer [48] and Dickson and Randle [45] are about 10% lower than ours, those of B r u n e t al. [50] and Gauvin et al. [51] are higher than ours by 10%. Below 50 MeV our data are disagreeingly higher than those of Lefort et al. [49] and B r u n e t al. [50]. Further disagreement with our data is found with the work of Dickson and Randle [45], their data being systematically lower by 20-40%. Also the cross section of Roche et al. [55] at 100 MeV is too low by 20%. Between 100 and 150 MeV quite a number of measured data exists [46-51], which agree within limits of errors, while the single data points by Rayudu [53] and Jung et al. [56] are too low, the latter by nearly an order of magnitude. For the production of 7Be from nitrogen (fig. 3b) there also exists a number of earlier reports [46,54,5661] describing the excitation function fairly well up to 42 MeV. The (p,24He)-peak in the excitation function has been investigated in detail by Epherre and Seide [57] from threshold to 42 MeV. Our new data extend these data to higher energies. They are in good agreement between 35 and 45 MeV with those of Epherre and Seide [57] giving a consistent excitation function up to 100 MeV. Also the measurements of Jacobs et al. [58] up to 24 MeV are in reasonable agreement with this excitation function though their data are slightly shifted to lower energies. However, the data by Laumer et al. [60] for energies between 25 and 35 MeV are too low by 30-40% and also the cross sections given by Bimbot and Gauvin [54] at 50 and 100 MeV deviate from our data. For higher energies, there are just a few measurements [46,56,59] of 7Be production from nitrogen, which show a large scatter. Among them the data point of Jung et al. at 125 MeV [56], again is too low by an order of magnitude. For the production of 7Be from oxygen the earlier reports [52,54,61] for energies up to 100 MeV are discrepant up to a factor of 6 in magnitude and even show different shapes of the excitation function (fig. 3c). The new data give a consistent excitation function from threshold up to 100 MeV and resolve these discrepancies. The new data are in moderate agreement with those of Lafleur et al. [61] except for their cross section at 30 MeV. The data of Albouy et al. [52] and of Bimbot and Gauvin [54] have to be ruled out from our results. It is to note here, that Albouy et al.

21

[52] irradiated BeO. So they had to correct for the contribution of the 9Be(p,aH)TBe-reaction. The authors themselves pointed out that there could be a large source of error in the cross sections for the latter reaction. Production of 7Be from Mg, Al and Si (fig. 4) starts at energies between 25 and 30 MeV with cross sections in the microbarn region. After a steep increase of the excitation functions, they level off between 40 and 70 MeV showing fairly constant cross sections around 1 mb up to 150 MeV. The early onset of the production can only be explained by emission of heavier particles, in particular by evaporation of 7Be, as has been discussed in detail by Lindsay and Neuzil [64]. Calculation of Q-values for the various possible reaction modes even shows that the near-threshold cross sections can only be explained by evaporation of 7Be with no further complex particles being emitted. A survey of cross sections at energies above 150 MeV shows [31] that a second production mode sets in roughly between 200 and 300 MeV exhibited by further increase of the excitation functions, followed by high energy plateaus with cross sections around 10 mb. This second plateaus has to be attributed to production of 7Be by fragmentation. Though there is a number of earlier investigations of the production of 7Be from Mg [53,54,63,64], Al [48,54,61,64-66,68-72] and Si [48,54,73], our new data fill in some important gaps and clear up some existing discrepancies. However, for 7Be from Mg, Al and Si further measurements are needed. For Mg(p,9pxn)TBe (fig. 4a), our data fill a gap between 50 MeV and 100 MeV. Two cross sections reported by Bimbot et al. [54] at 50 and 100 MeV are 30-40% lower than ours. Below 35 MeV Lindsay et al. [64] investigated this reaction, but there are still further measurements needed between 35 and 50 MeV to see whether their data are consistent with ours. Above 100 MeV there are two measurements by Heydegger et al. [63] at 119 MeV and by Rayudu [53] at 130 MeV which are compatible with our data. The earlier investigations of the production of 7Be from AI (fig. 4b) below 150 MeV [65-68] show some discrepancies for energies below 60 MeV. Thus, the consistent excitation functions reported by Griitter [66], Furukawa et al. [67] and Lindsay and Neuzil [61,68] are in contrast to the measurements up to 52 MeV by Miyano [65]. The new data agree within errors with the results of other authors for energies above 60 MeV. However, our cross section measured at 53 5:1 MeV is larger by a factor of about 1.5 compared to the data given by Griitter [66] and Furukawa et al. [67] and do support the Miyano [65] measurements. Our data point at 29.4 + 1.1 MeV is located at the lower end of the region of discrepancy and does not allow to decide about these uncertainties. For this reaction more mea-

R. Bodemann et aL / Proton-induced reactions on meteoritic elements

22

surements are also necessary. For energies between 150 and 200 MeV just a few but consistent measurements exist [69-72]. For Si(p,X)TBe (fig. 4c) there were just three measurements available up to 150 MeV up till now. Our data agree within the errors with those by Sheffey et al. [73] up to 60 MeV and extend the well investigated area up to 100 MeV. The measurements by Rayudu [53] at 130 MeV and Bimbot and Gauvin [54] at 153 MeV are compatible with the low energy data.

mation of l°Be via the reactions 14N(p,2p3He)l°Be (Q = - 3 2 . 0 MeV) and 160(p,3p4He)]°Be ( Q - - - 3 8 . 5 MeV). In the case of 1°Be from carbon (fig. 5a), earlier data by Fontes [74] are in reasonable agreement with our cross sections. Roche et al. [55] measured one cross section of 0.5 ( + 0.4, - 0 . 2 ) mb at 100 MeV for the reaction 12C(p,3p)l°Be, which is lower by a factor of 2 compared to our value of 0.939 + 0.072 mb at 96.3 MeV. There are just three more data points for higher energies, which are compatible with the shape of our excitation function. Fontes et al. [75] measured cross sections of 1.1 4- 0.1 mb at 150 MeV. Honda and Lal [76] reported a cross section of 1.8 + 0.6 mb for 220 MeV. For the N(p,4pxn)mBe reaction (fig. 5b), our measurement is the first one reported below 100 MeV. The results give a smooth transition to the results of Raisbeck and Yiou [77], who reported 0.6 4- 0.2 mb at 150

3.3.Z Cross sections for the production of i°Be Also for 1°Be from carbon, nitrogen and oxygen the production from the most abundant isotopes via the reactions 12C(p,3p)mBe ( Q - - - - 1 9 . 9 MeV), 14N(p, 4pn)mBe (Q = - 3 2 . 0 MeV) and 160(p,5p2n)mBe (Q = - 6 2 . 6 MeV) is dominating. The empirical thresholds of the excitation functions (fig. 5) clearly demonstrate the importance of complex particles in the for10

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Fig. 5. Excitation functions for the production of 1°Be from (a) carbon, (b) nitrogen and (c) oxygen. Data from our group (this work and ref. [31]) are given as open squares. Earlier measurements are coded as AM72 [78], FO71 [75], FO75 [74], HO64 [76], JU70 [56], RA74 [77], RO76 [55], YI68 [62]. Measurements by Jung et al. [56], the errors of which were larger then the actual data are omitted in this figure. Errors are plotted only if they exceed the symbol size. Hybrid model calculations are plotted as full lines (MS) and dashed lines (EX-MS).

23

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

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Fig. 7. Excitation functions for the production of 22Na from (a) magnesium, (b) aluminum and (c) silicon. For the target element AI only our results (open squares), the recent measurements by Steyn et al. [30] and the recommended data by Tobailem et al. [28,29] are given. For full references of earlier work see refs. [27-29]. For Mg and Si data from this work are given as open squares, earlier measurements of other authors are coded as BA51 [81], BA54 [83], BA84 [84], BI71 [54], FU65 [67], FU71 [87], HE76 [63], KO70 [43], ME51 [80], RA64 [53], SH68 [73], WA76 [85]. Hybrid model calculations are plotted as full lines (MS), dashed lines (EX-MS) and dashed-dotted lines (EX-MS-C). Errors are plotted only if they exceed the symbol size.

24

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

MeV. However, the value of 1.6 5:1.6 mb measured by Jung et al. [56] at 125 MeV must be discarded when compared with the new cross section of 0.51 5:0.03 mb at 94.9 MeV being lower by a factor of 3. The present cross sections for the reaction O ( p , 5 p x n ) l ° B e (fig. 5c) are in good agreement with the readjusted values from ref. [31] (table 8). Furthermore, Yiou et al. [62] measured a cross section of 0.37 5:0.12 mb at 135 MeV slightly different from 0.59 + 0.05 mb reported by Amin et al. [78] for the same energy, which after renormalization to the correct half-life of 1°Be, give a better agreement with our new cross sections than the earlier value [62]. A systematic study of roBe production cross sections from proton-induced reactions in oxygen was recently reported by Sisterson et al.

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R. Bodemann et al. / Proton-induced reactions on meteoritic elements

7Be, there is is no indication at low energies that evaporation of 1°Be is a significant reaction mode. The appearent thresholds can be explained by evaporation and preequilibrium emission of light (A < 4) nuclei with 1°Be as a residual nucleus. In spite of the importance of 1°Be in cosmochemistry there has been up to now an extreme lack of cross section measurements for the production of 1°Be from all relevant target elements. Data for the production of 1°Be from proton-induced reactions in Mg, Al and Si for energies up to 200 MeV have only been reported by our group [31]. There is a great need for further data, except for energies above 600 MeV for which energy region earlier measurements exist [31]. For the reaction Mg(p,9pxn)l°Be there is just one new cross section of 0.099 + 0.010 mb at 98.0 5:0.3 MeV from this work, which agrees with the renormalized cross sections from ref. [31]. The present data for the reaction 27Al(p,10pxn)1°Be are in good agreement with our earlier work [31], giving together a consistent excitation function up to 200 MeV (fig. 6a). Earlier measurements of this reaction exist only for energies above 600 MeV [31]. For the reaction Si(p,llpxn)l°Be (fig. 6b), the present data agree with earlier measurements [31] above 90 MeV. The cross sections earlier reported by our group [31] at 50.1 MeV and 68.2 MeV could not be confirmed, the new data being lower by more than an order of magnitude. From our present knowledge we attribute the earlier discrepant data to have suffered from contamination problems during the chemical separation. 3.3.3. Cross sections for the production o f 2eNa and 24Na 2~Na and ~ N a are produced from Mg, Al and Si by reactions ranging from (p,2pxn) to (p,4pxn), where the emission of protons, neutrons and 4He-particles is due to evaporation and preequilibrium processes. A number of earlier measurements exist for Z2Na production cross sections from proton-induced reactions in Mg, Al and Si, which together with the present data give a fairly good description of the excitation functions up to 200 MeV (fig. 7). Several earlier measurements exist of the 24Na production cross sections for Al (fig. 8), see refs. [27-29] for references, while just a few earlier references can be found for the production of this radionuclide for Mg [43,54,82] and Si [43,53,54,73]. Since the production of 22Na and 24Na from 27A1 was investigated by a large number of authors, [27-29] and references therein, and since evaluated excitation functions exist [28,29], we here will just refer to these recommended data and to selected works of other authors. The 22Na production cross sections from Mg have been measured for natural [43,54,63,81,83,86] and isotopically enriched targets [82,83]. The latter measure-

25

ments will not be considered here. The measurements of the present study fill a gap in the earlier data in the energy region between 50 MeV and 100 MeV (fig. 7a) and agree well at low energies with the excitation function given by Furukawa et al. [86] up to 52 MeV. At 100 MeV there is a smooth transition to the data of Bimbot and Gauvin [54] and Kortelling et al. [43], both at 100 MeV and of Heydegger et al. [63] at 119 MeV. In contrast, the results of Bartell and Sofftky [81] between 22 MeV and 31 MeV and of Bimbot et al. [54] at 50 MeV are higher than our results by factors of 3-30 and those of Rayudu [53] at 130 MeV lower by 30%. The low-energy data of Batzel and Coleman [83] are in agreement with those of Furukawa et al. [86] if shifted about 3 MeV to lower energies. The reaction 27Al(p,3p3n)22Na was used for beam monitoring in this work. The cross sections derived in this work for this reactions (table 3) were determined from the 0.125 mm Al foils of stacks I and II using proton fluxes determined from the first 0.036 mm Al catcher foils in each stack. The resulting excitation function was in good agreement with the recommended function by Tobailem and de Lassus St. Genies [28] and with the results of recent absolute measurements of Steyn et al. [30]. The result of this comparison confirms the recommendations by Tobailem and de Lassus St. Genies [28] and demonstrates the consistency of our procedures (fig. 7b). Also for 22Na from Si there is a number of earlier investigations [43,53,54,63,67,73,84-86], from which together with our new data the excitation function can be well described up to about 150 MeV (fig. 7c). At energies below 60 MeV our data agree within errors with the cross sections given by Furukawa et al. [67,86] and Sheffey et al. [73]. The low-energy data from threshold up to 45 MeV by Walton et al. [85] show large scatter and are in moderate agreement with other measurements [67,73,86], while the cross section given by Bimbot et al. [54] at 50 MeV disagrees. At higher energies, the present data are consistent with the resuits of Kortelling and Caretto [43], Heydegger et al. [63], Baros and Regnier [84] and the cross section by Bimbot and Gauvin [54] at 153 MeV. The result of Rayudu [53] at 130 MeV is too low by a factor of 2-3 compared to the above references. Our cross section data for the Mg(p,2pxn)24Na reaction are the first measured ones below 100 MeV, fitting well to the renormalized cross section given by Kortelling and Caretto [43] at 100 MeV (fig. 8a). Three data points exist [43,53] between 100 MeV and 200 MeV which are compatible with the data up to 100 MeV. Further measurements are needed to determine the excitation function above 100 MeV more accurately. The new cross sections for 27Al(p,3pn)24Na are 1015% higher than the evaluated data given by Tobailem

26

R. Bodemann et al. / Proton-induced reactions on meteoritic elements l o '~

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et al. [29], but lie well within the scatter of the experimental data and are in good agreement with earlier results of our group [18] obtained from irradiations at I P N / O r s a y (fig. 8b). T h e p r e s e n t cross section d a t a for the Si(p,4pxn)24Na reaction describe in detail the excitation functions from threshold up to 100 MeV (fig. 8c). The data obtained from different irradiation experiments and from pure Si and SiaN 4 targets are fully consistent within experimental errors. They also agree within errors with the measurements up to 60 MeV by Sheffey et al. [73]. Again, the cross section given at 50 MeV by Bimbot and Gauvin [54] is either too low or shifted to higher energies. The measurement by Kortelling et al. [43] at 100 MeV agrees well with ours, and those by Rayudu [53] at 130 MeV and Bimbot and Gauvin [54] at 153 MeV are compatible with the new cross sections. Also here more measurements above 100 MeV are needed.

[88] measured cross sections up to 52 MeV after chemical separation using counting techniques, while Schneider et al. [87] measured 26AI by AMS for p-energies between 50 MeV and 159 MeV. Our present data agree within errors with those of Furukawa et al. [86], while an explanation for the discrepancy between the data of Schneider et al. [87] and ours has not yet been found. Both, our data and those by Schneider al. [87] show excellent transition to the cross sections reported by Furukawa et al. [86] around 50 MeV those being

3.3.4. Cross sections for the production o f 26.4l from AI and Si The new measurements together with those of ref. [31] have provided 27 data points between threshold and 200 MeV for the reaction 27Al(p,pn)26Al (fig. 9a). Our data contains analyses of targets irradiated at I P N / O r s a y , Universit6 Catholique Louvain La Neuve and University of Uppsala. Several independent chemical separations were done. The AMS measurements were performed during various beam times and spread out in time more than one year. As shown in fig. 9a good consistency is obtained. However, some discrepancies are observed compared to the results of other measurements [86-88]. Furukawa et al. [86] and Evans

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R. Bodemann et al. / Proton-induced reactions on meteoritic elements significantly higher than the data of Evans et al. [88] in the low-energy region. The production of 26A1 from silicon is dominated by the reaction 28Si(p,2pn)26Al (Q - - 2 4 . 6 MeV). Significant contributions of the low-abundant Si isotopes by 29Si(p,4He)26Al (Q = - 4 . 8 MeV) and 3°Si(p,n4He)26Al (Q = - 15.4 MeV) are only seen below 30 MeV. For silicon, there were up to now only cross sections available up to 51 MeV measured by Furukawa et al. [86], though silicon is the most important target element for cosmogenic 26A1. The present investigation gives data for energies between 20 and 100 MeV measured from pure silicon and quartz targets. The obtainable accuracy of the cross section determinations was only 11-12%. But, because of the large number of measurements, the excitation function is now well described up to 100 MeV (fig. 9b). Our new data are slightly higher than those of Furukawa et al. [86] in the overlapping energy region. Further investigations will be carried out to clear up the situation [26].

3.3.5. Cross sections for the production of 28Mg from Si 28Mg is produced from silicon exclusively by the reaction 3°Si(p,3p)28Mg. The present measurements of 28Mg from Si are the only ones below 100 MeV (fig. 10). The results obtained from different irradiation experiments and different target materials agree within the limits of experimental errors. Above 100 MeV the data are compatible with the results of a measurement by Heydegger et al. [63] at 121 MeV, but they are in contradiction with a single cross section measurement by Morrison and Caretto [89], the latter result being to high by about a factor of three. Also here more measurements are needed at higher energies.

27

results as well as to find the best choice of parameters with respect to global a priori calculations of nuclide production by proton-induced reactions on light (A < 30) target elements. T h e code A L I C E L I V E R M O R E 87 [24] is an extended version of the code A L I C E L I V E R M O R E 82 [22]. It contains a number of new features compared to the older version. First, it allows experimental nuclide masses according to the Wapstra and Gove mass table [91] to be used. If this option is chosen, the Meyers and Swiatecki mass formula [92] is applied only for those nuclides for which no experimental masses exist. It allows for the choice of broken exciton numbers, thus taking into account the statistical distribution of different possible initial exciton configurations. A detailed discussion of this feature was given by Blann and Vonach [23]. Thirdly, in contrast to ALICE LIVERM O R E 82 the more recent ALICE L I V E R M O R E 87 takes into account multiple preequilibrium (PE) decay, allowing for both the emission of more than one nucleon from a single exciton configuration and for the PE emission of several nucleons in sequential exciton configurations. ALICE 900 is an extension of ALICE LIVERM O R E 87 allowing calculations to be performed up to proton energies of 900 MeV, but without taking into account new physical phenomena such as meson production, fragmentation or giant resonances which become important at intermediate energies above a few hundred MeV. In this version an option was implemented allowing for correction of shell effects onto the nuclear level densities [93]. First results of hybrid model calculations by this code above 200 MeV were reported by Pearlstein [94], who found it necessary to use improved reaction cross sections to describe p-induced reactions on 56Fe.

4. Hybrid model analysis 4.1. Choice of parameters The hybrid model of preequilibrium reactions [20] in combination with the statistical model of equilibrium reactions [90] in the form of the A L I C E code has found wide-spread applications for a priori calculations of integral excitation functions. However, there now exists a considerable number of A L I C E versions released during more than one decade [21-25], which make it difficult to understand the reasons for disagreement between different calculational results and experimental data. Moreover, there is a wide variety of choices with respect to parameters and calculational details, the influence of which does not always become clear from published reports. In this work hybrid model calculations were performed using two recent versions of ALICE, namely ALICE L I V E R M O R E 87 [24] and A L I C E 900 [25]. Various parameter combinations were chosen in order to investigate the sensitivity of their effect upon the

In the present work five different, but seemingly reasonable combinations of options and parameters were used for the calculations. Common to all these calculations were the following features: For the equilibrium reactions standard Weisskopf and Ewing [90] evaporation calculations with multiple particle emission (p, n, d, 4He) were performed. For preequilibrium reactions the geometry dependent hybrid model (GDH) was used. Preequilibrium emission of protons and neutrons, but not of complex particles was taken into account. Inverse cross sections were calculated from the optical model subroutine of the ALICE codes, which uses the Becchetti and Greenlees [95] optical model parameters. Since it is not possible in the code to use individual level density parameters for particular residual nuclides, a constant level density parameter of A / 9 was adopted. The initial exciton configuration was

28

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

also kept constant. Initial states with broken exciton numbers were used as discussed by Blann and Vonach [23] and as they were found to be very successful in our earlier work [18]. Intranuclear transition rates were calculated using the Pauli corrected nucleon nucleon scattering cross sections, since the optical model parameters are valid only up to 55 MeV [95]. However, the mean free paths for intranuclear transitions based upon nucleon cross sections are in quite resonable agreement with optical model analyses of p-induced reactions up to 200 MeV [18]. In accordance with our earlier work up to 45 MeV and 200 MeV [13-19] a mean free path multiplier k = 1 was adopted. For the excitation energies energy bins of 0.5 MeV were used up to Ep = 100 MeV and of 1 MeV for higher energies. In our earlier work [18] we observed an improvement of the calculations of production cross sections for p-induced reactions on medium mass elements (22 < Z _< 28) for energies above 50 MeV when switching from A L I C E L I V E R M O R E 82 to the 1987 version, that was attributed by us to both use of experimental nuclide masses and taking into account of multiple PE decay in combination with broken initial exciton numbers. When investigating which of these changes mainly cause the improvements, we found a wide variability of the results depending on the choice of nuclide masses. Therefore, in this work we performed a variety of calculations using the different options related to the choice of nuclide masses, which, however, influence the calculations in many respects, as there are e.g level densities and intra- and extranuclear transition rates. Moreover, calculations were performed using shell dependent level densities as introduced into the ALICE code by Kataria et al. [93]. Of the five types of calculations only three will be shown in this work. These calculations have the following combinations of options and will be referred to as MS, EX-MS and EX-MS-C, respectively: MS: masses according to the Meyers and Swiatecki mass formula [92] without shell or pairing corrections; EX-MS: experimental Wapstra and Gove [91] masses as far as available, MS masses else, but without shell and pairing corrections; EX-MS-C: experimental masses as far as available, else MS masses with shell and pairing corrections and shell corrected level densities according to Kataria et al. [93]. The calculations, which are not further discussed here, were (1) MS masses including corrections for shell and pairing effects and (2) experimental masses as far as available, otherwise MS masses with shell and pairing corrections but without shell corrected level densities. They gave much worse results than the above three calculational types.

4.2. Discussion o f calculational results

Excitation functions were calculated in 1 MeV steps up to 50 MeV, 5 MeV steps between 50 and 100 MeV and 10 MeV steps above 100 MeV. Hybrid model calculations of the production of 7Be and 1°Be from Mg, AI and Si are not feasible. The existing codes do not allow for such large charge differences between target and product because of limitations of storage space. The differences between the results of the five types of calculations were striking (figs. 3, 5-10). The different options drastically affected the shapes of the excitation functions near threshold and also the appearent threshold energies. There are differences between the calculated thresholds of up to 12 MeV. The calculations only occasionally match the experimental data near the thresholds and no systematically "best" calculational option exists. Moreover, at higher energies the calculated cross sections differed between a few percent and factors of up to 10, the differences being not systematic. Since the applicability of equilibrium and preequilibrium models has to be questioned in general for light target elements, the same calculations were performed for radionuclide production from the medium mass target elements Co and Cu, for which detailed measurements are available [18,96], and references therein. For these target elements some systematic trends in the differences could be observed. The calculations MS and EX-MS gave nearly identical result for Co and Cu, while EX-MS-C resulted in significantly different excitation functions (factors between 1.5 and 10). There was generally much better agreement between the excitation functions calculated by method MS or EX-MS with the experimental data for Co and Cu than for the other calculations. For the light target elements C, N and O dealt with in this work no such clear cut statements can be made. The quality of the calculations varied from one target element to another, probably exhibiting problems either with the statistical approach of the mass formula for small masses or with the transition from empirical masses to calculated ones in quite narrow parts of the valley of stability. MS and EX-MS did no longer give identical excitation functions. Calculations using exclusively the mass systematics (MS) are in better agreement with the experimental cross sections. We attribute this to the problem of transition from experimental to theoretical mass data, which might cause discontinuities in the excitation energies. It is to point out that for the target elements carbon, nitrogen and oxygen EX-MS-C generally gives the least adequate results. This may point to problems with the shell and pairing corrections for nuclides far from stability. It has to be pointed out that the energy binning used in the calculations generally had significant influ-

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

ence onto the results. For the calculated excitation functions an energy binning of 0.5 MeV was used up to Ep--- 100 MeV. Above 100 MeV 1.0 MeV energy bins were applied. For the light target elements of this work, energy bins of 1.0 MeV below 100 MeV caused unphysical, saw-toothed shapes in the excitation functions. This effect vanishes if one goes to target elements with higher masses (e.g. Co). It is most prominent for the target elements C, N, and O and it has also significantly affected the calculations for Mg. Further, it has a larger influence on calculations using shell corrected Meyers-Swiatecki masses or shell corrected level densities than on calculations using uncorrected masses. The excitation functions for the production of 7Be and l°Be from C, N and O can be completely explained by evaporation and PE decay of single nucleons and of light nuclei (A < 4) though the calculations do not always give satisfactory results (figs. 3 and 5). Generally, the MS calculations showed the best results, while EX-MS results where generally insufficient being lower up to an order of magnitude over the entire energy range for all six reactions. EX-MS-C calculations led even to unphysical discontinuities in the excitation functions. Therefore, only the MS and EX-MS calculations are presented in figs. 3 and 5. A further reduction of the energy binning for these calculations did not improve the calculations, it even led to incorrect terminations of the computer runs. There are particular shortcomings of the calculations for the reaction 14N(p,4p4n)7Be. The two-aHe evaporation peak in this reaction is underestimated by a factor of more than 2 (fig. 3b) and between 40 MeV and 100 MeV where the experimental data show a nearly constant plateau, the calculations underestimate this excitation function by more than an order of magnitude, probably due to neglect of PE emission of 4He particles. At higher energies, when nucleon emission becomes dominant, the MS calculations are compatible with the rare experimental data. There is no code available to calculate cross sections for the production of 7Be and 1°Be from Mg, AI and Si for energies below 100 MeV and it seems inadequate to extend spallation systematics to such low energies. Also intranuclear cascade evaporation calculations using the high energy transport (HET) code within the HERMES code system [97] did not give adequate description of these reactions, but underestimated the experimental data by about an order of magnitude [31]. Considering the production of the other radionuclides from Mg, Al and Si, in case of Mg the results generally were of low quality, while for AI and Si the MS, EX-MS and EX-MS-C calculations turned out to be significantly better. However, there is no generally recommendable set of calculational options and pa-

29

rameters, since the quality of the calculations changed from one reaction to the other (figs. 7-10). Generally, for 22Na from Mg, AI and Si (fig. 7) and for 26Al from Al and Si (fig. 9) the differences between the five different calculations are smaller (up to a factor of 2 for higher energies) than for the light target elements, EX-MS and EX-MS-C ones giving the best, but still not satisfying results. For 22Na the shapes of the excitation functions are partially rather well described, but for 26A1 from AI the high energy part of the excitation function is strongly underestimated. In the case of 24Na from Mg, AI and Si (fig. 8), all MS type calculations overestimate the experimental data. Moreover, they all show unphysical irregularites in their shapes which could not be avoided by any meaningful parameter combination. But again, the differences between EX-MS and EX-MS-C calculations are less than a factor of 2, if one neglects the energy shifts near the thresholds. These latter calculations describe the experimental data up to about 100 MeV fairly well, but strongly underestimate the high energy parts of the excitation functions. Finally, for the reaction 3°Si(p,3p)28Mg the different calculational results are strongly disagreeing (fig. 10). There is none, which adequately describes all the experimental data. Up to 80 MeV the experimental cross sections are best described by MS and EX-MS calculations. But for higher energies both, MS and EX-MS, strongly understimate the excitation function. The results from EX-MS-C calculations are completely insufficient for this reaction.

5. Conclusions

A consistent set of 21 excitation functions was measured for the p-induced production of 7Be and 1°Be from C, N, O, Mg, AI and Si, of 22Na and 24Na from Mg, AI and Si, of 26A1from ml and Si and of 28Mg from Si up .to 100 MeV. The new data fill gaps in existing data, allow to decide about earlier existing discrepancies and, as in cases of C(p,3pxn)l°Be, N(p,4pxn)l°Be and Si(p,3p)28Mg, are the first complete excitation functions reported for energies below 100 MeV. The comparison of experimental data with hybrid model calculations for a variety of calculational options demonstrated that the capability of this model to predict unknown excitation functions is for light elements not as clear cut as for medium mass target elements. Though the calculations show that modelling of the reactions is possible in the framework of equilibrium and preequilibrium theories even for the lightest elements dealt with here. It is, however, not possible to recommend a global set of calculationai options which allows for adequate a priori calculations. This result emphasizes the importance of further experimental

30

R. Bodemann et al. / Proton-induced reactions on meteoritic elements

measurements to satisfy the data needs for applications of production cross sections, here in particluar for the interpretation of SCR interactions with matter.

Acknowledgements The authors are grateful to the authorities of the Svedberg Laboratory for making available the irradiations at Uppsala, and to the staffs of the accelerator for their kind cooperation. The AMS measurements were supported financially and personally by the Paul Scherrer Institute/Villigen. This work was supported in part by the Deutsche Forschungsgemeinschaft, Bonn, by the Swedish Natural Science Research Council, Stockholm, and by the Swiss National Science Foundation, Bern. The Si targets were made from wafers, which were kindly provided by Wacker Chemitronic, Burghausen. All this support is gratefully acknowledged. The authors gratefully acknowledge the contribution of Dr. T. Wiedling in establishing this collaboration.

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