- Email: [email protected]
Production of 7Be, 22Na, 24Na and 10Be from Al in a 4π-irradiated meteorite model
Nuclear Instruments and Methods North-Holland, Amsterdam
PRODUCTION MODEL
P. ENGLERT, Institut
OF ‘Be,
in Physics
‘*Na, “Na
Research
415
B5 (1984) 415-419
AND “Be
FROM
Al IN A 4sr-IRRADIATED
METEORITE
S. THEIS and R. MICHEL
ftir Kernchemie der Universitiit zu K&t, D _5000Kiiln I, Fed. Rep. Germany
C. TUNIZ Istituto di Fisica, Universitb degli Studi, Trieste, Italy
R.K. MONIOT Division
of Science and Mathematics, Fordham University, New York, NJ, USA
S. VAJDA Department
of Physics, New Jersey Institute of Technology, Newark, NJ, USA
T.H. KRUSE Department
of Physics, Rutgers University, New Brunswick, NJ, USA
D.K. PAL and G.F. HERZOG Department
of Chemistry
Rutgers University, New Brunswick, NJ, USA
A model of a small meteorite, 10 cm in diameter, made of granodiorite was rotated in a beam of 600 MeV protons to simulate 4virradiation conditions in space. Analysis of AI foils placed at known locations within the model give depth profiles of high energy (‘Be, “Be) and low energy ( 24Na, **Na) spallation p roducts. These isotopes were measured by means of high-resolution gamma-ray or accelerator mass spectroscopy (AMS). While the production rate profiles of the high energy products are flat within the limits of uncertainty, the profiles of the low energy products increase from the surface to the center by a factor of approximately 1.2. This increase indicates the importance of secondary neutrons and protons for the production of low energy spallation products, even within small meteorites.
1. Introduction Since the first detection of its products in meteorites about 30 years ago, the interaction of the galactic cosmic radiation with matter has been the subject of continuous research. Since then, both the study of extraterrestrial objects exposed to the galactic cosmic radiation (GCR) and simulation experiments performed with charged particles have led to a better, but still imperfect, understanding of the phenomena observed. In particular, the development of the secondary particle cascade as a function of the dimensions of the extraterrestrial bodies and its influence on the production of so-called cosmogenic nuclides must be better understood. The goal of simulation experiments is to study, via energetic particle bombardment of an extended target, the development of the secondary particle cascade for the body irradiated, the spatial distribution of reaction 0168-583X/84/$03.00 0 Elsevier Science publishers B.V. (North-Holland Physics publishing Division)
products, elemental production rates and production rate ratios. The first attempt to simulate the 4+irradiation conditions to which a meteorite is exposed in space was reported by Lavrukhina et al. [l], who rotated an iron sphere (10 cm diameter) in a defocus& proton beam. However, only short-lived isotopes produced were then determined and the authors do not give evidence as to whether a homogeneous irradiation was achieved by this experimental setup. Many experiments of this kind have been done to simulate the 2a-geometry of the lunar surface (refs. [2,3] and references therein), the latest of which used a 16 t target [4]. The simulation experiment reported here is the first one of a series of 4s-meteorite model irradiations, in which short-lived and long-lived radioisotopes as well as stable nuclides are measured in cosmochemically relevant target elements at various locations within the “extraterrestrial” body. The goal is to determine depth profiles of nuclear V(c). ASTROPHYSICS
416
P. Engiert et al. / Prodwtion
reaction products from selected target elements, and then, using these results, to infer the development of the secondary cascade within the model. Long-lived cosmogenie radionuclides such as ‘*Be (T,,, = 1.6 x lo6 a), 26Al ( TI,2 = 7.2 x 10’ a), and s3Mn (T,,, = 3.8 X 10” a) play a major role as a link between the meteorite data and the simulation experiment.
2. Experimental A suitable material for the representation of extraterrestrial matter, whether synthetic or natural, has to be essentially water-free [4] and have a density of approximately 3 g/cm). Among the terrestrial rocks, certain granodiorites meet these requirements most closely (H,O < 10K3 g/g, density 2.98 g/cm3). Materials from the Odenwald (West Germany) and from San Diego County (California, USA) were used for this and previous experiments ]4,5]. A 10 cm diameter granodiorite sphere was made with two cores (8 mm diameter) drilled perpendicular to each other through its center [5] (see fig. 1). Isotropic irradiation of the target was achieved
4-$= IOcm
STACKS
OF
FOILS
:
Mg,Al,Si,Guarlz.Ti.Fe,Co. Ni,Cu,Rh,Bo,Lu.Au
I-20mm-I
Fe& Al
FOILS
ETTER
(ilEGASSED.SEALEO IN QUARTZ)
II
of ‘Be, “Na,
‘“Na and “Be
by superposition of two linear and two rotational movements of the target during the experiment. The diorite sphere contained in different positions within the drilled cores stacks of elemental foils, meteoritic material and chemical compounds (see fig. 1). A thin target stack and a circular Al foil 10 cm in diameter were located 50 cm and 10 cm in front of the sphere, respectively. The arrangement was irradiated with 2.6 pA of 600 MeV protons for 8 h 10 min at the synchrocyclotron at CERN. Homogeneous irradiation within 5% was achieved, as proved by 7Be, 22Na and 24Na in Al and Fe monitor foils. The irradiation produced at least 10” atoms/g of interesting product nuclides. The short-lived isotopes ‘Be, **Na and 24Na were measured by high-resolution gamma ray spectroscopy. Dete~nations of other stable and radioactive isotopes are in progress. For this study, aluminum samples (reactor grade, 0.881.4% Mg) from different locations of the granodiorite sphere (Core I, fig. 1) were available for “Be and 26Al determination after 7Be and 22Na were determined instrumentally. Two major problems had to be solved in the subsequent chemical separation: an almost complete removal of 22Na from the Al-fraction in order to avoid interferences for the final y-y coincidence counting and the removal of boron from the ‘Be, “Be-fraction. The separation scheme is given in fig. 2. Each sample of Be0 was mixed with a subequal volume of Ag-powder and mounted in the ion source of the Rutgers tandem Van de Graaff accelerator [6]. 9Be and “Be were analysed sequentially. A Wien filter at tbe object position eliminated the background due to In 30 mm of count9L3e during the t0Be measurements. ing a reagent grade Be0 blank, no events assignable to “‘Be were recorded. The l”B background was reduced to manageable levels by a charge-changing post-stripper and by differential absorption before dispersion in an Enge split-pole magnetic spectrograph. The specific activities of the samples were calculated by comparing their measured ‘“Be/9Be ratios to that of a laboratory standard.
3. Results @
1
PbS-PILL
1
C0F@lLL
!
Cu&Al
FOILS
AI-CONTAINERS n=l,S. LiF ” =2k: Kf+O, n=3
:MgF2
Fig. 1. Schematic view of the small granodiotite model. The elemental and meteoritic target materials and their location within Cores I-III are shown.
Results for 7Be, 22Na and the long-lived “Be in the aluminum targets exposed within the meteorite model are given in table 1. For the calculation of the production rates, a 4rr-integrated primary proton flux of 1 cm -’ s-r was used. Fig. 3 shows the production rates of 7Be, “Be and “Na in Al targets as a function of depth within the meteorite model. A depth dependence is observed for the low energy spallation nuclide *‘Na in Al, the production rate of which increases by a factor of 1.2 from the surface to the center. No depth dependence, however, is observed for the two high energy
P. Englert et al. / Production of ‘Be, 22Na, “Na
r
repreclpltote
417
and “Be
1-2X
Vacwm filtration through cooled filter precipitate
I
I
solution
I
discard t&sample
m Al-26 (7-7-okcidencel
9soT
c5 Be0
Be-7 (7-spectr.1 Be-10 (AM3
\
\
Fig. 2. Scheme for the separation of ‘Be, “Be and 26A1from aluminum foils.
spallation products ‘Be and “Be in Al. The ‘Be and “Be contents of samples occupying equivalent positions within the core through the granodiorite sphere agree within the limits of uncertainty, the average production rate being 11.6 f 1.2 (X 10M5 atoms ‘Be s-t g-‘) and 2.9 f 0.2 (X 10m5 atoms “Be s-l g-‘) for an incident 4rr-flux, $s, of 1 cmm2 s-l. The “Be/‘Be production rate ratio in Al for the given irradiation conditions is then 0.25 + 0.03. The ratio approximates the expected 600 MeV proton cross section ratio of ‘Be and “Be of - 0.2 [7,8].
Table 1 ‘Be, “Be and 22Na activities (dpm/kg) meteorite model Sample Al Al Al Al AI Al Al Al
10 I 10 III 11-2 12-3 13-4 14-5 15 I 15 II
Michel et al. (51 determined ‘Be, 22Na and 24Na by instrumental methods in high purity Al foils which were exposed within the small meteorite model at locations close to our own. Their production rates are shown as a function of depth in fig. 4 [5]. In addition, thin target production rates for the primary 600 MeV protons ( P6a,,) are given there. ‘Be shows the same flat profile as observed for the radiochemically separated ‘Be and “Be. Though the average p reduction rate of 10.1 f 0.7 (X lo-* atoms ‘Be s-l g-‘) for five Al foils adjacent to the samples analysed for “Be is some 12% lower than
and production rates (atoms s-’
Depth (mm)
‘Be ( X lo9 dpm/kg)
‘Be ‘) (X 10m5 s-’ g-‘)
“Be (dpm/kg)
0.2 2.5 23.5 44.5 41.5 22.0 2.1 1.5
4.8 + 0.7 3.6 f 0.5 5.2*0.8 5.2 + 0.8 5.4 f 0.8 5.1+0.7 5.2kO.8 5.0+0.7 average:
ll.Ok1.6 9.0*1.4 12.0*1.I3 12.0 f 1.8 13.0 f 2.0 12.0+1.s 12.0+ 1.8 12.0 rt 1.8 11.6+ 1.2
104 f 20 103 + 15 114*14 120f 15 103+13 122*12 119fll 125 f 18
average:
g-‘)
in Al from Core I of a small proton bombarded
‘aBe a) ( X10-s 2.6 + 0.5 2.6 f. 0.4 2.9 f 0.4 3.0*0.4 2.6 + 0.4 3.1 *IO.3 3.OkO.3 3.1 f 0.5 2.9 f 0.2
s-’ g-‘)
22Na (X lo9 dpm/kg)
22Na a)
1.1 kO.4 0.8 f 0.08 1.2kO.l 1.3+0.1 1.3fO.l 1.23tO.l 1.2*0.1 1.1*0.1
4.4kO.4 3.3 f 0.3 4.9*0.5 5.1*0.5 5.2rtO.5 4.9 f 0.5 4.7 f 0.5 4.6 f 0.4
(X 10m4 s-’
g-i)
‘) For the calculation of production rates a 4n-integrated flux of 600 MeV primary protons of 1 cmm2 s-l was used. V(c). ASTROPHYSICS
P. Englert et al. / Production
418
of 7Be, 2’Na, 2’Na and “3e -f-
I
40
y!! + + ) + + i’ +
T
“%
No-22 (AL1
6 ii 8 =i, -i m
+ t
+
,e-24(A’)
c
35
i, +
30
i
+
i
iftr
Be- 7 (Al)
Be-lO(Al)
i 0
5
DEPTH
0
[cm1
Fig. 3. Depth profiles of ‘Be, “Be and ‘*Na in Al from a 4r”proton irradiation (E, = 600 MeV) of a small meteorite model (diameter 10 cm, samples from Core I). Profiles of the high energy spallation products 7Be and “Be are essentially flat, revealing a production rate ratio of approximately 0.5. Production rates of the nuclide 22Na increase from the surface to the center by a factor of 1.2. Circles indicate that all samples analysed came from Core I (see fig. 1).
the radi~he~cal value here, they agree within their limits of uncertainty. The average over all 19 instrumentally measured Al foils from Cores I-III (fig. 1) is 9.5 f 0.4 (x10-’ atoms ‘Be s-l g-l). The relative increase of 22Na from the surface to the center by a factor of 1.25 differs only slightly from that of our sample set, however, 22Na production rates of the radiochemically treated AI are higher by appro~mately 30%. A reason for the difference could be the Mg-content of the reactor grade Al (- 1% Mg). Reactions on Mg could enhance 22Na production. No difference is observed between the average ‘Be production rates in Al of 11.8 & 1.2 (X lo-’ atoms s-r g-‘) and 10.1 rfr0.6 (~10~~ atoms s-r g-‘f and the respective thin target production rate for 600 MeV protons of 10.9 k 0.1 (X lo-’ atoms s-* g-‘) (see fig. 4). The same is expected for “Be, for which the determination of the thin target production rate is in progress. This behavior was expected for high energy spallation products such as ‘Be and “Be, since little variation has been observed for “Be in small meteorites f6]. The flat profiles also demonstrate that homogeneous 4n-irradiation was achieved. Low and medium energy products such as 22Na and 24Na in Al show increased production rates near the center of the target, as compared to the corresponding thin target production rates of 3.57 + 0.06 ( x 10e4 atoms
DEPTH
km1
Fig. 4. Depth profiles of ‘Be, “Na and 24Na in Al, as determined exclusively by instrumental methods. ‘Be and 22Na show the same trends as in fig. 3. However, the absolute **Na activity is approximately 30% lower here. The reason may be excess production of ‘*Na from a - 1% Mg impurity in the samples analysed in fig. 3. Circles indicate sample locations in Core I; squares and triangles indicate locations within Cores II and III of the granodiorite sphere, respectively. Diamonds represent thin target production rates for 600 MeV protons (Pea& obtained in this experiment.
2ZNa s-l g-*) and 2.52 f 0.04 (X 10S4 atoms 24Na s-r g-r), respectively (see also fig. 4). This increase demonstrates the importance of secondary neutrons and protons for the production of low and medium energy spallation products even within meteorites with diameters as small as 10 cm. As flat profiles of low energy spallation nuclides are found in small meteorites [9,10], they may be the result of a superposition of galactic and solar cosmic ray effects, which may have been underestimated Ill]. Interpretation of these findings by comparison with semiempi~cal and Monte Carlo model calculations is in progress.
4. Conclusions Depth profiles of high and low energy spallation products of Al exposed to 600 MeV protons within a core through a 10 cm diameter 4n-irradiated meteorite model were determined by means of high-resolution gamma ray and acceIerator mass spectrometry. Depth profiles of the high energy products 7Be and “Be are essentially flat resulting in an overall production rate
P. Englerr et al. / Production of ‘Be, “Na,
ratio of l/4, whereas those of the low energy products 22Na and 24Na increase from the surface to the center, i.e. over a distance of only 5 cm, by a factor of approximately 1.2. This increase indicates the importance of the contribution of secondary neutrons and protons to the production of low energy spallation products. Measurements of the same product isotopes in other target elements are in progress to establish a network of depth profiles, elemental production rates and production rate ratios. This information will help to improve our understanding of the GCR interaction with small meteorites. A further 4n-irradiation of a 50 cm diameter sphere has also been performed and a 30 cm diameter sphere will be irradiated in the fall of 1984.
References [l] A.K. Lavrukhina, G.K. Ustinova, V.V. Malyshev Saratova, At. Energiya 34 (1973) 23.
and L.M.
24Na and “Be
419
[2] R. Michel, H. Weigel, H. Kulus and W. Herr, Radiochim. Acta 21 (1974) 169. [3] T.P. Kohman and M.L. Bender, in: High energy reactions in astrophysics, ed., B.S.P. Shen (W.A. Benjamin, New York, 1967) p. 169. [4] P. Englert, R.C. Reedy, R. Fox and J.R. Arnold, Meteoritics 18 (1983), in press. [5] R. Michel, P. Dragovitsch, D. Filges and P. Cloth, Lunar Planet. Sci. Conf. 15, Abstr. (1984) p. 542. [6] R.K. Moniot, H. Kruse, W. Savin, G.S. Hall, T. Milazzo and G.F. Herzog, Nucl. Instr. and Meth. 203 (1982) 495. [7] J. Tobailem and C.H. de Lassus St.-Genies, CEA-N-1466 (4) (1977). [8] G.M. Raisbeck and F. Yiou, Phys. Rev. Cl2 (1975) 915. [9] P. Englert, U. Herpers, W. Herr and R. Sarafin, 5th Int. Conf. on Geochronology, cosmochronology and isotope geology, Nikko, Japan (1982) p. 89. [lo] 0. Braun, L. Schultz, H.W. Weber and F. Benjamin, ibid., p. 35. [ll] R. Michel, G. Brikmann and R. Stuck, Earth Plant. Sci. Lett. 59 (1982) 33.
V(c). ASTROPHYSICS
PRODUCTION MODEL
P. ENGLERT, Institut
OF ‘Be,
in Physics
‘*Na, “Na
Research
415
B5 (1984) 415-419
AND “Be
FROM
Al IN A 4sr-IRRADIATED
METEORITE
S. THEIS and R. MICHEL
ftir Kernchemie der Universitiit zu K&t, D _5000Kiiln I, Fed. Rep. Germany
C. TUNIZ Istituto di Fisica, Universitb degli Studi, Trieste, Italy
R.K. MONIOT Division
of Science and Mathematics, Fordham University, New York, NJ, USA
S. VAJDA Department
of Physics, New Jersey Institute of Technology, Newark, NJ, USA
T.H. KRUSE Department
of Physics, Rutgers University, New Brunswick, NJ, USA
D.K. PAL and G.F. HERZOG Department
of Chemistry
Rutgers University, New Brunswick, NJ, USA
A model of a small meteorite, 10 cm in diameter, made of granodiorite was rotated in a beam of 600 MeV protons to simulate 4virradiation conditions in space. Analysis of AI foils placed at known locations within the model give depth profiles of high energy (‘Be, “Be) and low energy ( 24Na, **Na) spallation p roducts. These isotopes were measured by means of high-resolution gamma-ray or accelerator mass spectroscopy (AMS). While the production rate profiles of the high energy products are flat within the limits of uncertainty, the profiles of the low energy products increase from the surface to the center by a factor of approximately 1.2. This increase indicates the importance of secondary neutrons and protons for the production of low energy spallation products, even within small meteorites.
1. Introduction Since the first detection of its products in meteorites about 30 years ago, the interaction of the galactic cosmic radiation with matter has been the subject of continuous research. Since then, both the study of extraterrestrial objects exposed to the galactic cosmic radiation (GCR) and simulation experiments performed with charged particles have led to a better, but still imperfect, understanding of the phenomena observed. In particular, the development of the secondary particle cascade as a function of the dimensions of the extraterrestrial bodies and its influence on the production of so-called cosmogenic nuclides must be better understood. The goal of simulation experiments is to study, via energetic particle bombardment of an extended target, the development of the secondary particle cascade for the body irradiated, the spatial distribution of reaction 0168-583X/84/$03.00 0 Elsevier Science publishers B.V. (North-Holland Physics publishing Division)
products, elemental production rates and production rate ratios. The first attempt to simulate the 4+irradiation conditions to which a meteorite is exposed in space was reported by Lavrukhina et al. [l], who rotated an iron sphere (10 cm diameter) in a defocus& proton beam. However, only short-lived isotopes produced were then determined and the authors do not give evidence as to whether a homogeneous irradiation was achieved by this experimental setup. Many experiments of this kind have been done to simulate the 2a-geometry of the lunar surface (refs. [2,3] and references therein), the latest of which used a 16 t target [4]. The simulation experiment reported here is the first one of a series of 4s-meteorite model irradiations, in which short-lived and long-lived radioisotopes as well as stable nuclides are measured in cosmochemically relevant target elements at various locations within the “extraterrestrial” body. The goal is to determine depth profiles of nuclear V(c). ASTROPHYSICS
416
P. Engiert et al. / Prodwtion
reaction products from selected target elements, and then, using these results, to infer the development of the secondary cascade within the model. Long-lived cosmogenie radionuclides such as ‘*Be (T,,, = 1.6 x lo6 a), 26Al ( TI,2 = 7.2 x 10’ a), and s3Mn (T,,, = 3.8 X 10” a) play a major role as a link between the meteorite data and the simulation experiment.
2. Experimental A suitable material for the representation of extraterrestrial matter, whether synthetic or natural, has to be essentially water-free [4] and have a density of approximately 3 g/cm). Among the terrestrial rocks, certain granodiorites meet these requirements most closely (H,O < 10K3 g/g, density 2.98 g/cm3). Materials from the Odenwald (West Germany) and from San Diego County (California, USA) were used for this and previous experiments ]4,5]. A 10 cm diameter granodiorite sphere was made with two cores (8 mm diameter) drilled perpendicular to each other through its center [5] (see fig. 1). Isotropic irradiation of the target was achieved
4-$= IOcm
STACKS
OF
FOILS
:
Mg,Al,Si,Guarlz.Ti.Fe,Co. Ni,Cu,Rh,Bo,Lu.Au
I-20mm-I
Fe& Al
FOILS
ETTER
(ilEGASSED.SEALEO IN QUARTZ)
II
of ‘Be, “Na,
‘“Na and “Be
by superposition of two linear and two rotational movements of the target during the experiment. The diorite sphere contained in different positions within the drilled cores stacks of elemental foils, meteoritic material and chemical compounds (see fig. 1). A thin target stack and a circular Al foil 10 cm in diameter were located 50 cm and 10 cm in front of the sphere, respectively. The arrangement was irradiated with 2.6 pA of 600 MeV protons for 8 h 10 min at the synchrocyclotron at CERN. Homogeneous irradiation within 5% was achieved, as proved by 7Be, 22Na and 24Na in Al and Fe monitor foils. The irradiation produced at least 10” atoms/g of interesting product nuclides. The short-lived isotopes ‘Be, **Na and 24Na were measured by high-resolution gamma ray spectroscopy. Dete~nations of other stable and radioactive isotopes are in progress. For this study, aluminum samples (reactor grade, 0.881.4% Mg) from different locations of the granodiorite sphere (Core I, fig. 1) were available for “Be and 26Al determination after 7Be and 22Na were determined instrumentally. Two major problems had to be solved in the subsequent chemical separation: an almost complete removal of 22Na from the Al-fraction in order to avoid interferences for the final y-y coincidence counting and the removal of boron from the ‘Be, “Be-fraction. The separation scheme is given in fig. 2. Each sample of Be0 was mixed with a subequal volume of Ag-powder and mounted in the ion source of the Rutgers tandem Van de Graaff accelerator [6]. 9Be and “Be were analysed sequentially. A Wien filter at tbe object position eliminated the background due to In 30 mm of count9L3e during the t0Be measurements. ing a reagent grade Be0 blank, no events assignable to “‘Be were recorded. The l”B background was reduced to manageable levels by a charge-changing post-stripper and by differential absorption before dispersion in an Enge split-pole magnetic spectrograph. The specific activities of the samples were calculated by comparing their measured ‘“Be/9Be ratios to that of a laboratory standard.
3. Results @
1
PbS-PILL
1
C0F@lLL
!
Cu&Al
FOILS
AI-CONTAINERS n=l,S. LiF ” =2k: Kf+O, n=3
:MgF2
Fig. 1. Schematic view of the small granodiotite model. The elemental and meteoritic target materials and their location within Cores I-III are shown.
Results for 7Be, 22Na and the long-lived “Be in the aluminum targets exposed within the meteorite model are given in table 1. For the calculation of the production rates, a 4rr-integrated primary proton flux of 1 cm -’ s-r was used. Fig. 3 shows the production rates of 7Be, “Be and “Na in Al targets as a function of depth within the meteorite model. A depth dependence is observed for the low energy spallation nuclide *‘Na in Al, the production rate of which increases by a factor of 1.2 from the surface to the center. No depth dependence, however, is observed for the two high energy
P. Englert et al. / Production of ‘Be, 22Na, “Na
r
repreclpltote
417
and “Be
1-2X
Vacwm filtration through cooled filter precipitate
I
I
solution
I
discard t&sample
m Al-26 (7-7-okcidencel
9soT
c5 Be0
Be-7 (7-spectr.1 Be-10 (AM3
\
\
Fig. 2. Scheme for the separation of ‘Be, “Be and 26A1from aluminum foils.
spallation products ‘Be and “Be in Al. The ‘Be and “Be contents of samples occupying equivalent positions within the core through the granodiorite sphere agree within the limits of uncertainty, the average production rate being 11.6 f 1.2 (X 10M5 atoms ‘Be s-t g-‘) and 2.9 f 0.2 (X 10m5 atoms “Be s-l g-‘) for an incident 4rr-flux, $s, of 1 cmm2 s-l. The “Be/‘Be production rate ratio in Al for the given irradiation conditions is then 0.25 + 0.03. The ratio approximates the expected 600 MeV proton cross section ratio of ‘Be and “Be of - 0.2 [7,8].
Table 1 ‘Be, “Be and 22Na activities (dpm/kg) meteorite model Sample Al Al Al Al AI Al Al Al
10 I 10 III 11-2 12-3 13-4 14-5 15 I 15 II
Michel et al. (51 determined ‘Be, 22Na and 24Na by instrumental methods in high purity Al foils which were exposed within the small meteorite model at locations close to our own. Their production rates are shown as a function of depth in fig. 4 [5]. In addition, thin target production rates for the primary 600 MeV protons ( P6a,,) are given there. ‘Be shows the same flat profile as observed for the radiochemically separated ‘Be and “Be. Though the average p reduction rate of 10.1 f 0.7 (X lo-* atoms ‘Be s-l g-‘) for five Al foils adjacent to the samples analysed for “Be is some 12% lower than
and production rates (atoms s-’
Depth (mm)
‘Be ( X lo9 dpm/kg)
‘Be ‘) (X 10m5 s-’ g-‘)
“Be (dpm/kg)
0.2 2.5 23.5 44.5 41.5 22.0 2.1 1.5
4.8 + 0.7 3.6 f 0.5 5.2*0.8 5.2 + 0.8 5.4 f 0.8 5.1+0.7 5.2kO.8 5.0+0.7 average:
ll.Ok1.6 9.0*1.4 12.0*1.I3 12.0 f 1.8 13.0 f 2.0 12.0+1.s 12.0+ 1.8 12.0 rt 1.8 11.6+ 1.2
104 f 20 103 + 15 114*14 120f 15 103+13 122*12 119fll 125 f 18
average:
g-‘)
in Al from Core I of a small proton bombarded
‘aBe a) ( X10-s 2.6 + 0.5 2.6 f. 0.4 2.9 f 0.4 3.0*0.4 2.6 + 0.4 3.1 *IO.3 3.OkO.3 3.1 f 0.5 2.9 f 0.2
s-’ g-‘)
22Na (X lo9 dpm/kg)
22Na a)
1.1 kO.4 0.8 f 0.08 1.2kO.l 1.3+0.1 1.3fO.l 1.23tO.l 1.2*0.1 1.1*0.1
4.4kO.4 3.3 f 0.3 4.9*0.5 5.1*0.5 5.2rtO.5 4.9 f 0.5 4.7 f 0.5 4.6 f 0.4
(X 10m4 s-’
g-i)
‘) For the calculation of production rates a 4n-integrated flux of 600 MeV primary protons of 1 cmm2 s-l was used. V(c). ASTROPHYSICS
P. Englert et al. / Production
418
of 7Be, 2’Na, 2’Na and “3e -f-
I
40
y!! + + ) + + i’ +
T
“%
No-22 (AL1
6 ii 8 =i, -i m
+ t
+
,e-24(A’)
c
35
i, +
30
i
+
i
iftr
Be- 7 (Al)
Be-lO(Al)
i 0
5
DEPTH
0
[cm1
Fig. 3. Depth profiles of ‘Be, “Be and ‘*Na in Al from a 4r”proton irradiation (E, = 600 MeV) of a small meteorite model (diameter 10 cm, samples from Core I). Profiles of the high energy spallation products 7Be and “Be are essentially flat, revealing a production rate ratio of approximately 0.5. Production rates of the nuclide 22Na increase from the surface to the center by a factor of 1.2. Circles indicate that all samples analysed came from Core I (see fig. 1).
the radi~he~cal value here, they agree within their limits of uncertainty. The average over all 19 instrumentally measured Al foils from Cores I-III (fig. 1) is 9.5 f 0.4 (x10-’ atoms ‘Be s-l g-l). The relative increase of 22Na from the surface to the center by a factor of 1.25 differs only slightly from that of our sample set, however, 22Na production rates of the radiochemically treated AI are higher by appro~mately 30%. A reason for the difference could be the Mg-content of the reactor grade Al (- 1% Mg). Reactions on Mg could enhance 22Na production. No difference is observed between the average ‘Be production rates in Al of 11.8 & 1.2 (X lo-’ atoms s-r g-‘) and 10.1 rfr0.6 (~10~~ atoms s-r g-‘f and the respective thin target production rate for 600 MeV protons of 10.9 k 0.1 (X lo-’ atoms s-* g-‘) (see fig. 4). The same is expected for “Be, for which the determination of the thin target production rate is in progress. This behavior was expected for high energy spallation products such as ‘Be and “Be, since little variation has been observed for “Be in small meteorites f6]. The flat profiles also demonstrate that homogeneous 4n-irradiation was achieved. Low and medium energy products such as 22Na and 24Na in Al show increased production rates near the center of the target, as compared to the corresponding thin target production rates of 3.57 + 0.06 ( x 10e4 atoms
DEPTH
km1
Fig. 4. Depth profiles of ‘Be, “Na and 24Na in Al, as determined exclusively by instrumental methods. ‘Be and 22Na show the same trends as in fig. 3. However, the absolute **Na activity is approximately 30% lower here. The reason may be excess production of ‘*Na from a - 1% Mg impurity in the samples analysed in fig. 3. Circles indicate sample locations in Core I; squares and triangles indicate locations within Cores II and III of the granodiorite sphere, respectively. Diamonds represent thin target production rates for 600 MeV protons (Pea& obtained in this experiment.
2ZNa s-l g-*) and 2.52 f 0.04 (X 10S4 atoms 24Na s-r g-r), respectively (see also fig. 4). This increase demonstrates the importance of secondary neutrons and protons for the production of low and medium energy spallation products even within meteorites with diameters as small as 10 cm. As flat profiles of low energy spallation nuclides are found in small meteorites [9,10], they may be the result of a superposition of galactic and solar cosmic ray effects, which may have been underestimated Ill]. Interpretation of these findings by comparison with semiempi~cal and Monte Carlo model calculations is in progress.
4. Conclusions Depth profiles of high and low energy spallation products of Al exposed to 600 MeV protons within a core through a 10 cm diameter 4n-irradiated meteorite model were determined by means of high-resolution gamma ray and acceIerator mass spectrometry. Depth profiles of the high energy products 7Be and “Be are essentially flat resulting in an overall production rate
P. Englerr et al. / Production of ‘Be, “Na,
ratio of l/4, whereas those of the low energy products 22Na and 24Na increase from the surface to the center, i.e. over a distance of only 5 cm, by a factor of approximately 1.2. This increase indicates the importance of the contribution of secondary neutrons and protons to the production of low energy spallation products. Measurements of the same product isotopes in other target elements are in progress to establish a network of depth profiles, elemental production rates and production rate ratios. This information will help to improve our understanding of the GCR interaction with small meteorites. A further 4n-irradiation of a 50 cm diameter sphere has also been performed and a 30 cm diameter sphere will be irradiated in the fall of 1984.
References [l] A.K. Lavrukhina, G.K. Ustinova, V.V. Malyshev Saratova, At. Energiya 34 (1973) 23.
and L.M.
24Na and “Be
419
[2] R. Michel, H. Weigel, H. Kulus and W. Herr, Radiochim. Acta 21 (1974) 169. [3] T.P. Kohman and M.L. Bender, in: High energy reactions in astrophysics, ed., B.S.P. Shen (W.A. Benjamin, New York, 1967) p. 169. [4] P. Englert, R.C. Reedy, R. Fox and J.R. Arnold, Meteoritics 18 (1983), in press. [5] R. Michel, P. Dragovitsch, D. Filges and P. Cloth, Lunar Planet. Sci. Conf. 15, Abstr. (1984) p. 542. [6] R.K. Moniot, H. Kruse, W. Savin, G.S. Hall, T. Milazzo and G.F. Herzog, Nucl. Instr. and Meth. 203 (1982) 495. [7] J. Tobailem and C.H. de Lassus St.-Genies, CEA-N-1466 (4) (1977). [8] G.M. Raisbeck and F. Yiou, Phys. Rev. Cl2 (1975) 915. [9] P. Englert, U. Herpers, W. Herr and R. Sarafin, 5th Int. Conf. on Geochronology, cosmochronology and isotope geology, Nikko, Japan (1982) p. 89. [lo] 0. Braun, L. Schultz, H.W. Weber and F. Benjamin, ibid., p. 35. [ll] R. Michel, G. Brikmann and R. Stuck, Earth Plant. Sci. Lett. 59 (1982) 33.
V(c). ASTROPHYSICS