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Measurements of the Isotopes of Lithium, Beryllium

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Measurements of the Isotopes of Lithium, Beryllium Measurement Isotopee th f so Lithiumf so , Berylliumd ,an Boron from ACE/CRIS Nolfo*e d . A . , G N.E. Yanasakf, W.R. Binns**, A.C. Cummingst, E.R. Christian*, J.S. Georget, P.L. Hink**, M.H. Israel**, R.A. Leskef, M. Lijowski**, R.A. Mewaldtf, E.G. Stone*, T.T. von Rosenvinge M.Ed *an . Wiedenbeck4 * Laboratory for High Energy Astrophysics, NASAIGoddard Space Flight Center, Greenbelt, MD, 20771 USA ^ Space Radiation Laboratory, California Institute of Technology, Pasadena, CA, 91125USA ** Department of Physics andMcDonnell CenterSpacethe for Sciences, Washington University, Louis,St. MO, 63130 USA * Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109 USA Abstract. The isotopes of lithium, beryllium, and boron (LiBeB) are known in nature to be produced primarily a fusiospallatio O - ba y nCN d fronan m interactions between cosmic rayinterstellad san r nuclei. While eth dominant source of LiBeB isotopes in the present epoch is cosmic-ray interactions, other sources are known to exist, including the production of 7Li from big bang nucleosynthesis. Precise observations of galactic cosmic-ray LiBeB in addition to accurate modeling of cosmic-ray transport can help to constrain the relative importance among the different production mechanisms. The Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) has measured nuclei with 2 < Z < 30 in the energy range ~30-500 MeV/nucleon since 1997 with good statistical accuracy. We present measurements of the isotopic abundances of LiBeB and discuss these observations in the context of previous cosmic-ray measurements and spectroscopic observations. INTRODUCTION assessed through observations of low-metallicity halo stars [6], [7]. These observations indicate an approxi- The elements lithium, beryllium, and boron (LiBeB) are mately linear correlation between the elemental ratios of rare in our galaxy and offer important constraints on Be/H or B/H and Fe/H (or metallicity of the star), in con- r understandinou f nucleago r astrophysics. These frag- tradiction with the expected quadratic behavior for the elemente il generatet no e sar appreciabln di e amountn si productio LiBef no B nuclei from GCRs released intn oa stellar nucleosynthesi destroyee ar t sbu stellan di r inte- ISM, which is itself increasing in metallicity with time riors thid reflectes si an , theiy abundancdw b rlo - na n ei fro contributione mth f evolvino s g stars lineaA . r cor- ture (as measured in meteorites and stellar photospheres) relation suggests a primary origin of LiBeB nuclei not [1]. Galactic cosmic-ray (OCR) LiBeB e nucleith n o , coupled to the ISM metallicity, such as the fragmenta- other significanhandn i foune e b ar , o dt t excess over tion products from freshly synthesized supernova ejecta. their solar-system abundances. This exces quits si e large Several scenarios have been postulate explai- o dt ob e nth (LiBeB/CNO -0.2 r OCR5fo s compared with ~10~6 served correlation, including the origin of LiBeB species for solar-system values) and is attributed to the spalla- from within superbubble regions [8], [9], [10] thin I . s tion of CNO OCRs on the interstellar medium (ISM) [2]. model, supernova eII) N Typ, (S whic I eI commoe har n isotopiw fe A c discrepancies betwee abundanceR nGC s in the early Galaxy and originate from stars in large and solar-system abundances, specifically the ratios of associations, ten sweeo dbubblt a t pou e filled witha 6Li/ n d 7t B/consistenLan ino 10 e Bar , t with spallation warm densitw lo , y mixtur freshlf eo y synthesized super- production, suggesting the need for other sources of the novae ejecta and remaining interstellar material. Low- lithiu borod man n isotopes suc stellas ha r nucleosynthe- energy carbon and oxygen (< 100 MeV/nucleon) are pro- sis [3] and neutrino spallation [4]. In addition, some 7Li duced and accelerated within SN II, perhaps at the re- productio expectes ni d from primordial nucleosynthesis verse shock [11],[12]. Once ejected, these low-energy [5]. carbo oxyged nan n nucle subsequentle ar i y accelerated evolutioe Th LiBef no B ovebee s lase ha r th nt r 10Gy by massiv forware eth start a thes n r i ds o I shockI e N S f so CP598, Solar Galacticd an Composition, . Wimmer-Schweingrube F edite . R y db r © 2001 American Institute of Physics 0-7354-0042-3/017$ 18.00 251 Downloaded 02 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp superbubble regions and interact with interstellar hydro- DATA ANALYSIS gen and helium to produce LiBeB abundances via frag- mentation in agreement with observations of old Popula- CRIS identifie charge th smas d ean f inciden o s t parti- tion II stars [13], [14]. Other models have been proposed, cles using multiple dE/dx measurements and the total en- including the origin of Be and B nuclei by OCRs accel- ergy deposited within stacked silicon solid-state detec- erated as debris from grains formed in SNII ejecta [15], tors [19]mase Th . s resolutio refines ni d wit meae hth - [16]. surement of particle trajectory via a scintillating optical Observations of GCR LiBeB nuclei help to con- fiber hodoscope (SOFT) [19]. In the present study, CRIS strain modele origi th d evolutio f an no s f presenno t observations of LiBeB are separated into two separate epoch abundances. The Cosmic Ray Isotope Spectrom- time intervals, corresponding to differing levels of solar s beeeteha nr E measurin(CRISAC n o ) isotopie gth c modulation firse .tTh time period ranges from December composition of 2 < Z < 30 in the energy range ~30-500 , 19920 Januaro 7t , 1999y23 , which corresponda o st MeV/nucleon since 1997, including the abundances of tota 31f o l 1 days, excluding period intensf so e sola- rac LiBeB between -30-200 MeV/nucleon. CRIS observa- tivity. The second period ranges from January 24, 1999 tions of LiBeB nuclei originate almost exclusively from to April 18, 2000 correspondin days0 totaa 27 ,f o gt o l OCRO fasCN ts interactin produco t M g IS ewit e hth again excluding periods of intense solar activity. The ef- secondary LiBeB nuclei between 200-500 MeV/nucleon. fec f solao t r modulation experienced during theso etw The resulting GCR LiBeB nuclei depend on the frag- time intervals is determined using a spherically symmet- mentation rate and the rate of escape from the Galaxy. ric model described by Fisk [20]. The solar modulation Indeed LiBeR GC B , e mos escapth f to e fro Galaxye mth . parameter, <|), is determined by fitting elemental spectra As the escape rate is essentially the same for all LiBeB from CRIS at low energies and from HEAO-3 at high en- species of equal energy per nucleon [17], the relative ergies with the predictions of a propagation model [21]. abundances are a direct measure of the relative produc- firse Th t intervae lth correspondd an V M (|o 0 )st ~40 tion rates. Approximatel LiBeye halth f Bo f nuclei pro- second time interval correspond. MV (|0 o )st ~59 duced by GCRs, however, result from the fragmentation protonR heliuGC d y san b mM whosofIS e energy spec- 1000 tra peak around 1 GeV. Provided the cross-sections are not too different between the interactions of ISM (p,He) 800 (CNOR +inversGC LiBee > )- th d Bean arounV Ge d1 interactions of ISM (CNO) + GCR (p,He) -> LiBeB in the energy range covered by CRIS, the observed abun- 600 dances of LiBeB isotopic ratios from CRIS should re- flect the actual isotopic ratios observed in the ISM. In- 400 deed, base reviea cross-sectione n th d o f wo f Reaso d and Viola [18] by Yanasak et al. (these proceedings), the 200 cross-sections between 200-500 MeV/nucleo thosd nan e between 500-2000 MeV/nucleo e inversth r nfo e reac- 0 averagn tiono e sar e les sdifferent M tha% IS 0 n2 e Th . ratio 6Li/7Li, however, may not be similar to the GCR 0 100 200 300 400 ratio sinc cross-sectioe e th fusioa + A=6,o a nt r nfo 7 E (MeV) increases by more than 2 orders of magnitude between 10 and 100 MeV/nucleon, implying that a large frac- tion of ISM lithium may be produced at these low ener- FIGUR . E1 Energy loss (AE) versu energe sth deposite) y(E d gies. Thus, GCR LiBeB observations over a wide range i ndetectoa r element midwa, Be y , througLi , stace He hth r kfo and B events. The solid lines indicate the region excluded from in energ providn yca e important constraint cosmicn so - the analysi excludo st e events duty-cycl withiw lo e nth e helium ray origi evolutiod nan addition ni decipherino nt e gth event buffer. dominant source LiBef so B over time thin I . s papere w , CRI designes Si transmio dt t onl ysmala l samplf eo present the measurements of GCR LiBeB from Decem- highle th y abundant helium events [19]- . Sincon e eth ber 1997 through April 200 compard 0an e these obser- board classification of an event as helium is rather coarse vations with previously published results. and based only on pulse heights, and since lithium is ad- jacent to helium, lithium events within certain energy in- terval treatee sar helius da sampled man reducea t da d duty cycle. Figur show1 e exampln a senerg e th f eo y loss versus the energy deposited in two detectors mid- 252 Downloaded 02 Oct 2007 to 131.215.225.176.
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