Heil, M

Käppeler, F

Plag, R

Reifarth, R

Walter, S

CERN Document Server

PoS(NIC-IX)188Neutron capture studies with a short flight pathStephan Walter∗1, Michael Heil2, Franz Käppeler1, Ralf Plag1, and René Reifarth31 Institut für Kernphysik, Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe,Germany2 Gesellschaft für Schwerionenforschung, Planckstr. 1, D-64291 Darmstadt, Germany3 Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, USAE-mail: stephan.walter@ik.fzk.deThe time of flight (TOF) method is an important tool for the experimental determination of neu-tron capture cross sections which are needed for s-process nucleosynthesis in general, and foranalyses of branchings in the s-process reaction path in particular. So far, sample masses of atleast several milligrams are required to compensate limitations in the currently available neutronfluxes. This constraint leads to unacceptable backgrounds for most of the relevant unstable branchpoint nuclei, due to the decay activity of the sample. A possible solution has been proposed bythe NCAP project at the University of Frankfurt. A first step in this direction is reported here,which aims at enhancing the sensitivity of the Karlsruhe TOF array by reducing the neutron flightpath to only a few centimeters. Though sample masses in the microgram regime can be used bythis approach, the increase in neutron flux has to be paid by a higher background from the promptflash related to neutron production. Test measurements with Au samples are reported.International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX25-30 June 2006CERN∗Speaker.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC-IX)188Neutron capture studies with a short flight path Stephan Walter1. The s process and neutron capture cross sectionsNeutron capture reactions in the s process contribute to about 50% of the isotopic abundancesbetween Fe and Bi. Since the time span between two neutron capture reactions (1-10 years) is muchlonger than the time scale for β−decays, the reaction path follows closely the valley of stability.Sometimes radioactive isotopes with half-lives comparable to the time between two neutroncapture reactions are encountered, leading to a branching in the nucleosynthesis path. The know-ledge of the corresponding (n,γ)-cross sections allows one to obtain information about the physicalconditions in the s process during stellar helium burning [1, 2], but are also important for thenucleosynthesis in the r and p process during supernova explosions [3].The neutron capture cross sections can be measured via the activation method [4] or the timeof flight (TOF) technique. In activation experiments, one obtains the Maxwellian averaged crosssection (MAC), whereas in TOF measurements the energy dependent cross section can be extractedover a certain energy range.2. The Karlsruhe 4pi BaF2-detectorIn TOF experiments, total absorption calorimeters (TAC) are meanwhile used in differentplaces, taking advantage of the high efficiency and good timing properties of the detector crystals.The Karlsruhe 4pi calorimeter consists of 42 BaF2 crystals, and the nominal flight path betweenthe neutron production target and the sample is 77 cm. Neutrons are produced via the 7Li(p,n)7Bereaction, using a pulsed proton beam with an energy of about 2 MeV. For background reduction, thetarget is surrounded by a lead enforced collimator, which reduces the neutron beam to a diameterof about 20 mm [5].Neutron productiontarget ( Li)7Sample42-fold segmentedBaF -detector2Neutron absorberPulsed proton beam125 kHz, ~ 1 nsVdG accelerator4 cm flight pathFigure 1: New setup of the Karlsruhe 4pi-detector: the distance between the 7Li neutron production tar-get and the sample is reduced to 4 cm, leading to an expected increase in neutron flux by two orders ofmagnitude.2PoS(NIC-IX)188Neutron capture studies with a short flight path Stephan WalterRadioactive samples can only be used in very small quantities – otherwise the background dueto the activity of the target would be too high. For the measurement of such samples, the sensitivityof the setup needs to be increased.One possibility is to move the neutron production target inside the 4pi detector as shown inFig. 1. The reduction in flight path from 77 cm down to 4 cm leads to an expected increase inneutron flux by two orders of magnitude. This would allow to use sample masses in the microgramregion.The drawback of this modification is a higher background – mainly due to the γ-flash whenthe proton beam hits the neutron production target. In an idealized picture, a time window betweenthe prompt γ-rays and the background from scattered and captured neutrons inside the detectormaterial should allow undisturbed measurements. For 100 keV neutrons, this time window wouldstart 9 ns after the γ-flash, ending 17 ns later when the fastest neutrons reach the detector material.3. Background reduction and first test measurementsThe background has two different origins: the γ-flash itself and γ-rays produced by scatteredor captured neutrons inside the detector material.The amount of photons that are produced in the γ-flash depends on the backing material of theLi target, and on the thickness of the lithium layer itself. While Cu was used in the past as backingdue to its high heat conductivity, Ta will be used for the short flight path, resulting in a reductionin the number of produced photons by a factor of about 400.However, by far most of the γ-rays are produced in the lithium layer (about a factor 150 morethan in Cu). Since 7Li is needed for neutron production, the only way of reducing the backgroundis a reduction in Li layer thickness. For 5 µm of Li, the protons are still slowed down from 30 keVabove the neutron production threshold at 1.881 MeV below the threshold, thus resulting in a quasiMaxwellian neutron energy distribution of 25 keV. The expected count rate using such a Li layer –the accelerator working with a pulse repetition rate of 125 kHz and 1.5 µA beam current – is 1–2γ-rays per pulse and per crystal.Several possibilities for further background reduction were considered as well. The scintilla-tion light of BaF2 crystals consists of two components: one with a peak emission at λ=220 nm anda decay time constant τ=0.6 ns, the other with λ=310 nm and τ=620 ns [6]. For high count rateapplications, only the fast component is of interest. The fast to slow ratio can be influenced by thecrystal temperature. Increasing the temperature from 20◦C to 65◦C increases this ratio by about afactor of 1.6, but such high crystal temperatures are not suited for the real measurements.Additionally, the possibility of using UV-filters or wavelength shifters such as p-Terphenylwas also considered. It turned out that these devices would either reduce the signal strength toomuch or would influence both components – more or less – equally. Therefore, these ideas had tobe abandoned.Although passive shieldings inside the detector are difficult for geometrical reasons, the use ofa lead absorber around the Li target was investigated and the influence on the γ-rays both from thetarget and the sample was simulated with the GEANT4 toolkit [7].The lead shield consists of a massive sphere 66 mm in diameter with two openings: a cylindri-cal on one side for the beamline, and a cone with 120◦ opening angle on the other side in order to3PoS(NIC-IX)188Neutron capture studies with a short flight path Stephan WalterFigure 2: In order to reduce the background, the 7Li target can be surrounded by a lead shield (with anopening for the beamline and the neutrons opposite to each other). The influence on γ-rays with differentenergies was simulated with GEANT4: the upper curve shows the results without the Pb shield (squares), thecurve in the middle the influence on γ’s from the sample (circles) and the lower curve on γ’s from the target(triangles).avoid interactions of the neutron beam with the shielding (Fig. 2). For γ-ray energies of 2 MeV – atypical capture γ-ray energy considering a Q-value of 6 MeV and a multiplicity of 3 – the numberof photons from the target are reduced by 30%, whereas sample γ-rays are only reduced by 7%.In order to verify the simulated properties of the new setup, test measurements with a goldsample have been performed. Time of flight spectra showed that the γ-flash is clearly separatedfrom the rest of the spectrum. Further test measurements concerning the influence of a neutronabsorber inside the 4pi-detector in addition to the lead shield, but also background subtraction anddata reduction are planned for the near future.4. 63NiThe s process consists of two components: the main component, responsible for isotopes withA>90 that occurs in the He-rich intershell of thermally pulsing low mass asymptotic giant stars,and the weak component, that contributes to the isotopes with 56<A<90, and operates during theconvective core He burning in massive stars. Especially neutron capture cross sections in the lattermass region have to be very accurate since the local approximation <σ>N=const is not valid anymore. Changes in the capture cross sections can therefore have a large impact on the abundancesof the heavier isotopes [8].In the case of the Ni isotopes, it was not clear if the resulting overabundances in stellar modelcalculations are due to residual uncertainties in these models or to inaccuracies of the nuclearphysics input [9]. Recent activation measurements showed the influence of different (n,γ)-crosssections on the nucleosynthesis yield in the case of 62Ni [10], but also TOF measurements on thesame isotope have been performed [11, 12].Figure 3 shows a part of the chart of nuclides around 63Ni. This isotope is the first branch-ing point in the weak s-process path. Hence, its decay and neutron capture rates are directlyinfluencencing the abundance of 63Cu. Up to now, only theoretical predictions with fairly largeuncertainties exist for this (n,γ)-cross section (MACS(30)=31(6) mb [13]).4PoS(NIC-IX)188Neutron capture studies with a short flight path Stephan Walterr-processp-processb--decayb+-decay(n, )-reactionsg59Co60Ni 61Ni 62Ni63Cu64Ni65Cu60Co63Ni 65Ni66Cu62Cu 64Cu631/2Ni: t = 100 astheo = 31 (6) mbFigure 3: 63Ni is the first branching point in the s process path. The branching has direct influence on theabundance of 63Cu.First TOF measurements with the new setup of the Karlsruhe 4pi-detector are planned as soonas the necessary test measurements and the verification phase have been completed.References[1] F. Käppeler, Nuclear Physics A 752, 500c–509c (2005).[2] F. Käppeler, M. Wiescher, U. Giesen, J. Görres, I. Baraffe, M. E. Eid, C. M. Raiteri, M. Busso,R. Gallino, M. Limongi, and A. Chieffi, Ap. J. 437, 396–409 (1994).[3] S. E. Woosley, A. Heger, and T. A. Weaver, Rev. Mod. Phys. 74, 1015–1071 (2002).[4] W. Ratynski and F. Käppeler, Phys. Rev. C 37, 595-605 (1988).[5] K. Wisshak, K. Guber, F. Käppeler, J. Krisch, H. Müller, G. Rupp, and F. Voss, Nucl. Instr. Meth. A595–618 (1990).[6] M. Laval, M. Moszyn´ski, R. Allemand, E. Cormoreche, P. Guinet, R. Odru, and J. Vacher, Nucl. Instr.Methods 206, 169–176 (1983).[7] S. Agostinelli, J. Allison, K. Amako, J. Apostolakis, H. Araujo, P. Arce, M. Asai, D. Axen,S. Banerjee, G. Barrand, F. Behner, L. Bellagamba et. al, Nucl. Instr. Meth. A 506, 250–303 (2003).[8] M. Heil, and M. Pignatari AIP Conf. Proc. 819, 265–272 (2006).[9] T. Rauscher, A. Heger, R. D. Hoffman, and S. E. Woosley Astrophys. J. 576, 323–348 (2002).[10] H. Nassar, M. Paul, I. Ahmad, D. Berkovits, M. Bettan, P. Collon, S. Dababneh, S. Ghelberg,J. P. Greene, A. Heger, M. Heil, D. J. Henderson, C. L. Jiang, F. Käppeler, H. Koivisto, S. O’Brien,R. C. Pardo, N. Patronis, T. Pennington, R. Plag, K. E. Rehm, R. Reifarth, R. Scott, S. Sinha, X. Tang,and R. Vondrasek Phys. Rev. Lett. 94, 092504 (2005).[11] A. Tomyo, Y. Temma, M. Segawa, Y. Nagai, H. Makii, T. Shima, T. Ohsaki, and M. IgashiraAstrophys. J. 623, L153–L156 (2005).[12] A. M. Alpizar-Vicente, T. A. Bredeweg, E.-I. Esch, U. Greife, R. C. Haight, R. Hatarik,J. M. O’Donnell, R. Reifarth, R. S. Rundberg, J. L. Ullmann, D. J. Vieira, J. B. Wilhelmy, andJ. M. Wouters AIP Conf. Proc. 819, 273–277 (2006).[13] Karlsruhe Astrophysical Database of Nucleosynthesis in Stars (KADoNiS),http://nuclear-astrophysics.fzk.de/kadonis5

Neutron capture studies with a short flight path

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