Amaudruz, P

Buchmann, L

Fox, S P

Fulton, B R

Laird, A M

Mumby-Croft, P

Openshaw, R

Pavan, M

Pearson, J

Ruprecht, G

Walden, P

CERN Document Server

PoS(NIC-IX)170The TRIUMF Annular Chamber of Tracking andIdentification of Charged Particles (TACTIC)G. Ruprecht∗b, P. Amaudruzb, P. Mumby-Crofta, L. Buchmannb, S. Foxa, B. Fultona,A. M. Lairda, R. Openshawb, M. M. Pavanb, J. Pearsonb, P. Waldenba University of York, U.K.b TRIUMF national laboratory, CanadaE-mail: ruprecht@triumf.caMany nuclear reactions important for nucleosynthesis are investigated by transfer reactions usinga radioactive ion beam on gas targets such as 1H, 2H, 3He and 4He. Since these measurementsare performed in inverse kinematics, the energies of the ejectiles are usually low with a strongangular dependence in the laboratory system. Therefore, a new kind of detector is needed thatallows the detection of low-energy ions from a gas target with good energy and angle resolutionand a large angular coverage. TACTIC is a cylindrical time-projection chamber that can utilizethe target gas also as the drift medium so that no separation window is required. Moreover, thereaction region is still separated from the drift region which allows for the higher beam currentsneeded for impact energies far below the Coulomb barrier. The cylindrical design is possible bythe application of gas electron-multiplier (GEM) foils for the amplification of the weak electron-drift signals. From the electron- drift times the ion track can be reconstructed while the collectedcharge provides additional information about the ion species.International Symposium on Nuclear Astrophysics — Nuclei in the Cosmos — IXJune 25-30 2006CERN, Geneva, Switzerland∗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)170TACTIC status G. Ruprechtdetection regionbeamtarget regionGEManodestripsdriftelectronsejectilesFigure 1: Schematic view of the TACTIC chamber. Wires are stretched along the target tube and biased witha negative high voltage . Another (inner) system of wires suppresses beam induced electrons. The electricalconnectors on the right hand side provide the high voltages for the wires and the GEM and lead the signalsto the outside. The anode cylinder is divided into 48 strips. The detection cylinder is 24 cm long and has adiameter of 6 cm.1. Chamber designThe principal design of TACTIC [1, 2] is shown in figure 1. The beam particles coming fromthe left move into the gas filled target tube, while the ejectiles move into the detection region wherethey induce secondary electrons. An electric field between the target tube and the anode stripsforces them to drift to the gas electron multiplier (GEM) [3, 4] foil where the signal is pre-amplifiedand then picked up by the anode strips. From the measured drift times the radial coordinates of theparticle tracks can be deduced, and from the measured charge the energy loss for each anode stripcan be derived. The GEMmakes it possible to give the chamber a cylindrical design and, therefore,to separate target and detection region. Since the target region is "blind", higher beam currents canbe applied than in other time projection detectors. For many reactions (see e.g. 8Li(α ,n) describedbelow) target gas and detection are the same, so that no separation foil is needed and low-energyejectiles can be detected.There is a complicated dependence between the reaction kinematics, the detection/target gaspressure, the reaction yield, and the energy loss of the ejectiles (and projectiles). All these de-pendencies fix the geometry of the chamber. For this reason a simulation has been implementedusing CERN’s GEANT4 framework. A Coulomb single-scattering process (not yet available inGEANT4) has been programmed and the ion physics has been improved. An example of simulatedion tracks for the 8Li+4He reaction as detected by the 48 anode pads is shown in fig. 2.2. Gas mixture dependenceA series of measurements with a planar test chamber has been performed in order to determinethe best composition of the detection gas and the performance of the GEM. Fig. 3 (left) shows theoperation range of the GEM for a 90% He / 10% CO2 volume gas mixture at different pressures.The tests show that even at lower pressures (which are necessary for low-energy ejectiles) there isa good amplification. The noise limit will be even lower for the TACTIC prototype, so that a large2PoS(NIC-IX)170TACTIC status G. Ruprecht10 15 20 25 30 35 40 450204060anode strip #radius [mm]Figure 2: Simulated ion tracks for the 8Li+4He reaction as seen by the 48 anode pads which can detect onlyelectrons from the drift region (r = 10-60 mm). Circles = 11B, squares = 8Li, triangles = 4He. The size of thesymbols reflects the energy collected per anode strip.0 200 400 600 800 10006008001000120014001600Pressure [mbar]GEM+Anode voltage [V]BreakdownNoise limit-0.20 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 40.10.20.30.40.50.60.70.80.911.11.21.31.41.51.6Drift timeDistance from track start [cm]Drift time [µsec]Cell: TacticGas: CO2 10%, He-4 90%, T=300 K, p=0.29608 atmUnknownPlotted at 23.54.14 on 02/03/06 with Garfield version 7.10.Figure 3: Left: Voltage range in which an amplified signal can be detected vs. gas pressure. Right:GARFILED calculation of the electron drift times.pressure range is available, sufficient for most astrophysical applications. Fig. 3 (right) shows aGARFIELD [5] calculation of the electron drift times vs. the radial distance in the chamber wherethey are produced at a pressure of 300 mbar as needed for the 8Li(α ,n) reaction. Ionization cross-sections from Magboltz [6] have been used for this calculation.The linearity is excellent for thispurpose.3. First measurementsThe first planned reactions to be measured are 8Li(α ,n)11B and 7Be + p elastic scattering.They both can be measured with a radioactive beam in inverse kinematics. The low cross-sectionand the large ejectile cone favour TACTIC for these experiments.3.1 8Li(α ,n)11BR-process calculations of nucleosynthesis in neutrino driven winds in supernovae also includ-3PoS(NIC-IX)170TACTIC status G. Ruprechting light elements [7] found two new reaction chains that can change the heavy element synthesisby one order of magnitude. A very large uncertainty in these chains comes from the only poorlyknown 8Li(α ,n)11B cross-section. The lowest important temperature of T9 = 0.6 corresponds to a8Li impact energy of 100-200 keV/u. In previous measurements [8] a time projection chamber wasused as well. However, since there was no separation between target and detection region only alimited beam intensity could be applied. At the ISAC/TRIUMF facility, higher 8Li intensities areavailable at low energies (120 keV/u), and with TACTIC the available data set could be greatlyimproved.3.2 7Be + p elastic scatteringThe famous 7Be(p,γ)8B still has uncertainties at low energies mainly arising from conflictingresults of direct and Coulomb dissociation measurements (see [9] for a description of the problem).With the high angular range covered by TACTIC, 7Be(p,p) scattering phase shifts (there are 16 in-volved) could be measured which contributes to a better understanding of the 7Be(p,γ)8B reaction.This measurement is more challenging since the very high proton energy requires a heavier driftgas. Therefore, the detection gas is not the same as the target gas and a separation foil surroundingthe target region is needed.The chamber prototype is now being built at the University of York, U.K., and will be testedat the end of this year at TRIUMF with a stable beam.References[1] TACTIC website http://tactic.triumf.ca.[2] G. Ruprecht, et al., Europ. Phys. J. A 27(2006)315[3] F. Sauli and A. Sharma, Ann. Rev. Nucl. Part. Sc. 49 (1999) 341[4] A. Sharma and F. Pauli, Nuc. Inst. Meth. A 350, (1994) 470[5] R. Veenhof, GARFIELD web site: http://garfield.web.cern.ch[6] S. Biagi, MAGBOLTZ web site: http://consult.cern.ch/writeup/magboltz/[7] M. Terasawa, et al., Astrop. J. 562(2001)470[8] T. Hasimoto, et al., Nuc. Phys. A 764(2004)330[9] C. A. Bertulani, Phys. Rev. Lett. 94(2005)0727014

The TRIUMF Annular Chamber of Tracking and Identification of Charged Particles (TACTIC)

The TRIUMF Annular Chamber of Tracking and Identification of Charged Particles (TACTIC)

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