Charged particle detection in GE6 To stop high energy particles need large thickness of Germanium (GE6 has ~13 cm) Charged particle detection in Ge

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1 Using stacked germanium detectors for charged hadron detection Daniel Watts Edinburgh University Derek Branford, Klaus Foehl Charged particle detection in GE To stop high energy particles need large thickness of Germanium (GE has ~3 cm) In contrast to γ, charged particles deposit energy all along their path measure energy deposited in all material passed through Conventional Ge technology: use thick /2mm rear contact on the crystals Prototype detectors commissioned using thin contacts (< micron) from: Eurisys Mesures Detector Systems GmbH Considerable effort but in the end has been successful! Eurisys won contract to build 4 more Charged particle detection in Ge The GE - Array Employ Planar crystals Little dependence of pulse shape on entry point. Easier signal analysis. Band gap small at voltages required to fully deplete the crystal 3 Vcm - Need to cool with liquid N 2 to avoid large leakage current Ge detectors good charged particle energy resolution and particle identification. Excellent alternative to expensive magnetic spectrometer systems

2 GE Double sided Silicon strip detectors Give (i) position information and (ii) energy signal for use in E/E p side GE Double sided Silicon strip detectors ve Tigre cm x cm (28 x28 strips) SiO 2 Al BB2s 2.5cm x 2.5cm (24 x 24 strips) Donor impurities in perpendicular strips on the p and n sides 57 pixels Electrons drift to the anode (horizontal n-type) Holes drift towards cathode (vertical p-type) Use two detectors to get track information Thickness p type Pitch n type n side Micron BB2 Si Strip Photo of TIGRE??? Ge detector Some of the BB2 strips in position

3 GE: Particle identification GE electronic readout E/E technique. Obtain E from Si strip detector(s) and from Ge(sum) Ge versus Ge if > Ge fired (also useful to reject nuclear scattered events see later) Identify π + from afterpulses arising from delayed muon decays. Michel spectrum of positron energies ν Si strip π + µ + + ν µ (τ /2 =2 ns) µ + e + + ν e + ν µ (τ /2 =2.2 µs) e + π + µ ν ν Crystals 2,3 4,5, TFA 82 PRE AMP 855 PRE AMP 855 PRE AMP 855 TFA 82 V42 V42 delay QDC Strobe 2,3 4,5, TDC Gate 4,5, Timing Filter Amplifier (pulse length 4ns) Ortec 855 Amplifier CAEN V42 Gate generator Delay box First test of GE at PSI First test of GE at PSI: Muon afterpulse Test of prototype Ge detector characteristics 59 MeV proton beam passed through 2 C target to produce secondary beam (cocktail of alpha, protons, pions, muons) Beam focussed by quadropole magnets Detector Position Intermediate focus π + µ + + ν µ e + + ν e + ν µ Counts Afterpulse TDC TDC spectrum (combined for crystals 4-) 5 Protons Target Quadrupole magnet 2m Michel spectrum -positron energy Time (µs) Fit with exponential Half life =2.2±.2µs Recover muon lifetime Electrostatic separator Adjustable slits Dipole magnet Beam stop

4 Scale in millimetres First test of GE at PSI: Energy resolution MeV Single crystal resolution (FWHM) < 3 kev ± 8 kev (from proton events which stop in first crystal) π + resolution consistent with spread of momenta in the beam ~.4 MeV at 2 MeV (FWHM) Majestix 55 degrees 2 mm Obelix degrees 9 mm Idefix 5 degrees 7 mm γ Asterix 75 degrees 5 mm Panoramix 25 degrees 9 mm Proof of principle Run period ~ days on Li(γ,π + ) He Calibrations and X-section normalisation based on p(γ,π + )n reaction (CH2 target) Strip detectors were all Micron BB2s Ge resolution measured with γs 5 kev (Edinburgh) 3 kev (Mainz) The Mainz Facility Three cascaded microtrons delivering 85 MeV electrons + the Glasgow photon tagging facility Schematic diagram of the tagger Dipole Extraction 8 MeV Main Beam (855 MeV) Photon Beam 45 MeV Quadrupole Magnet FOCAL PLANE Linear Accelerators Dipole Magnet MeV Injection Scale (m) Quadrupole Schematic diagram of a microtron Radiator Main Beam Input Microtron gives 4 Mev Microtron 2 gives MeV Microtron 3 gives 85 MeV Direct-Current Beam ( = % Duty Factor) Resolution ~ 2keV E γ = E e E tagged-e Main tagger resolution ~2 MeV Tagger microscope ~7-22 MeV had nominal resolution ~4 kev

5 GE: Trigger logic GE element gain matching Trigger split into two levels controlled by Lecroy LRS458 Memory lookup unit First Level: Pulse from Crystal one of any GE detector > 3 mv Kinematically overdetermined Reconstructed Pion Energy (MeV) p(γ,π+)n reaction Measure Eγ, θπ Calculate Eπ Source of energy tagged π+ Uncalibrated Pion Ridge n γ π+ Reconstructed Pion Energy (MeV) Second Level: (i) Energy in start detector and sum used to reject most high energy electrons (ii) Hit in Tagger OR (iii) Afterpulse signal between and 8 µs after initial signal Pion Vertex Energy (MeV) Calibrated Pion Ridge Pion Vertex Energy (MeV) E/E for Ge versus Ge Germanium Energy (MeV) x y distribution on Ge frontface Energy in First Crystal (MeV) Particle ID using DSSD & GE Aligned Strip Signals 2 8 DEUTERONS 4 2 PIONS PROTONS Total Particle Energy (MeV)

6 Estimation of Losses due to Hadronic Interactions in the Ge Hp-Ge tagger TDC spectrum HpGe Tagger TDC Spectrum Deduced from the p(g,p+)n Results Counts Relative Detection Efficiency.8..4 Comparison GE with known p(γ,π + )n cross section Prompt Region Random Regions Pion Energy (MeV) Tagger Time (µs) Pions can interact with the Ge nuclei: scatter, knockout protons etc. Cross section rises as pions acquire sufficient energy to excite the Imply timing resolution Ge ~ 3-4ns Physics motivation: : Halo halo Nuclei nuclei GE@MAMI results BE last nucleon (n or p) close to separation energy. Sn J π = MeV.759 MeV Li(γ,π + ) He (ground state) reaction at E γ = 2 MeV Core with a low density extended tail of n- or p-matter e.g. He = alpha + 2neutrons. Halos are interesting as (i) they allow pure (neutron) or proton matter to be studied (ii) play an important role in determining astrophysical reaction rates and (iii) are a unique test of cluster models and three-body theories Photopion reactions - complimentary to ion reactions & also allow excited state halos to be accessed π J = + He Protons Neutrons. MeV p 3/2 s /2 (µb/sr) dσ dω lab - -2 Young - halo Kartaglidis - halo Kartaglidis no halo θ π Blue Present Data Red Shaw et al. Phys. Rev. C43 8 (99) Green Shoda et al. Phys. Lett. B 24 (98) Young Surrey PhD Thesis (24) Uses cluster model for Ψ Kartaglidis PRC 2439 (2) Uses WS and HO Ψ

7 (µb/sr) dσ dω lab Angular Distribution for the He 2 + Excited State at.8 MeV CK Sask C GE@MAMI results θ π Red squares Present data Black triangles Shoda et al. Phys. Lett. B 24 (98) CK Curve Cohen and Kurath Calcn. presented in Shoda et al. Sask C Curve Bergstrom Phys. Rev. C2 249 (98) GE: Summary and Future programme GE performed well in commissioning measurements In 2 will be one of the primary detector systems at MAXLAB@Lund Upgraded Lund facility has Eγ ~2-5 kev (FWHM) Halo measurements : Li(γ,π + ) He, B(γ,π + ) Be and 7 O(γ,π ) 7 F Short range nuclear structure A(γ,pp) Pre-formed + in nuclei A(γ,pπ + ) Three-body forces A(γ,pd) Large angle cross sections comparable to g.s. results Suggests st excited state also a halo state. Summary GE performed well in commissioning measurements Will play very active role in a range of experiments at the upgraded Lund facility at MAX-LAB, Sweden