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Boron-GEM3 – First measurements

Boron-GEM3 – First measurements. Dorothea Pfeiffer. 27.02.2014. Content. Neutron sources and Geant4 simulations Neutron conversion and Boron-GEM3 Differences to standard GEM3 with respect to energy calibration First measurements. 241 Am Be 370 MBq neutron source.

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Boron-GEM3 – First measurements

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  1. Boron-GEM3 – First measurements Dorothea Pfeiffer 27.02.2014

  2. Content • Neutron sources and Geant4 simulations • Neutron conversion and Boron-GEM3 • Differences to standard GEM3 with respect to energy calibration • First measurements Dorothea Pfeiffer

  3. 241 Am Be 370 MBq neutron source • 370 MBq = 0.01 Curie [Ci] • 241 Am Be sources emit 2.28E+06 neutrons per second per Ci • Remainder of activity is due to gamma emission • Our source emits hence ca. 22800 neutrons per second • Source is shielded by 5 cm – 10 cm of PE 5 cm 10 cm 10 cm Dorothea Pfeiffer

  4. Geant4 simulation of source • Physics list QGSP_BERT_HP • QGSP: quark gluon string model for high energy interactions of protons, neutrons, pions, and Kaons and nuclei • BERT: Bertini cascade for primary protons, neutrons, pions and Kaons below ~10GeV • HP: data driven high precision neutron package (NeutronHP) to transport neutrons below 20 MeV down to thermal energies • 106 primaries (neutrons) simulated • Assumption: Source is a cylindrical volume source that emits isotropic radiation • PE thermalizes the higher energetic neutrons by elastic scattering Dorothea Pfeiffer

  5. Neutron source emission Dorothea Pfeiffer

  6. Thermal neutron flux at detector Flux of neutrons with E <= 25 meV: 85 Hz Flux of neutrons with E <= 50 meV: 200 Hz 25 meV Dorothea Pfeiffer

  7. Neutron capturecrosssectionsat 1.82 Å (25meV) Conversion efficiency of 1um of 10B4C: about 2% at En = 25 meV=> using the simulated rates of 85-200Hz thermal neutrons, the rate of charged particles from neutron conversion should be between 1.7 Hz and 4 Hz Dorothea Pfeiffer

  8. Standard Triple GEM Drift cathode: 50 um of Kapton with 5 um Cu on bottom 3 mm 2 mm 2 mm 2 mm Dorothea Pfeiffer

  9. Triple GEM with Boron-10 converter (variant 1) Drift cathode: 300 um Al coated with 1 um10B4C on both sides 3 mm 2 mm 2 mm 2 mm Dorothea Pfeiffer

  10. Triple GEM with Boron-10 converter (variant 2) Drift cathode: 18 um Al foil coated with 1.3 um10B4C one side 3 mm 2 mm 2 mm 2 mm Dorothea Pfeiffer

  11. Boron GEM3: detectorandsupport Readout x direction Readout for integrated signal induced at bottom of third GEM Readout y direction Gas flow (in) Drift with10B4C Support with O-ring Voltage divider for HV for drift and 3 GEM foils Gas flow (out) Dorothea Pfeiffer

  12. GEM3: spectrum 55 Fe (calibrationspectrum) But with Boron-GEM3, one sees … NOTHING! Why? Dorothea Pfeiffer

  13. Mean free path of photons in matter • Beer-Lambert law of light attenuation I = I0 e (-mx) • I0: Intensity before passage through matter • I: Intensity after passage through matter • m: linear attenuation coefficient [cm-1] • x: thickness of matter [cm] • r: density of matter [g cm-3] • m/r: mass attenuation coefficient [cm2 g-1] (taken from NIST XCOM database) • 1/m: mean free path [cm] (matter with thickness of mean free path attenuates 1/e = 37% of the photons) • Concept of mean free path for gas molecules is different • (average distance travelled between collision with other moving particle) • Mean free path = kBT/(20.5 p d2 p) [m] • kB: Boltzmann constant [J/K] • T: Temperature [K] • P: pressure [Pa] • D: diameter of gas particle [m] Dorothea Pfeiffer

  14. Standard Triple GEM – meanfreepath For standard GEM3, 55 Fe useful for calibration ! Dorothea Pfeiffer

  15. Boron GEM3 – meanfreepath For Boron-GEM3 with 300 um cathode, calibration should be done with Au or Ag X-ray!

  16. Measurement setup Dorothea Pfeiffer

  17. Measurement with gain ~2000 • GEM3 • HV U=4200 V, Im = 730 uA • Boron-GEM3 • HV U=4180 V, Im = 730 uA • With a 10 mm lead shield most of the gammas disappear • Boron-10 GEM shows a signal with very high amplitude on the oscilloscope that saturates the preamp and the MCA • After attenuation of 12 dB at entrance of pre-amplifier signal could be read by the MCA • Assumption: signal in peak is discharges (to be confirmed) Dorothea Pfeiffer

  18. Spectrum 241AmBe source: 1 cm lead, gain 2000 Rate in peak: ~ 6 Hz

  19. Spectraof 241Am and 109 Cd: gain ~200 Peak position: 55 ?? Energy 60 keV No real peak – crappy energy calibration! Peak position: 65 Energy 23 keV

  20. Spectrum 241Am Besource: 1 cm lead, gain ~200 Peak position: 2650Rate in peak: 3.9 Hz Peak position: 5200Rate in peak: 4.1 Hz

  21. Measurement with gain ~200 • HV: U=3720 V, Im = 646 uA • Visible energies due to converted neutrons should be 0.84 MeV (7Li ion) and 1.48 MeV (alpha particle) • Energy calibration gives peak energy of 2.8 MeV at preamplifier gain 5 using 60 keV gammas of 241 Am. Factor 2 off, BAD! • Energy calibration gives peak energy of 1.8 MeV at preamplifier gain 10 using 23 keV gammas of 109 Cd. BETTER! • Energy calibration is difficult with 241 Am and 109 Cd source • Rate agrees with simulated flux of thermal neutrons Dorothea Pfeiffer

  22. Spectrumfromreadingwire in Boron Blade Detectorat ILL(4 mm of gas) Looks similar to our spectrum, also only one peak due to gas gap of 4 mm (not two peaks for alpha and Li ion) Dorothea Pfeiffer

  23. Next steps • Improve energy calibration • Optimize detector gain • Test Boron-GEM variant 2 (18 um Al foil) • Increase gap between cathode and first GEM to 7mm Dorothea Pfeiffer

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