Boron gem3 first measurements
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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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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

Dorothea Pfeiffer


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


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


Neutron source emission

Dorothea Pfeiffer


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


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


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


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


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


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


GEM3: spectrum 55 Fe (calibrationspectrum)

But with Boron-GEM3, one sees … NOTHING! Why?

Dorothea Pfeiffer


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


Standard Triple GEM – meanfreepath

For standard GEM3, 55 Fe useful for calibration !

Dorothea Pfeiffer


Boron GEM3 – meanfreepath

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


Measurement setup

Dorothea Pfeiffer


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


Spectrum 241AmBe source: 1 cm lead, gain 2000

Rate in peak: ~ 6 Hz


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


Spectrum 241Am Besource: 1 cm lead, gain ~200

Peak position: 2650Rate in peak: 3.9 Hz

Peak position: 5200Rate in peak: 4.1 Hz


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


    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


    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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