BORON-GEM3 – FIRST MEASUREMENTS
27.02.2014
Dorothea Pfeiffer
Content
2
Neutron sources and Geant4 simulations
 Neutron conversion and Boron-GEM3
 Differences to standard GEM3 with
respect to energy calibration
 First measurements
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Dorothea Pfeiffer
27.02.2014
241 Am Be 370 MBq neutron source
3
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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
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Our source emits hence ca. 22800
neutrons per second
Source is shielded by 5 cm – 10
cm of PE
Dorothea Pfeiffer
5 cm
10 cm
10 cm
Geant4 simulation of source
4
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Physics list QGSP_BERT_HP
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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
27.02.2014
Neutron source emission
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Dorothea Pfeiffer
27.02.2014
Thermal neutron flux at detector
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Flux of neutrons with E <= 25 meV: 85 Hz
Flux of neutrons with E <= 50 meV: 200 Hz
25 meV
Dorothea Pfeiffer
27.02.2014
Neutron capture crosssections at 1.82 Å (25meV)
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Isot
ope
Crosss Reaction
ection
[barns]
3He
5333
n + 3He -> 3H (191 keV) + 1H (573 keV) Q= 0.76MeV
Rp = 5.7 bar cm
6Li
940
n + 6Li -> a (2.06 MeV) + 3H (2.73 MeV) Q = 4.79 MeV
Rt=130 mm
10B
3835
n + 10B
-> 7Li*(0.84 MeV) + a (1.47 MeV) + g (0.48 MeV) (93%)
Q=2.3 MeV
-> 7Li (1.16 MeV) + a (1.78 MeV) (7%) Q=2.79 MeV
Ra = 3.14 mm
157
25900
0
n + 157Gd -> 158Gd + g (79, 181, 944 keV) + conversion electron
spectrum (29-182 keV) Q=7.94 MeV
lce = 11.6 mm
Gd
Range
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
27.02.2014
Standard Triple GEM
8
Drift cathode:
50 um of Kapton
with 5 um Cu on
bottom
3 mm
2 mm
2 mm
2 mm
Dorothea Pfeiffer
27.02.2014
Triple GEM with Boron-10 converter (variant 1)
9
Drift cathode:
300 um Al coated
with 1 um10B4C on
both sides
3 mm
2 mm
2 mm
2 mm
Dorothea Pfeiffer
27.02.2014
Triple GEM with Boron-10 converter (variant 2)
10
Drift cathode:
18 um Al foil coated
with 1.3 um10B4C
one side
3 mm
2 mm
2 mm
2 mm
Dorothea Pfeiffer
21.02.2014
Boron GEM3: detector and support
11
Readout x direction
Readout y direction
Readout for integrated
signal induced at
bottom of third GEM
Gas flow (in)
Drift with10B4C
Support with
O-ring
Voltage divider
for HV for drift
and 3 GEM foils
Gas flow (out)
Dorothea Pfeiffer
27.02.2014
GEM3: spectrum 55 Fe (calibration spectrum)
12
But with Boron-GEM3, one
sees … NOTHING!
Why?
Dorothea Pfeiffer
27.02.2014
Mean free path of photons in matter
13
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Beer-Lambert law of light attenuation I = I0 e (-mx)
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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)
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Mean free path = kB T/(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
27.02.2014
Standard Triple GEM – mean free path
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Source
55 Fe
Au X-ray
109 Cd,
Ag X-ray
241 Am
22 Na
Energy [keV]
6
10
20
60
500
mean free path [mm]
0.51
2.35
13.5
37.7
76.6
Transmission through 50 um [%]
90.7
97.9
99.6
99.9
99.9
mean free path [mm]
0.0097
0.0052
0.0332
0.704
13.4
Transmission through 5 um [%]
59.7
38.2
86
99.3
100
Kapton + Cu Transmission [%]
54.2
37.3
85.7
99.2
99.9
ArCO2
mean free path [mm]
27.6
114
831
13600
57800
Interaction in 3 mm [%]
10.3
2.6
0.36
0.02
0.005
5.58
0.97
0.31
0.02
0.005
Kapton
Cu
Kapton + Cu Interaction of unattenuated
+ ArCO2
source photons [%]
For standard GEM3, 55 Fe useful for calibration !
Dorothea Pfeiffer
27.02.2014
Boron GEM3 – mean free path
15
Source
55 Fe
Au X-ray
109 Cd,
Ag X-ray
241 Am
22 Na
Energy [keV]
6
10
20
60
500
Al
mean free path [mm]
0.51
2.35
13.5
37.7
76.6
Variant 1:
Transmission through 300 um [%]
0.01
11.97
75.67
97.77
99.32
Variant 2:
Transmission through 18 um [%]
57.1
88.0
98.3
99.9
100
10B4C
mean free path [mm]
0.0097
0.0052
0.0332
0.704
13.4
Transmission through 2 um [%]
99.66
99.92
99.98
99.99
100
Variant 1 transmission [%]
0.01
11.96
75.66
97.77
99.31
Variant 2 transmission [%]
57
88
98.33
99.86
99.96
mean free path [mm]
27.6
114
831
13600
57800
Interaction in 3 mm [%]
10.3
2.6
0.36
0.02
0.005
Variant 1: Interaction of
unattenuated source photons [%]
0.001
0.31
0.27
0.02
0.005
Variant 2: Interaction of
unattenuated source photons [%]
5.7
2.29
0.35
0.02
0.0
Al + 10B4C
ArCO2
Al + 10B4C +
ArCO2
For Boron-GEM3 with 300 um cathode, calibration should be done with Au or Ag X-ray!
Measurement setup
16
Dorothea Pfeiffer
27.02.2014
Measurement with gain ~2000
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GEM3
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Boron-GEM3
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HV U=4200 V, Im = 730 uA
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
27.02.2014
Spectrum 241AmBe source: 1 cm lead, gain 2000
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Rate in peak: ~ 6 Hz
Spectra of 241Am and 109 Cd: gain ~200
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Peak position: 55 ??
Energy 60 keV
Peak position: 65
Energy 23 keV
No real peak – crappy
energy calibration!
Spectrum 241Am Be source: 1 cm lead, gain ~200
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Peak position: 2650
Rate in peak: 3.9 Hz
Peak position: 5200
Rate in peak: 4.1 Hz
Measurement with gain ~200
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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
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Rate agrees with simulated flux of thermal neutrons
Dorothea Pfeiffer
27.02.2014
Spectrum from reading wire in Boron Blade Detector at ILL
(4 mm of gas)
22
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
27.02.2014
Next steps
23
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
27.02.2014