ddep 2012 | c. bisch – study of beta shape spectra 1 study of the shape of spectra development of...
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DDEP 2012 | C. Bisch – Study of beta shape spectra 1
Study of the shape of spectra Development of a Si spectrometer for measurement of
spectra
Introduction Beta detectors Experimental device Monte Carlo calculations Conclusion and perspectives
Charlène Bisch
LNHB/CDF : M.-M. Bé, C. Bisch, C. Dulieu, M. A. Kellett, X. Mougeot
In collaboration with IPHC, Ramses, Strasbourg (A.-M. Nourreddine)
DDEP 2012 | C. Bisch – Study of beta shape spectra 2
Introduction
Users : Nuclear Power Industry (decay heat calculations), medical care sector (dose calculations), ionizing radiation metrology (liquid scintillation and ionization chamber techniques)
Test and constrain calculations with perfectly controlled experiments
Calculations are necessary : very short T1/2, multiple beta decays, cascades, …
Understand the theory to make it evolve
Growth of computing power more complex models
Beta spectra shapes evaluation
Experiments are necessary : validation of the calculations, uncertainties of the models
Subtle understanding of the phenomena that distorting beta spectra
Growth of computing power Monte-Carlo simulations
DDEP 2012 | C. Bisch – Study of beta shape spectra
Beta detectors
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Detector Proportional Counter
Scintillation Counter
Magnetic spectrometer
Semi-conductor (Si)
Metallic magnetic
calorimeter
Energy resolution at
100 keV30 keV 15 keV 10 eV to 1 keV
~ 8 keV (300 K) ~ 3 keV (77 K)
~ 50 eV
DDEP 2012 | C. Bisch – Study of beta shape spectra
Measurements – metallic magnetic calorimeters
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Very promising technique:
• Detection efficiency > 99,9 %
• Energy threshold of about 200 eV
• Energy resolution of 30 eV @ 6 keV
• Non-linearity of 0,1 % in 6 – 80 keV
But:
• Activity ≤ 15 Bq
• Measurements at 10 mK
cooling time of about 3 days
• Bremsstrahlung from 800 keV
deacrease of efficiency
• Quality of the source
distortion of the spectrum?
Detectors floors
Dilution cooler
DDEP 2012 | C. Bisch – Study of beta shape spectra
Measurements – Silicon detector
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Our detector specifications:• PIPS: Passivated Implanted Planar Silicon Detector• Window thickness (Si eq.): < 50 nm• Active diameter: 23,9 mm• Active thickness: 500 µm
More classical technique: • Good energy resolution of 8 keV @ 100 keV (300 K) • Linear response • Easy to implement
But:• Dead zones• Bremsstrahlung• Backscattering • High quality of vacuum • Detector thickness
Si(Li)
PIPS
DDEP 2012 | C. Bisch – Study of beta shape spectra
Experimental aspects
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• Experimental spectra may be distorted by the detection system
• Experimental aspects to limit sources of distortion
- Detector cooled to liquid nitrogen temperature thermal noise
- Ultra high vacuum interactions e-/environment and dead layer due to water steam condensation
- Reduction of vibrations microphonics (additional component to electronic noise)
- Distance from source to detector and centring of the source
solid angle, reproducibility, simulations
- Source: ultra-thin reduction of auto-absorption
quality minimisation of impurities
homogeneous reproducibility, simulations
• Any remaining factors will be quantified by Monte-Carlo simulations
DDEP 2012 | C. Bisch – Study of beta shape spectra 7
Gate valve
Experimental device - General
PUMP
Linear
feed-through
GAUGE
100 cm
17 cm
Detection chamber “The Cube” with the
PIPS detector
DDEP 2012 | C. Bisch – Study of beta shape spectra 8
DEWAR
JAUGEPOMPE
Vanne à vide
Canne de
translation
Experimental device - The source holder
Source holder Source support
Screen
Influence of X-rays
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Experimental device - The detection chamber
Electrical BNC/microdot connector
Detector holder in copper
detector cooled uniformly
An electrical wire connects the detector to the BNC/microdot connector to avoid thermal transfer
DDEP 2012 | C. Bisch – Study of beta shape spectra
Monte Carlo simulations
• Utilisation of GEANT4 to optimise the source holder and the detection chamber
• Influence of the source-detector distance • Geometry and materials least likely to scatter electrons
• Code validation:
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Monte Carlo simulationsVS
Co
un
tsS-D distance (mm)
Theory
MC
DDEP 2012 | C. Bisch – Study of beta shape spectra
Influence of the source-detector distance
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10 mm
20 mm
30 mm
40 mm
PIPS 500 µm
24 m
m
90 Y
• Four source – detector distances : 10 mm, 20 mm, 30 mm, 40 mm
• 106 particles emitted from 90Y (MetroMRT project) isotropic source
• Thickness of active volume: 500 µm
Detector thickness too small
Huge influence of the solid angle
DDEP 2012 | C. Bisch – Study of beta shape spectra
Influence of the thickness of active volume
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90 Y
• Source – detector distance: 10 mm
• 106 particles emitted from 90 Y isotropic source
• Four thicknesses of active volume (500 µm, 2 mm, 5 mm, 8 mm)
5 mm thickness active volume is necessary for measuring 90 Y spectra
10 mm
500 µm
2 mm
5 mm
8 mm
DDEP 2012 | C. Bisch – Study of beta shape spectra
Geometry and materials of the cube
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250 mm
No cube
Steel cube 250 mm
Steel cube 170 mm
Aluminium cube 170 mm
Source – detector distance: 10 mm Source – detector distance: 40 mm
250 mm
DDEP 2012 | C. Bisch – Study of beta shape spectra
Sources of distortion
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• Four main sources of distortion:
- Solid angle source – detector distance
- Detector thickness effect depth of active volume
- Geometry effect geometry of active volume
- Scattering and backscattering energy, Z of the material, incidence angle
Geometry effect
DDEP 2012 | C. Bisch – Study of beta shape spectra
Conclusion and perspectives
• Development phase of experimental device is complete
• The experimental setup is currently being assembled
• We intend to do our first measurements of beta spectra early 2013
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DDEP 2012 | C. Bisch – Study of beta shape spectra 16
Thank you for your attention
DDEP 2012 | C. Bisch – Study of beta shape spectra
Theory
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• Fermi (1933):
Energy P
roba
bilit
y
85Kr
• Beta decay: the electron and the antineutrino
share the momentum and energy of the decay
continuous kinetic energy spectra