Study of the shape of b spectra Development of a Si spectrometer for measurement of b spectra

1. Study of the shape of b spectra Development of a Si spectrometer for measurement of b spectra Charlène Bisch • Introduction • Beta detectors • Experimental device • Monte Carlo calculations • Conclusion and perspectives LNHB/CDF : M.-M. Bé, C. Bisch, C. Dulieu, M. A. Kellett, X. Mougeot In collaboration with IPHC, Ramses, Strasbourg (A.-M. Nourreddine)

2. Beta spectra shapes evaluation Experiments are necessary : validation of the calculations, uncertainties of the models Calculations are necessary : very short T1/2, multiple beta decays, cascades, …  Subtle understanding of the phenomena that distorting beta spectra Understand the theory to make it evolve Growth of computing power more complex models Growth of computing power Monte-Carlo simulations Test and constrain calculations with perfectly controlled experiments Introduction • Users : Nuclear Power Industry (decay heat calculations), medical care sector (dose calculations), ionizing radiation metrology (liquid scintillation and ionization chamber techniques)

3. Beta detectors

4. Measurements – metallic magnetic calorimeters Dilution cooler • 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 timeof about 3 days • Bremsstrahlung from 800 keV •  deacrease of efficiency • Quality of the source •  distortion of the spectrum? Detectors floors

5. Measurements – Silicon detector • 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 • Our detector specifications: • PIPS: Passivated Implanted Planar Silicon Detector • Window thickness (Si eq.): < 50 nm • Active diameter: 23,9 mm • Active thickness: 500 µm

6. Experimental aspects • 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

7. Experimental device - General Detection chamber “The Cube” with the PIPS detector Gate valve PUMP GAUGE Linear feed-through 17 cm 100 cm

8. Experimental device - The source holder Source holder Source support POMPE DEWAR JAUGE Vanne à vide Screen Influence of X-rays Canne de translation

9. Experimental device - The detection chamber ElectricalBNC/microdot connector An electrical wire connects the detector to the BNC/microdot connector  to avoid thermal transfer Detector holder in copper  detector cooled uniformly

10. 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: Theory MC Monte Carlo simulations VS Counts S-D distance (mm)

11. Influence of the source-detector distance • 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 90 Y PIPS 500 µm 10 mm 30 mm 24 mm 20 mm 40 mm Huge influence of the solid angle Detector thickness too small

12. Influence of the thickness of active volume • 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) 8 mm 90 Y 5 mm 2 mm 500 µm 10 mm 5 mm thickness active volume is necessary for measuring 90 Y spectra

13. Geometry and materials of the cube Source – detector distance: 40 mm Source – detector distance: 10 mm No cube Steel cube 250 mm Steel cube 170 mm Aluminium cube 170 mm 170 mm 170 mm 250 mm 250 mm

14. Sources of distortion • 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

15. 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

16. Thank you for your attention

17. Theory • Beta decay: the electron and the antineutrino • share the momentum and energy of the decay •  continuous kinetic energy spectra 85Kr • Fermi (1933): Probability Energy