Ultra High Resolution Gamma-Ray Spectroscopy with Crystal Spectrometers

1. Ultra High Resolution Gamma-Ray Spectroscopy with Crystal Spectrometers Michael Jentschel Institut Laue-Langevin, Grenoble, France

3. Outline • Diffraction of g-rays • Perfect crystals • Diffraction geometries • Crystal Spectrometers • Design, Components • Performances • Example Experiments • Outlook

4. Diffractivion of g-rays

5. Diffraction of photons on perfect crystals Resolution is constant over energy! Zachariasen, 1967 Dynamical Diffraction Theory

6. Acceptance width of a perfect crystal Single crystal rocking curves for Si220 2.5 mm thickness 500 keV 10-6monochromatisation possible! 1000 keV 10-8 rad 10-2rad

7. Single crystal diffraction geometries

8. Alternative Approach: Bent crystals

9. The ILL bent crystals Two crystals, 2.5 mm thich R=17m and 17.7 m The crystals are bent with an accuracy of about 4 microradian • Problems: • R ~ Dd3 -> very flat crystals needed • Polishing might introduce strains • Anticlastic bending • Acceptance angle of Si220 @ 1 MeVis nrad, while imperfection is mrad • => Double bent crystal for Gamma Rays not possible

10. Perfect Crystals for GAMS6 (Dd/d < 10-6)

11. Double flat crystal geometry Non-dispersive dispersive

12. How much photons do we diffract? 4p source 1016 10-6 10-5 10-2 103 Xtal reflectivity Mismatch of divergence and acceptance Can be avoided by different geometry for the price of resolution Solid angle due to collimation

13. Integral reflectivity of a perfect crystal

14. Comparison of Energy resolution of X-tal spectrometers GAMS5 in single bent crystal mode HpGe detector GAMS5/GAMS4/GAMS6 in double flat crystal mode

15. Crystal spectrometers

16. General Layout of a double flat crystal spectrometer 2.Crystal 1.Crystal Movable Collimation Detector Source Beam Collimation Spectrometer Table with angle interferometers These beams need to be separated Large distances needed

17. Angle Interferometer for Absolute Angle Measurements • Solution: • displacement interferometer: 10-11 m resolution • Dynamic range: 11 orders • Angle Calibration using Polygon • 2p absolute angle • stepwise measurement of 2p Needed: • 2×10-11rad resolution • Absolute calibration • Dynamic range: of 6 orders of magnitude Angle interferometer Linear displacement interferometer q k Dl Dl

18. Alignment of a double crystal spectrometer Additional broadening (rad) Relative crystal Alignment (rad)

19. Double flat crystal geometry(optimized for resolution and accuracy) Well defined angle Measures Instrument Response Measures Additional Broadening

20. PN3: General Layout Grenoble Solid angle: 10-6 Solid angle: 10-7 • In-pile target position • 5×1014 neutrons cm-2 s-1 • Capture rate: <1016 • Gamma emission rate <1016

21. Realisation of the spectrometer Curved crystal: R=17 m

22. Example Experiments

23. Using the resolution: GRID for nuclear state lifetimes Emission of g1 starts two clocks: • Atomic clock: • collision sequence • sensitive to atomic interaction • Nuclear clock: • Emission probability of g2 Doppler shift of g2 contains information of the two clocks Gamma Ray Induced Doppler broadening

24. Comparinig clocks: Atomic movements and nuclear de-excitation Atomic collision cascade Thermal spike Nuclear Lifetime Measurement from Doppler Broadening: t < 5 ps Presence of thermal motion sets upper limit for t

25. Absolute Energy Measurements for a new mass definition Hz Mass Energy Triangle Well defined angle kg mol Inconsistency: 10-7 Thermal energies ~10-2eV Excitation energies ~8×106eV Intrinsic uncertainty: 10-9 Can be absolutely measured lg can be absolutely measured Penning Trap measurements PN3 measurements

26. Comparison of single bent crystal spectrometer to Ge-detector array 188Re Program started to study odd-odd nuclei

27. PN3 as monochromator for sP(Eg) 10 g of Gd2O3 1016 captures per second Two copper crystals as tunable monochromator 8 fold segmented BGO as pair spectrometer

28. New results of the sP(Eg) n A clear enhancement of all experimental data with respect to classical Bethe-Heitler calculation Normalization to calculations At present no theoretical explanation available

29. Measuring the refractive Index for g-Rays Si prism, 160 degree, faces optically polished Alignment stages Prism installed between the two crystals

30. Measuring the refractive Index We use crystal as collimator to Generate beam with < mrad divergence Deviation is detected by second crystal as analyzer

31. Results For higher energies we find sign change This is what we expect from virtual photo effect only More on this: Habs et al. PRL to be published 03/05/2012

32. Outlook

33. Effective Monochromator via refractive optics

34. Consequences for focusing lenses

35. Efficient monochromator for Compton back scattering sources Requires massive source ELI-NP is more point like source ! Combining refractive and diffractive optics allows to get perfect result

36. Summary Crystal spectrometers are perfect tools if resolution and dynamic range is of interest • Double flat crystal spectrometer provide DE/E~10-6 • Doppler broadened line profiles • absolute energy measurements • monochromator • Bent crystal spectrometer provide DE/E~10-6 for E < 1 MeV • spectroscopy • search for very weak transitions

38. Diffraction efficiency of a perfect crystal How does a crystal reflect, if the beam divergence fits its acceptance width ? 3rd order 45% 22% 1st order

39. Reflected Intensity of double flat crystal spectrometer h higher orders thick crystals