GAMMA RAY OPTICS. Michael Jentschel Institut Laue-Langevin, Grenoble, France

Transcription

1 GAMMA RAY OPTICS Michael Jentschel Institut Laue-Langevin, Grenoble, France

2 Acknowledgement ILL: W. Urbam M.J Ludwigs Maximilian University Munich D. Habs MPQ Munich M. Guenther

3 Outline Motivation Gamma Ray Optics Diffractive Refractive Measurements Refractive index Pair creation threshold Outlook Conclusion

4 Motivation: nuclear Photonics measurement of many E level scheme from coincidences or ritz Hadron (p, n, d,,...) + Energy produces excitated nucleus: Only partially selective Limited penetration power Complex excitations Photon h Scanning photon energy and selective excitation from ground state Measurement of absorption or deexcitation Requirement: brilliant tunable gamma ray source

5 New brilliant sources based on Compton back scattering Courtesy of C. Barty LLNL 2 0 E0 / mec yields ~4 2 Doppler upshift

6 Motivation

7 Spectrum delivered by Compton Backscattering source Source: C. Barty, ELI-NP Meeting, August 2011 Monochromatization: E/E~10-3 This means we have MeV

8 Spectral requirements nuclear point of view E ev ~ s Broadening due to thermal motion: E Th E v c Th E 3k BT ETh mc E 5 In most cases dominating Example: > s, E = 1 MeV, T = 300 K E = 50 ev

9 What is really required from a nuclear point of view Example: > s, E = 1 MeV, T = 300 K E = 50 ev Resolution of HPGe detector Photon source sample Detector The dominating number of photons is not 1MeV: we absorb max. 5% of photons E Peak/Background bad Detector might be overloaded (95% of 10 9 photons/s )

10 What is this talk about? Photon source Optics Sample Optics Detector ELI-NP new brilliant source for -rays Do we need to something here? What can we do here?

11 Monochromatization via Crystal Diffraction 2 sin A 1 I( ) 2 1 y y B, A y 2, E hc n B n 2d sin hc E B E E B B const FWHM 2 hc E resolution constant over energy 1000 kev rad 10-2 rad

12 How to realize diffraction Resolution is limited by divergence of incoming beam A beam with 10-8 rad divergence is needed

13 Double crystal geometry Non-dispersive dispersive

14 General Layout Grenoble In-pile target position neutrons cm -2 s -1 Capture rate: <10 16 Gamma emission rate <10 16 Solid angle: 10-6 Solid angle: 10-7

15 Beam Brilliance ILL as photon source 1,0E % band width % band width 1,0E+18 1,0E+16 1,0E+14 ESRF ID15 APS 1,0E+12 1,0E+10 1,0E+08 1,0E Energy (kev) 10000

16 GAMS5 operating in dispersive and nondispersive geometry

17 Counts / 20 Sec. /15 Chan. Counts / 20 sec. / 15 Chan. Diffraction efficiency of a perfect crystal How does a crystal reflect, if the beam divergence fits its acceptance width? 815 kev (1,1) 2.5 mm Si <220> 184KeV (1,1) 2.5 mm Si <220> rd 1 st order order 45% 22% Angle (Arc Sec) Angle (Arc Sec)

18 How much do we really diffract? 10 9 s -1 for 1keV s -1 for 50 ev ~10-8 rad Diff I 0 R Div 2 50 s Max. count rate for HpGe detectors Could be 10 4 s -1 We need to improve throughput by 2 orders of magnitude

19 Approach 1: Correcting completely the divergence by refractive optics Diff I 0 R Div 2 Normal single crystals scheme: Lens - single crystals scheme: Could be pushed to by 1000 Could be pushed to by 10 Requires lens system for g-rays: -ray Energies?

20 Refractive Index n( E) 1 ( E) i ( E) If 0 then one can build refractive opitcs f n 2 A. Snigirev et al., Nature Vol 384, 1996 For X-rays = f 2 n N

21 For X-rays refractive optics is known M. Wegener, Karlsruhe Institute of Technology C. Schroer, TU-Dresden Snigerev, ESRF, Grenoble B. Lengeler, RWTA Aachen

22 Measuring the refractive Index We use crystal as collimator to generate beam with < rad divergence Deviation is detected by second crystal as analyzer

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

24 Results For higher energies we find sign change This is what we expect from virtual photo effect only

25 Possible Explanation (D. Habs) Refractive index is related to forward scattering amplitude n( E ) 1 ( E ) i ( E ) ( E ) 2 N C A rf 2 Kramers-Kronig dispersion relation: A f A rf ia if A rf 2 E Aif ( E) ( E ) l im de c E E E i 0 A rf (E g ) = If we want to explain positive index of refraction we need to look for processes, which increase strongly with energy E 2 g 2p 2 c lim e 0+ 0 s abs (E)dE E 2 -(E g +ie) 2

26 Possible explanation (D. Habs) d(e g ) = 2 c 2 2pE g 2 N C A rf (E g ) Calculated with XCOM from NIST A rf (E g ) = E 2 g 2p 2 c lim e 0+ 0 s abs (E)dE E 2 -(E g +ie) 2 There is a correlation between absorption and refraction Absorption cross section: Up to 500 kev Photo effect is dominating Pair creation sets in from 1022 kev Rising with E would produce positive A rf Exact knowledge of Pair needed

27 New ILL 10 g of Gd 2 O captures per second Two copper crystals as tunable monochromator 8 fold segmented BGO as pair spectrometer

28 Measurement of the Pair creation cross section A clear enhancement of all experimental data with respect to classical Bethe-Heitler calculation Normalization to calculations At present no theoretical explanation available M. Jentschel et al, Phys. Rev. C. 84(5) 2011

29 The complete picture More processes to be considered: elastic and inelastic Delbrück scattering (Calculations by D. Habs and M. Günther from LMU/MPQ Munich) These components Are responsible for increase of at high E

30 Results (comparison with expectation) Material depending cross over 1/E scaling 1/E 2 scaling D. Habs et al., The Refractive Index of Silicon at γ-ray energies, Phys. Rev. Lett. 108, (2012).

31 Next measurements: very near future Physics: measurement of Z-dependence precision Prism would be needed thick material homogeinity? 5mm Si phase modulator for 1 MeV Interferometric measurement Possible working range: Calculation for Si: ~ 0.8 period phase shift enough to detect sign for higher Z materials stronger effect expected

32 First test of techniques and applications: still this year 10 B 11 B 7 Li -decay = 73 fs 478 kev We can work on nuclear resonance fluorescence of 7 Li and test all techniques right From PhD Tesis S. Baechler, Uni Fribourg

33 Conclusion Crystals can be used as optical elements for -rays Beam splitter Collimator Monochromator (lens) Refractive Optics for gamma rays is possible Refractive index 0 for -rays Investigations of more materials planned Combining Refracting and Diffracting Optics very promising ILL is perfect test bench to develop technologies