1. GD AND GD2O3 COATINGS AS NEUTRON CONVERTERS Dorothea Pfeiffer 23.09.2014

2. Content 2  Instrument for which Gd detectors are foreseen  Principle of GEM detectors  Neutron converters  Gd-Neutron GEM simulations and prototypes  Coating needs and schedule Dorothea Pfeiffer 23.09.2014

3. Neutron Macromolecular Crystallography 0.2mm Resolution 60x60cm modules? Good time resolution > 30% efficiency Gamma rejection not particularly important E. Oksanen

4. Gd-Based MSGC/GEM Detector HZB in-kind contribution to ESS originally developed within context of DETNI /NMI3 Substitute MSGC with GEMs: now a collaboration CERN-HZB- ESS

5. GEM detector principle 5 Courtesy: F. Sauli Dorothea Pfeiffer 23.09.2014

6. GEM Foil 6 Courtesy: F. Sauli Dorothea Pfeiffer 23.09.2014

7. Neutron converters  Good neutron converters have a high cross section for thermal neutrons  The neutron capture creates a charged particle that can be easily detected  The converter has to have the correct thickness so that a maximum of the charged particles can escape and reach the gas volume => Gd has a high cross section and produces conversion electrons during the capture of thermal neutrons, thickness has to be optimized 7 Dorothea Pfeiffer 23.09.2014

8. Geant4 Gadolinium Simulations 8 Converter Drift backwards 0.25 – 50 um 25 meV neutrons Scoring of electrons that cross boundary between converter and drift Drift forwards  Geant4 simulations to evaluate different converter materials and thicknesses  Natural Gd, 155 Gd, 157 Gd, Gd2O3 and enriched Gd2O3 were simulated Dorothea Pfeiffer 23.09.2014

9. Neutron capture and conversion electrons 9

10. Electron spectra of natural Gd and 157 Gd 10 conversion electrons (in converter) Electrons arrived in drift natural Gd 157 Gd MeV Mean: 67 keV Mean: 69 keV Mean: 60 keV Mean: 54 keV

11. Conversion electrons in drift space 11 44 % 35 % 17 % 157 Gd: 3 um optimal 155 Gd: 9 um optimal natural Gd: 6 um optimal

12. Gd GEM – first simulation results  Oxides lead to comparable results for number of captured neutrons and conversion electrons created, but are not conductive. Need to have conductive layer added if they are used in forwards and backwards direction  Contrary to what is found in the literature, 155 Gd has a higher percentage of conversion electrons per captured neutron than 157 Gd  The capture cross section of 155 Gd is smaller than that of 157 Gd, therefore a thicker converter is needed. But since the spectrum of 155Gd (mean 83 keV) is considerably harder than that of 157 Gd (mean 61 keV), the conversion electrons can exit the converter  Assuming the simulations are correct, the upper threshold for the detection efficiency lies at 45 %  Needs to be verified by measurements! 12 Dorothea Pfeiffer 23.09.2014

13. Neutron GEM in backwards configuration 13 Dorothea Pfeiffer 23.09.2014

14. Neutron GEM in coated GEM configuration 14 3 -10 mm 2 mm Gd coating 25 meV neutrons Dorothea Pfeiffer 23.09.2014

15. Gd coating requirements  According to the simulations, the following thicknesses would be ideal:  For forwards/backwards configuration with two readouts 155 Gd: 9 um Natural Gd and Gd2O3: 6 um 157 Gd and Gd2O3: 3 um  For backwards configuration with one readout: 1 55 Gd: 15 um Natural Gd and Gd2O3: 12 um 157 Gd and Gd2O3: 5 um  Layers of metallic Gd could be a freestanding foil, but due to small thickness a support foil might be necessary  Gd2O3 coating as paint is soft and need support foil  Gd2O3 needs an additional thin conductive coating  Backwards modus also possible with coated GEM foil 15 10.09.2014 Gd2O3 Natural Gd Dorothea Pfeiffer

16. Coating challenges  Create metal coating starting from Gd2O3 (in case of enriched Gd2O3)?  Embed Gd2O3 in resin etc, but achieve a high density  Create stable coatings of >= 5 um  Coating of a GEM foil without destroying it (e.g. Kapton by heat)  …. 16 Dorothea Pfeiffer 23.09.2014

17. Shortterm R&D Schedule  Fall/winter 2014: Characterize Gd2O3 and Gd GEM, investigate CsI option. Simulate complete detector with Geant4/Garfield++ interface, develop first test prototype (prototype 1)  End 2014: Have Boron GEM and Gd GEM plus crate ready to take to test beam.  Q1/2 2015: Verify simulations, determine in which ways the detector has to be improved, decide whether Gd is feasible  Late 2015: Refined prototype  2016: Test samples Gd-155, 157  2017: Decision on technology  2017: Technology demonstrator  Late 2017: Start of construction 17 05.05.2014 10.09.2014 Dorothea Pfeiffer 23.09.2014

19. Backup slides

20. Geant4-Garfield Simulation Setup 20 Converter Drift (3mm – 10 mm) 50 um of natural Gd 25 meV neutrons  To simulation the position resolution, the deposited energy of the conversion electrons and the sensitivity to gamma background, a Geant4/Garfield++ interface was created  As a first step of a complete detector simulation, the primary ionization clusters in the drift were simulated 100 keV – 1GeV gamma (forwards) electrons 100 keV – 1GeV gamma (backwards) Dorothea Pfeiffer – NMX STAP Review 10.09.2014

21. Tracks left by conversion electrons in ArCO 2 21 50 um natural Gadolinium 10 6 thermal neutrons simulated, of which 16% had conversion electrons create a track in the drift Simulated with Heed, which assumes a thin absorber and does not include multiple scattering

22. Conversion electrons - Primary ionization spectrum 22