1. Neutron detectors for the NMX instrument
Dorothea Pfeiffer on behalf of the ESS detector group

2. Content Collaboration for NMX detector development
NMX instrument and requirements
Detectors for NMX
Gd-Neutron measurements
Schedule

3. Collaborations and people
ESS detector group
Dorothea Pfeiffer (CERN) – Coordinator
Filippo Resnati (CERN) – Detector R&D
Thomas Kittelmann (Lund) – Computing and Simulations
Scott Kolya (Lund) - Electronics
Carina Hoeglund (Linkoping) - Coatings
Linda Robinson (Linkoping) - Coatings
ESS joined RD51 in 2013
CERN MPGD group world wide leading in R&D
Discussions with CERN thin film experts from TE-VSC

4. The macromolecular crystallography instrument NMX

5.
Dorothea Pfeiffer

6. Detectors and sample environment

7. Reflections of example crystal
Purpose of the NMX instrument is to determine structures of proteins, in particular location of hydrogen atoms in the structure

8. TOF separation of reflections

9. ESS pulse shape and intensity
16.66 Hz pulse rate, duty cycle of 1:25
Time averaged flux at NMX sample

10. Detectors for NMX

11. Challenges and History
200 um beyond state of art for time resolved detector
High rate requirements with up to MHz/cm2
In original instrument proposal for NMX: Gd-MSGC developed by HZB as in-kind partner with a 10 yr history
Ca. 24 months ago: HZB ordered to cease development
ESS detector group prioritized this development
Idea: Replace MSGC with GEM
Engaged with CERN as strong, stable, long term collaborative partner
Joined RD51, Strong R&D collaboration and community

12. Thermal neutron converters
Isotope
Crosssection [barns]
Reaction
Range
3He
5333
n + 3He -> 3H (191 keV) + 1H (573 keV) Q= 0.76MeV
Rp = 5.7 bar cm
6Li
940
n + 6Li -> a (2.06 MeV) + 3H (2.73 MeV) Q = 4.79 MeV
Rt=130 mm
10B
3835
n + 10B
-> 7Li*(0.84 MeV) + a (1.47 MeV) + g (0.48 MeV) (93%) Q=2.3 MeV
-> 7Li (1.16 MeV) + a (1.78 MeV) (7%) Q=2.79 MeV
Ra = 3.14 mm
157Gd
259000
n + 157Gd -> 158Gd + g (79, 181, 944 keV) + conversion electron spectrum ( keV) Q=7.94 MeV
lce = 11.6 mm
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

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

14. Gd thickness and efficiency
155 Gd: 9 um optimal (44%)
157 Gd: 3 um optimal (35%)
Nat. Gd: 6 um optimal (19%)
15 % with 1 detector

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

16. Measurement at IFE in Norway

17. Gd-GEM backwards setup

18. Gd-GEM configuration
Standard triple-GEM detector operated at gain of 5000 (730 uA)
250 um natural Gd as cathode with 50 um thick Cu tape
10 mm drift gap
Drift field of 700 V/cm

19. IFE measurement setup
Slits 1
Slits 2
Gd-GEM
neutrons
He3 tube

20. Spectra and detection efficiency
Spectra comparable to simulated spectra for 10 mm drift
First estimation of neutrons detection efficiency: >= 9 %
Neutron rate with beam of 2cm x 2cm and He3 tube in front of GEM: 11 kHz
Rate in GEM with beam of 2cm x 2cm (without He3 tube): 4.7 kHz
Background rate in GEM with beam of 2cm x 2cm with Cd sheet in front: 3.7 kHz

21. Electron tracking
The conversion electrons leave long more or less straight tracks in the drift space
Centroid calculation not sufficient to determine position
Tracking method like uTPC method is needed to determine the start point of the track
Fit more complicated than in case of alpha particles
(see talk by F. Resnati)
X - view
Y- view

22. Track map Beam collimated to 3mm x 10mm
Detector rotated with respect to collimation
Effects of 50 um copper tape clearly visible
Graph shows start of track in x versus start of track in y
Signal to background ratio of around 10:1

23. Position resolution 2cm x 2cm beam
Resolution with very simple method (last time bin over threshold): 1.4 mm
Improvement likely with better fit, better collimation and less background

24. Position resolution 2mm x 2mm beam
Resolution with very simple method (last time bin over threshold): < 1 mm
Improvement likely with better fit, better collimation and less background

25. Results and outlook Gd-GEM detector works
Detection efficiency is with >= 9% around the expected one of 15% from simulations
Tracking of electrons works in principle, but method has to be refined
Position resolution in the mm range does not meet requirements of 200 um yet
Setup limited: Improvements likely with better collimation, lower background and better fit
In backwards configuration, scattering of neutrons from readout board and three GEM foils influences resolution
Faster readout electronics needed

26. Electronics - ASICs Being evaluated ASIC used for R&D Being evaluated
Courtesy: S. Kolya
Dorothea Pfeiffer – Second Special Workshop on Neutron Detection with MPGDs

27. Sam says: Thank you for your attention !