1. A 10B-based neutron detector with stacked MWPCs with macrostructured cathodes
I. Stefanescu, C. Höglund, R. Hall-Wilton
ESS AB, Lund, Sweden
J. Birch, L. Hultman
Thin Film Physics Division, Linköping University, Sweden
Y. Abdullahi, I. Defendi,
M. Zee, K. Zeitelhack
MLZ-TUM, Garching, Germany

2. R&D in the framework of
German In-Kind Contribution to the ESS Upgrade Phase (WP 2.3),
FP7 – NMI3-II (Task 21.2),
and
International Collaboration for Neutron Detector Development (ICND).
Main objective: development of alternative technologies to replace the 3He tubes in large-area detectors for neutron scattering applications.
Work funded by the German Federal Ministry of Education and Research (BMBF) and EU (FP7 NMI3-II).

3. 10B-based neutron detectors: status, end of 2013
List not exhaustive n
Θ=90° at normal incidence
Boron-lined proportional counters
Oldest of all technologies, brought back in use due to the He-3 crisis.
Suppliers: Proportional Technologies, LND, Centronics, Reuter-Stokes, etc.
MultiGrid detector, ILL+ESS 
Detector concepts based on inclined Boron layers (CDT Heidelberg, HZG, ILL)
F. Piscitelli, accepted JINST (2014).

4. Detector concept n n n εabsorption  d α
Sense wire planes n
Stack of MWPCs with Boron-lined “macrostructured” cathodes Al
10B t n
εabsorption  d t n
α</2
εabsorption  d/sin(α) d‘
mm α
7Li(α)
α(7Li)
3D regular pattern consisting of grooves that can be created in the substrate material by milling, extrusion, forming, etc.
Similar concept (“microstructuring”) used to increase the efficiency of the semiconductor neutron detectors.
J. Uher et al., NIMA 576(2007)32.
μm

5. Model simulations
Issues with Boron-coated macrostructured surfaces:
Large wall effects close to the bottom of the grooves.
Distortion of the electric field near the surface of the cathodes.
Non-conformal coating if coating applied after the grooves are created in the substrate material.
Issues minimized, if not solved, by choosing the appropriate size and shape for the grooves.
Simulations with GEANT4, GARFIELD as the prerequisite to the experimental work.

6. Model simulations GEANT4 setup h or 10B(n,α)7Li2α α or h
Shapes predicted to have with similar efficiency. The grooves with triangular cross-section chosen to manufacture (milling) and use in proof-of-principle measurements.
GEANT4 setup
Ar/CO2 (70/30)
10B(n,α)7Li
Histograms of the energy deposited in gas by the reaction products (7Li and α)

7. GEANT4 simulation
Possible scenarios for the conformality of the coating of a macrostructured plate
tside = ttop
tside = ttop * sin(α/2)
tside = ttop * (x/h)
(forming after coating)
(coating after grooving, ideal)
(coating after grooving, realistic) h
Calculations for 45° grooves, 2 mm deep) and  = 4.7 Å
Results used in a general sense to understand trends in the performance of macrostructured cathodes.

8. Electron drift lines calculated with GARFIELD
wires
Ar/CO2 (70/30)
5 mm
MWPC with flat parallel plates
Drift time ions  0.4 μs
MWPC with macrostructured parallel plates
Drift time ions  0.7 μs
Drift of charges in an 1” tube filled with 6 atm of 3He is 4.5 μs => limitation of the maximum countrate of the detector

9. Proof-of-concept measurements
Coatings of 10B4C by dc magnetron sputtering at the Univ. of Linköping, Sweden.
Test detector for the macrostructured plates.
Active area 10x10 cm2, continous gas flow.

10. Proof-of-concept measurements
Beam, =4.7 Å
The first in-beam results were used to validate and refine the model simulations.
 = 4.7 Å
I. Stefanescu et al., NIMA727 (2013) and JINST 8, P12003(2013).

11. Efficiency of stacked MWPCs
 = 4.7 Å
Flat
Macrostructured
A realistic detector must incorporate several Boron layers in order to be competitive with the He-3 tube.
Detector stack used to test the efficiency of up to 5 MWPCs.
5 MWPCs = 10 Boron layers n
Anode wire planes
10B4C converter
I. Stefanescu et al., JINST 8, P12003 (2013).

12. n Position resolution with a MWPC h s Rp s2 s1 L=L1 + L2 s1+s2
Position resolution of a MWPC determined by:
geometry of the counter (s, h);
gas type (stopping power) and pressure.
2D position resolution with a MWPC could be achieved by:
cathode strips perpendicular
to the wires;
resistive wires;
delay-line readout. s1 s2 L1 L2
L1 = L ·
s1+s2
L=L1 + L2 n s h Rp
Counting gas
Smallest position resolution achievable with a MWPC (limited by the wire pitch) is ~1 mm2.

13. Resistive wire readout concept*B
Only 6 readout channels per MWPC.
5 mm
Concept:
groups of 4 resistive wires coupled to a common bus to reduce number of channels.
Assumption (1 atm Ar-CO2)
Charge collected by max 2 neighboring wires for each neutron.
* Thanks to R. Schneider, mesytec.com A B
GlobalGroup

14. Resistive wire readout concept
Task list
test the validity of the resistive readout concept with a small anode wire plane with 5 mm pitch (in-house); see next slides
build a realistic size demonstrator with stacked macrostructured cathodes and resistive wire readout (in-house); in progress
Readout electronics:
Mesytec will deliver off-the-shelf 16-channel preamplifier (MPR-16) and a shaper; done
development and integration of the front-end readout PCB (Mesytec/in-house);
In-beam tests demonstrator + electronics (Mesytec, in-house.

15. Concept for position resolution
QAL
QAR
QBL
QBR n
Resistive readout concept and event multiplicity tested with a small wire plane fabricated according to the proposed readout scheme.
Wire: Stableohm 875, =17 μm
2 independent wire chains
R = 5kΩ/chain
Lchain = 78 cm
Expected position precision: ~1% of length of the chain.

16. Position resolution: proof-of-concept
Chain B
FHWM = 5.2 mm
(0.7%*Lchain)
Chain A
wire#8
wire#10

17. n The FRM2 demonstrator 40 cm
The real demonstrator incorporates a stack of 3 macrostructured planes and 2 wire planes.
It runs in continuous flow of Ar/CO2 gas mixture.
Goal: prove that the components can be procured in real time and cost-effective.

18. Detector housing and macrostructured plates
High-quality and cost-effective macrostructured plates of size 5 cm x 40 cm were made by extrusion by MIFA® (Holland).
Coated with 1.4 μm 10B4C by magnetron sputtering in Linkoping, Sweden.
Mechanical components made in the Mechanical Workshop of the Physics Department.
Lid
Vessel
Frame
10B4C-coated plates for the demonstrator.
Cathode panel, 40 cm x 40 cm (8 plates).
MIFA macrostructured plate, 5 cm x 40 cm, not coated.
Material: Al 6060-T6, Price: ~3500 €/80 plates.

19. Making the wire frames
2 wires frames can be produced at the same time
Wire winding machine
No previous experience at FRM2 with resistive wires.
Wire: Stableohm 875 (AlCrFe), 
=17 μm (human hair ~70 μm)
5 mm pitch
72 wires/frame x 40 cm = 28.8 m wire/frame
Preliminary steps (required)
built test wires frames in order to get familiar with the winding technique;
test properties of Stycast;
measured the linear elongation of the wire with the weight applied in order to determine the wire tension needed to be used in order to avoid electrostatic instabilities when HV is applied.
Frame with the wires wound, not yet glued

20. Making the wire frames
glue the wires in position with Stycast and with conductive glue for the electrical contacts;
and allow for a couple of days for the glues to cure and polymerize at NTP;
determine the tension in each wire by measuring the mechanical resonance frequency of the wire;
replace the first and last 2 resistive wires by thick guard wires (50 µm) in order to avoid large electric fields at the counter border.
Gluing the wires on the frame
Wire frame, final
Stycast 1266
Conductive glue

21. Assembling the demonstrator
Inserting the coated plates in the frames
2-counter detector
Demonstrator, back side
Demonstrator, front side

22. Performance tests with the 252Cf source
Unfortunately, the debugging of the improvised electronics was very time consuming  little time left for testing.
Working voltage 1.5 kV
Gas mixture: P-10
Gas mixtures used: P-10, Ar-CO2 (20/80) in continuous flow.
Electronics: Mesytec preamps (MPR-1), Silena Amps.

23. Performance tests with the 252Cf source
Shaping time set to 1 μs for all other measurements.
Counter 1
Center of detector
72 wires per counter at 5 mm distance.
Wires joined in 6 groups of 12 wires each (12 pin Sub-D connectors).
Difference in count rate between sides and center due to the variation in the coating thickness and possibly distorted electric field near edges.

24. Performance tests @ TREFF (4.7 Å)
Counter 1
Counter 2

25. Conclusions
In the framework of the ESS Design Upgrade and FP7-NMI3-II programs we explored a detector concept based on stacked MPWCs with 10B-coated macrostructured plates.
Extensive simulations and proof-of-concept measurements demonstrated the validity of the concept.
Realistic demonstrator based on this concept successfully constructed. Preliminary tests show that in principle all initial specifications (5 mm x 5 mm position resolution and ~30% 4.7 A) are met.
Perspectives: continuation of the project must be decided by the FRM2. The priority should be the readout electronics. Meeting set up in Munich on March 3rd to talk about the continuation of this project.