10 B-based Multi-Grid Detectors as an alternative to 3 He In-beam test on the IN6 ToF spectrometer 10 B-based Multi-Grid Detectors as an alternative to.

10B-based Multi-Grid Detectors as an alternative to 3He

In-beam test on the IN6 ToF spectrometer

10B-based Multi-Grid Detectors as an alternative to 3He

ILL

Mathieu Ferraton, Jean-Claude Buffet, JeF Clergeau, Jonathan Correa, Sylvain Cuccaro, Bruno Guérard,

Francesco Pitcitelli, Patrick Van Esch

ESS

Carina Höglund, Richard Hall Wilton, Anton Khaplanov

14/10/2013 Special Workshop on Neutron Detection with MPGDs

Outline

• Basic principles and design of the IN6 Multi-Grid prototype– mechanics– electronics

• In-beam measurement on the IN6 ToF spectrometer– Neutron efficiency and comparison with 3He tubes– Gamma sensitivity– Background– Time resolution– Scattering in aluminum

• Construction of an IN5 Multi-Grid prototype

10B-based Multi-Grid Detectors as an alternative to 3He14/10/2013 Special Workshop on Neutron Detection with MPGDs

Basic principles of the Multi-Grid and design of the IN6 prototype


Principle and Mechanical Design• Base elements are grids (1 grid = frame + blades)

– 1 frame is mounted with 14 blades coated on each side with 10B4C (1 µm thick)

-> 15 cells in depth direction (1 cm each)– 1 grid = 15 × 4 cells (2 × 2 × 1 cm) -> 60 cells– grid are stacked to form 60 gas proportional counters

neutrons

Al substrate (0.5 mm)

neutrons

10B4C layers (1 µm)

Fissio

n fragm

ents

The IN6 prototype :• 96 frames arranged as 6 stacks of 16 frames

(sensitive area 0.15 m²)• 60 anode wires per stack (total 360 wires) • Position Sensitive Detector -> coincidence mode :

– Frames : vertical position– Wires : horizontal position and depth

• Gas mixture : 1 bar Ar/CO2 (90/10) flushed

neutrons

MeV .792αLi%) (6

MeV 2.3 MeV) (0.48 γα*Li%)94(nB7

710

neutrons

Amplification gas

Neutron conversion in 10B :

15 cm

8 cm

2 cm


Readout Electronics : reduction of channel number

• 96 frames + 360 wires = 456 channels

• Reduction of channel number :– 30 wires connected together via resistive networks -> charge division allows determination of the targeted wire

• 12 resistive networks × 2 ends = 24 readout channels

– 3 frames connected together horizontally• 96 / 3 = 32 readout channels

-> 56 readout channels to determine position and depth (reduction by a factor 8)

R

2R

R R R

anode wires

Charge Preamplifier

R R

Charge division

card


Readout Electronics : data acquisition

Cathode signal (grids)

Anode signal (resistive networks) Charge division

(or energy) card

MCC

Time signal (from IN6 chopper)

iSDN PC

Actual position of the beam

Result of the CoG algorithm

x (cm)

x (cm)

0

MCC (Multi Channel Controller)• Coincidence reconstruction• Position resolution improvement

trough CoG algorithm -> ½ grid (1 cm), ¼ grid (0.5 cm)


In-beam measurement on the IN6 time-of-flight spectrometer at ILL


Interest of this measurement

• Direct comparison between 10B Multi-Grid detector and 3He tubes performances

• More observables available thanks to the timing information from the IN6 chopper

• Low background instrument


The IN6 instrument

• ToF spectrometer in the range of thermal neutrons

• Available incident wavelengths : 4.1, 4.6, 5.1, 5.6 Å

• Detection system : 337 3He tubes (sensitive area 4 m²)

• Total background : 4.3 Hz

3He tubes (3 bar, diameter 3.2 cm, length

30 cm)

• 3 weeks measurement @5.1 Å with users’ samples• 24 h dedicated beam time @4.1 Å and 4.6 Å


Neutron efficiency : comparison with IN6

• 3He tubes : higher intrinsic efficiency (see theoretical curves)

• 10B Multi-Grid : less dead spaces thanks to optimized geometry

Bragg peaks in the detector -> position spectra match 3He tubes’ (average on top and bottom 3He tubes rows)-> better position resolution in the prototype (higher granularity)

Ratio of integrated rates in Bragg peaks :- 4.1 Å : rate(10B) / rate(3He) = 1.08- 4.6 Å : rate(10B) / rate(3He) = 0.97

-> measured efficiencies are similar in both detectors @4.1 and 4.6 Å !!

YGarnet @4.1 Å Si @4.6 Å


53% efficiency was measured @2.5 Å

Time spectrum NaI

Time spectrum multigrid

plateau measurement

Gamma sensitivity3He (gas) 10B (solid)

gammas and fission fragments well

separated

gamma and fission fragment spectra

widely overlapping

Multi-Grid detector :-> a similar time structure is observed for large values of the high voltage (above 900 V)-> no sign of this structure down from 900V

NaI scintillator :-> Strong gamma background from IN6 (see spectrum)-> well defined time structure


Background measurement• 4.4 Hz flat background (30 times higher than IN6’s) was observed (no time structure)

-> independent of the IN6 instrument

• Energy structure of the background (Pulse Height Spectrum measured on wires) :

– significant rate up to more than 3 MeV (excluding neutrons and gammas)

– background energy spectrum overlaps fission fragments spectrum (no threshold discrimination possible)

[email protected] Å

How do we explain this background ?

PHS measured on wires


Source of the background• Aluminum is known to be contaminated by alpha

emitters (238U, 232Th and daughters)– corresponding measured rate : 0.05 to 0.25 cm-2.h-1 – our detector is made of 360 Al tubes, 192 cm² each -> expected total rate between 0.96 Hz and 4.8 Hz (whole

detector)-> the measured 4.4 Hz background rate is coherent with

these values

• Two solutions have been investigated :- Ni layer ( > 23 µm) deposited on the Al surface- ultra-pure Al (U,Th concentration < 1 ppb)

Background PHS on wires (no B4C coating)

alpha energy (from MC simulation)

-> Background reduced by a factor of 20


Time resolution : effect of time correction• Measured ToF depends on the interaction depth in the detector• ToF value of interest is at the entrance window of the detector

-> ToF value has to be corrected for depth

Wavelength (Å)FWHM (µs)

(10B -3He) / 3He3He 10B4,1 45,3 50,7 12,0%4,6 45,9 53,0 15,5%5,1 57,3 65,2 13,7%


δdepth = 1cm → δToF = 8.6 µs (@4.6 Å)

Elastic neutrons


Time resolution : effect of scattering in Al

• Base of the elastic peak is widening toward low energy for 4.1 Å and 4.6 Å data

• No such effect @5.1 Å




-> consequence of scattering in aluminum (Bragg cutoff @4.67 Å)


4.6 Åz = 6

4.6 Åz = 12

• Dedicated experiment (24 h beamtime), Bragg peaks in the detector (higher rate and peak/background ratio, well defined angular position)

– 2 wavelengths and 2 samples ([email protected] Å, [email protected] Å) – Bragg scattering angles :

• 4.1 Å : θ = 122°• 4.6 Å : θ = 160°

Neutron scattering in aluminum

Some neutrons have negative speed (traveling from the back to the front of the detector) -> Bragg scattering at the back of the detector (thick exit Al window to cross)

θ

Cd shielding

• Only neutrons with λ < 4.67 Å (Bragg cutoff) induce coherent Bragg-scattering (σincoherent << σcoherent)

• Shielding the back of the detector will suppress the observed bump in time-of-flight spectra


Summary of the results

• The Multi-Grid concept has confirmed the good expected performances (efficiency comparable with IN6’s, low gamma-sensitivity, good time-of-flight resolution)

• Scattering in aluminum was observed

• The question of alpha emission in aluminum was raised

-> It is now time to go to the next step!


The IN5 Multi-Grid prototype• The construction phase of a much larger

prototype has started (size of an IN5 module, 1/12 of the instrument’s detector)

– Sensitive area will be 3 * 0.8 m²• 1024 grids (8 stacks of 128 grids), ~17500 blades• 480 wires , 3 m long (~1.5 km of wire)

– 1500 readout channel (to be reduced to 288)


The IN5 Multi-Grid prototype

• Design, inner mechanics and assembly at ILL

– Final design now settled on– Vessel under fabrication by the German

company ProBeam (electron-beam welding)

• Detector’s components supplied by ESS and Linkoping University

– 1024 Ni-plated frames

– 17500 two-side 10B4C-coated blades (35 m²)

-> Assembly will start in January 2014


Thank you for attention !!


10 B-based Multi-Grid Detectors as an alternative to 3 He In-beam test on the IN6 ToF spectrometer 10 B-based Multi-Grid Detectors as an alternative to.

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