Final preparations for the MIGDAL experiment at the NILE facility at RAL

Pawel Majewski

(STFC/Rutherford Appleton Laboratory)

for the MIGDAL collaboration

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RD-51 Mini-Week (February 2022)

1

Migdal effect

• Looking for a rare (10^-5) atomic phenomenon never before observed in the nuclear scattering
• Migdal effect increases sensitivity of DM experiments to low mass WIMPs
• Aim of the MIGDAL experiment - unambiguous observation and measurement of the Migdal effect using a low pressure Optical TPC
• Signal signature: “V-like” shaped event with two tracks from electron and NR with different dE/dx and sharing the same vertex

2

Simulated Migdal events in 100% CF₄ at 50 Torr

• Ionisation from electron using DEGRAD and full recoil cascade simulation using TRIM
• 150 keV Fluorine recoil and 7.5 keV electron
• 150 keV Fluorine recoils and 5 keV electron (secondary recoil clearly visible)

3

Secondary recoil

Electron

Primary recoil

dE/dx and required dynamic range

• Distinctive difference in dE/dx between NRs and electrons
• Huge dynamic range required at vertex where dE/dx at maximum for NRs and at minimum for electrons
• With high gain to see low energy electrons can lead to approaching the Raether limit and unwanted gaseous discharges in the lower GEM
• Plan for testing different THGEMs structures for robustness with fission fragments (RD-51 2 year grant)

4

Electronic dE/dx

Nuclear dE/dx

Expected rates of NR, non-NRs, MIGDAL events

from DD and DT generators in 50 Torr 100% CF₄

5

Detector

• Gas : 100% CF₄ at 50 Torr
• (planned mixtures with all noble gases)
• Operating with low drift E-field for minimum diffusion
• Signal amplification with two glass-GEMs
• Light readout with fast CMOS camera; charge readout using ITO strips

6

Copper collimator for DT generator

Aluminium chamber with O-TPC

TPC

• TPC inside of the central aluminium cube
• Drift gap: 3 cm between woven mesh and cascade of two glass-GEMs
• Transfer and signal induction gaps : 2 mm

Two glass-GEMs:

• 570 um thick
• OD /pitch: 180/280 um
• active area: 10x10 cm²

ITO strips wire bonded to readout

• 120 strips
• width/pitch: 0.65/0.83 mm

Three field shaping copper wires

7

TPC

8

Direction

of neutrons

Electric field uniformity in the active volume simulated with COMSOL : fiducial area 8 cm x 8 cm

Viewport for PMT

Viewport for camera

Detector (windows)

Neutron beam exit and entrance aluminium (100 um thick) windows :

• From outside and inside

9

Detector Metrology

• Chamber with the O-TPC check for parallelism and alignment between all the elements
• Main focus on alignment between beam entrance/exit window and the drift gap of the TPC
• Largest deviation found - 0.5 mm

10

Gas system

Gas system for single or two gases mixture operation almost ready.

11

DAQ

• Camera - continuous exposure with rolling shutter and 89 f/s (11 ms exposure)
• Acqiris digitizer (4 ns sampling) with 60 readout channels triggered by the signal from PMT (scintillation from GEM)
• Synchronisation :
• Timing of the trigger and cameras output signals registered by FPGA
• Timestamp recorded in every frame’s metadata and Acqiris’ waveform
• Recodriding triggers during Acqiris dead time

12

NILE facility and neutron generators

• Commissioning of both DD and DT neutron generators at the NILE facility at ISIS (RAL) is underway.
• MIGDAL experiment will start with DD (lower energy of neutrons) generator.

13

MIGDAL shield (w/o top) and detector

DD generator

DT

generator

Shield and collimator

• Full shielding for the experiment with DT generator
• Front shielding: Iron + borated HDPE + Pb
• 1 m long copper collimator
• Full shielding for the experiment with DD generator
• Front shielding for the experiment with DD generator
• borated HDPE + Pb
• 35 cm long borated HDPE+Pb envelope collimator

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Copper 1 m long collimator

Conclusions

• Detector and gas system assemblies almost complete
• Most of the shield parts are on site
• Collimator for DT on site and DD in the production
• Commissioning of DD and DT generators underway

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