A negative ion TPC with GridPix readout C. Ligtenberg , M. van - - PowerPoint PPT Presentation

A negative ion TPC with GridPix readout

• C. Ligtenberg, M. van Beuzekom, Y. Bilevych, K. Desch, H. van der

Graaf, M. Gruber, F. Hartjes, K. Heijhoff, J. Kaminski, P.M. Kluit,

• N. van der Kolk, G. Raven, T. Schiffer, J. Timmermans

Lepcol / LCTPC-WP meeting

June 15, 2020

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 1 / 18

The negative ion TPC

In a negative ion TPC, ionisation are captured shortly after creation by electronenative molecules (CS2) and drift to the readout plane as negative ions In the amplification region, the electron detaches and a normal avalanche occurs The negative ion TPC was introduced to reduce diffusion without the need for a magnetic field 1 The negative ion TPC has been applied to directional dark matter search experiments (Drift IId 2) From multiple types of ions with different drift velocities, the absolute drift distance can be reconstructed without a trigger

1see C. Martoff et al (2000) https://doi.org/10.1016/S0168-9002(99)00955-9 2see J. Battat et al (2017) https://arxiv.org/abs/1701.00171 Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 2 / 18

Introduction

Gas at atmospheric pressure is used in an existing setup: 93.6% Argon 5% iC4H10 as a quencher 1.4% CS2 to capture the electrons and form negative ions The gas contains a small amount of oxygen (650 ppm to 1150 ppm) and water vapor (about 4000 ppm). The oxygen is required to make a second type of ions: the minority carrier(s).

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 3 / 18

Experimental setup

TPX3 TPX3 TPX3 TPX3 G u a r d LV regulator C O C A Wire bonds

Guard TPX3 TPX3 TPX3 TPX3

Ionisation in the gas volume is created using a pulsed N2 laser, directed in the gas volume by a remotely controlled stage One quad (4 chips) is read out

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 4 / 18

Event display

x position [mm] 10 15 20 25 30 35 y position [mm] 5 10 15 20 25 30 35

1 2 3 4 5 6 7

Drift time [ms]

Timepix hits Laser track Timepix hits Laser track

Preliminary Event display of negative ion track 64 hits Edrift = 300 V/cm

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 5 / 18

Run parameters

Number of runs 9 Run duration 17 minutes Edrift 100 – 500 V/cm Vgrid −380 V Threshold 515 e− Temperature 295.9 – 297.0 K Pressure 1030 – 1029 mbar Oxygen concentration 650 – 1150 ppm Water vapor concentration ∼ 4000 ppm

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 6 / 18

Number of hits per laser pulse

Number of hits 20 40 60 80 100 120 140 Entries 50 100 150 200 250 300

Number of hits per laser pulse

Preliminary

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 7 / 18

Drift time spectrum

Drift time [ms] 1 2 3 4 5 6 7 8 9 Hits 200 400 600 800 1000 1200 1400 1600 1800 4.7 mm 10.7 mm 16.7 mm 22.7 mm 28.7 mm 35.7 mm

Edrift = 300 V/cm Preliminary The majority carrier and minority carrier(s) cause two distinct peaks

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 8 / 18

Fit of double Gauss

Do a ‘global’ fit per run of two Gaussians per drift distance: (1 − f2 − fnoise)nhits σ1 √ 2π exp

• −(t − µ1)2

2σ2

1

• + f2nhits

σ2 √ 2π exp

• −(t − r2µ1)2

2σ2

2

• + fnoisenhits,

(1) Fit per run: ratio of Gaussian constants f2) ratio of mobility r2 Fit per drift distance: standard deviations σ1 and σ2 mean µ1

• ffset fnoise

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 9 / 18

Drift velocity

Drift distance [mm] 5 10 15 20 25 30 35 40 Drift time [ms] 2 4 6 8 10

Majority carrier Minority carrier

drift

v 4.18 m/s

drift

E 300 V/cm

Preliminary The drift velocity is a few m/s The minority carrier(s) are 8.1% faster

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 10 / 18

Mobility

Drift velocity [m/s] 1 2 3 4 5 6 7 8 9 mobility /V/s

2

1.391 cm Drift field [V/cm] 100 200 300 400 500 600 /V/s]

2

Mobility [cm 1.38 1.39 1.4

Preliminary The mobility is 1.391(3) cm2/V/s

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 11 / 18

Diffusion

The transverse and longitudinal diffusion ( i = x, z ) are described by: σ2

i = σ2 i0 + D2 i z,

(2) where σi0 is the standard deviation at zero drift, Di the diffusion coefficient, and z the drift distance. In the thermal limit the diffusion coefficient is given by: Dthermal =

• 2kBT

eE , (3) where kB is Boltzmann constant, T is the temperature, e is the charge of the ion, and E is the electric field strength.

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 12 / 18

Diffusion

Drift distance [mm] 5 10 15 20 25 30 35 40 from fit [mm]

z

σ and

x

σ 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

Transverse diffusion Longitudinal diffusion

drift

E 300 V/cm

x

D cm m/ µ 130

x0

σ m µ 93

z

D cm m/ µ 152

z0

σ m µ 131

Preliminary The resolution at zero drift is explained by the laser beam width Plus for longitudinal diffusion the distance drifted by electrons before they are captured by the CS2 molecules, or unrecognised minority carriers

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 13 / 18

Diffusion as a function of drift field strength

Drift field [V/cm] 100 200 300 400 500 ] cm m/ µ Diffusion coefficient [ 50 100 150 200 250 300 350

Transverse diffusion coefficient Longitudinal diffusion coefficient Transverse diffusion temperature: 305 K Longitudinal diffusion temperature: 383 K Thermal diffusion temperature: 297 K Transverse diffusion coefficient Longitudinal diffusion coefficient Transverse diffusion temperature: 305 K Longitudinal diffusion temperature: 383 K Thermal diffusion temperature: 297 K

Preliminary The diffusion follows the thermal 1/√Edrift dependence well The transverse diffusion is close to the thermal limit

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 14 / 18

Fiducialisation

The difference in drift velocity between the majority carrier and minority carrier(s) can be used to reconstruct the drift distance without a trigger About 4.4% of the hits are attributed to the minority carrier(s), whose mobility is 8.1% higher than that the majority carrier. The reconstruction proceeds by performing per event a maximum likelihood fit of Equation (1) (the double Gaus) to the measured relative arrival time of ions from one or more laser pulses

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 15 / 18

Fiducialisation

Residual of reconstructed z-position [mm] 10 − 8 − 6 − 4 − 2 − 2 4 6 8 10 Normalised entries 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0.22

1 laser pulse (RMS = 3.9 mm) Superposition of 10 laser pulses (RMS = 1.29 mm)

Preliminary Efficiency is 66% for 1 pulse, and 100% for 10 pulses

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 16 / 18

A negative ion TPC at the ILC?

The negative ions have reduced diffusion, which is advantageous in the longitudinal direction but not small enough in the transverse direction for the ILD TPC The magnetic field does not reduce the diffusion much further because of the small ωτ (this also means little E × B effects) The slow drift velocity is not a problem for the collection of charge, but different bunch crossings may not be well separated This negative ion TPC does not meet the requirements for the ILD TPC

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 17 / 18

Conclusions

The GridPix quad was used as a negative ion TPC readout The mobility was measured to be 1.391(3) cm2/V/s for 93.6/5/1.4 gas mixture of Ar/iC4H10/CS2 with a small amount of oxygen and water vapor at a pressure of 1030 mbar and a temperature of 297 K The transverse and longitudinal diffusion have an effective thermal diffusion temperature of 383 K and 305 K Fiducialisation was applied and has an expected precision of 1.29 mm The small diffusion without the need for a magnetic field might be of interest to e.g. directional dark matter search experiments The full paper will be released soon

Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 18 / 18