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 (CS 2 ) 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 1 see C. Martoff et al (2000) https://doi.org/10.1016/S0168-9002(99)00955-9 2 see 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% iC 4 H 10 as a quencher 1.4% CS 2 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 G u a r d TPX3 TPX3 TPX3 Wire bonds A C O C LV regulator TPX3 TPX3 Guard TPX3 TPX3 Ionisation in the gas volume is created using a pulsed N 2 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 Timepix hits Timepix hits y position [mm] 35 Laser track Laser track 7 30 6 25 5 Event display of negative ion 20 Drift time [ms] track 4 15 64 hits 3 E drift = 300 V / cm 10 2 5 1 Preliminary 0 0 10 15 20 25 30 35 x position [mm] Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 5 / 18
Run parameters Number of runs 9 Run duration 17 minutes 100 – 500 V / cm E drift − 380 V V grid 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 300 Entries Number of hits per laser pulse 250 200 150 100 50 Preliminary 0 0 20 40 60 80 100 120 140 Number of hits Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 7 / 18
Drift time spectrum Hits 4.7 mm 1800 1600 E drift = 300 V / cm 10.7 mm 1400 16.7 mm 1200 22.7 mm 28.7 mm 1000 35.7 mm 800 600 400 200 Preliminary 0 1 2 3 4 5 6 7 8 9 Drift time [ms] 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: − ( t − µ 1 ) 2 (1 − f 2 − f noise ) n hits � � exp √ 2 σ 2 σ 1 2 π 1 (1) − ( t − r 2 µ 1 ) 2 + f 2 n hits � � exp + f noise n hits , √ 2 σ 2 σ 2 2 π 2 Fit per run: ratio of Gaussian constants f 2 ) ratio of mobility r 2 Fit per drift distance: standard deviations σ 1 and σ 2 mean µ 1 offset f noise Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 9 / 18
Drift velocity Drift time [ms] Majority carrier Minority carrier 10 8 6 4 v 4.18 m/s drift 2 E 300 V/cm drift Preliminary 0 0 5 10 15 20 25 30 35 40 Drift distance [mm] 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 9 Drift velocity [m/s] 8 7 6 5 4 3 2 mobility 1.391 cm /V/s 2 1 0 /V/s] 1.4 2 Mobility [cm 1.39 Preliminary 1.38 0 100 200 300 400 500 600 Drift field [V/cm] The mobility is 1 . 391(3) cm 2 / 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: σ 2 i = σ 2 i 0 + D 2 i z , (2) where σ i 0 is the standard deviation at zero drift, D i the diffusion coefficient, and z the drift distance. In the thermal limit the diffusion coefficient is given by: � 2 k B T D thermal = , (3) eE where k B 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 from fit [mm] 0.4 Transverse diffusion Longitudinal diffusion 0.35 0.3 z σ and 0.25 x σ 0.2 E 300 V/cm drift 0.15 µ D 130 m/ cm x σ µ 0.1 93 m x0 µ D 152 m/ cm z 0.05 σ Preliminary µ 131 m z0 0 0 5 10 15 20 25 30 35 40 Drift distance [mm] 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 CS 2 molecules, or unrecognised minority carriers Kees Ligtenberg (Nikhef) Negative Ion TPC June 15, 2020 13 / 18
Diffusion as a function of drift field strength 350 ] cm Transverse diffusion coefficient Transverse diffusion coefficient m/ 300 Longitudinal diffusion coefficient Longitudinal diffusion coefficient µ Diffusion coefficient [ Transverse diffusion temperature: 305 K Transverse diffusion temperature: 305 K Longitudinal diffusion temperature: 383 K Longitudinal diffusion temperature: 383 K 250 Thermal diffusion temperature: 297 K Thermal diffusion temperature: 297 K 200 150 100 50 Preliminary 0 0 100 200 300 400 500 Drift field [V/cm] The diffusion follows the thermal 1 / √ E drift 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 Normalised entries 0.22 1 laser pulse (RMS = 3.9 mm) 0.2 Superposition of 10 laser pulses (RMS = 1.29 mm) 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 Preliminary 0 − − − − − 10 8 6 4 2 0 2 4 6 8 10 Residual of reconstructed z-position [mm] 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) cm 2 / V / s for 93.6/5/1.4 gas mixture of Ar/iC 4 H 10 /CS 2 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
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