Gas Detector Developments

Jin Li

Liquid Xenon caseLiquid Xenon can be considered as a gaseous xenon of 520 atm.

K.Masuda, S. Takasu, T.Doke et al. (Doke Group) NIM,160(1979)247

Fit assuming gaseous xenon.

Proportional scintillation signal

Direct scintillation (50 ns/div)

Proportional scintillation (100 ns/div)

Known method from gaseous xenonPhoton-Assisted Cascaded Electron Multipliers (PACEM)

• Ion-backflow is reduced.• Optical signals are mechanically and electrically decoupled from the detector,

insensitive to electronic noise.Micro Hole & Strip plate

measured charge Gain=350 for the second-stage

1/350

2006 JINST 1 P08003.

Signal at 5.9keV Optical gain = #photoelectrons/#primary electrons

~ 60

For total gain = 2x104 and drift fields = 100V/cm,Ion backflow fraction = 2x103 are reachable.

Liquid argon caseNo stable gain on thin wires in Liquid Argon.

Small mixture of xenon (<100ppm) in LAr is needed to stabilize the electron avalanche process, when tested using a single sharp needle as an anode.

Gain ~ 100

Two kind of pulses when the xenon concentration is < 1000 ppm.

J.G. Kim, S.M. Dardin, K.H. Jackson, R.W. Kadel, J.A. Kadyk, V. Peskov et al., Studies of electron avalanche behavior in liquid argon, IEEE Trans. Nuc. Sci. 49 (2002) 1851.

Saturation in LAr57Co source (maximum photon energy 135 keV) produces identical pulse heights with the 241Am source (maximum photon energy 60 keV) => pulse is saturated.

However, the current v.s. voltage agrees with avalanche multiplication process for gaseous detectors: And there are two kinds of pulses:

Array of tipsG. Bressi, M. Cambiaghi, G. Carugno, E. Conti and E. D’Uscio, Electron multiplication in liquid argon on a tip array, Nucl. Instrum. Meth. A 310 (1991) 613.

• Electron multiplication process.• Gain ~100 in pure argon.• Array of microtips of 0.25 m.• A plateau region when HV around 1.4 kV.

Proportionality not studied.

Secondary scintillation in THGEM

Argon emits VUV light at 128 nm, so a 50% concentration of the tetraphenyl butadiene (TPB) is mineral oil is applied to the SiPM surface.

P.K. Lightfoot, et al, 2009 JINST 4 P04002.

Secondary scintillation spectrumDouble phase system.

Single phase system.

5.9keV 86 photoelectrons.

5.9keV 62 photoelectrons.

Integration time for signal: 10 s.

VTHGEM = 9.91 kV

Discussion

• The proportion of events containing discharge sparking increased with the voltage across the THGEM, causing greater spread of photoelectrons.

• Corona discharge from local field instabilities lead to positive photon feedback or premature breakdown. It’s also responsible for the narrow operating high voltage range.

• Background gamma increase because increased density of liquid argon.

Why there is a worse performance in single phase liquid argon?

Single phase system.

VTHGEM = 10.15 kV

Directional sensitivity of Columnar Recombination

A R&D program is going in LBNL and FNAL.

Fraction of Recombination to Ionization: R/I = 4 for perfect columnar alignment,R/I=0.67 for complete columnar perpendicularity.R/I=0.15 for electron recoils.

Why Liquid xenon does not work

• In 10 bar gas xenon, the track length for 30keV nuclear recoil is about 2100 nm, the Onsager radius for recombination is 70 nm.

• But, because of closely spaced ions, the effective Onsager radius will grow about to 6 times. So the length/radius for the track is about 5.

• In liquid xenon, the track length will decrease and the ion density will increase for a significant amount (two orders of magnitude), so essentially the track is not a line shape anymore.

Simulation tools

Need to understand the electron cooling, Penning transfer, and recombination processes, and add to simulation or calculation.

• SRIM provides the ionization topology information.• Garfield can simulate the electron transport and drifting.

No reliable simulation tool in liquid phase yet.