A few words about the IBF
Thorsten Lux
IFAE-BIST
1
Motivation
At the last collaboration meeting there were two talks related to IBF:
•Luciano from CIEMAT presented a study about effect of IBF on electron
attachment for a wide range of IBF/gain values
• Silvestro from ETH studied the track distortions for 0, 0.02 and 0.1 and
came to the conclusion of track distortions of 20 cm for 0.1 of IBF (?)
The values from Luciano seemed me too high, while the ones from Silvestro
too low.
Based on my experience with MPGDs and some simulations/results for
GEMs I try to motivate why I assume that the IBF in reality is between 0.5
and 1.
I propose also a measurement of the IBF which also should answer the
question if the ions enter the liquid or not.
2
Some Definitions
Since Luciano and Silvestro use different naming schemes which make
comparison a bit complicated. I would propose to define:
IBF = (#ions at cathode)/(#electrons at anode)
The IBF alone is not very useful and when it is used as parameter it should
be clear that it is for Geff=20 (for electrons) and primary charge density of
WA105.
The ion gain defined by Luciano is simply: Gion = IBF*Geff
=> For WA105 the interesting curves are for Gion=<20
Alternatively one could use directly the charge density in the liquid as
parameter for plots.
3
Extreme Case: No Electron Diffusion
Cathode
1) An electron follows perfectly the
field lines inside the hole
2) Charge amplification takes place at
the end of the hole
3) 2 electrons drift towards the anode
and one ion follows back the field
lines to the cathode
1)
3)
2)
In this case the IBF would be 1
3)
Anode
(and even a MWPC readout would be
better with an IBF of about 0.5)
4
Realistic Case
Cathode
1) An electron drifts towards the hole
and enters it
2) Charge amplification takes place at
the end of the hole but not at the
original field line but closer to the
edge of the hole
3) One electrons drifts towards the
anode, one electron to the bottom
of the GEM and one ion drifts back
through the hole ending on the top
of the GEM
Anode
In this case the IBF <1
1)
3)
3)
2)
5
Important IBF Parameters
Cathode
Drift/ Extraction field
Hole
field
Induction/ Collection field
Thumb rules:
1) Strength of induction field affects IBF
only slightly but is important for
electron collection
2) Increasing the hole field (= GEM/LEM
voltage), decreases the IBF
3) Increasing the drift field, increases the
IBF
Quite common sense looking at field lines
Anode
6
Simulations for GEMs
Did not find measurements for single THGEM
and my simple simulations still not finished
but at MPGD2015 was an interesting talk by
Purba Bhattacharya about simulation with
single GEM (50 m thick).
=> Simulations should consider worst
case scenario of IBF of 0.5 to 1!
Linear
Hole
field of extrapolation
LEM
https://agenda.infn.it/contributionDisplay.py?contribId=39&confId=8839
Measurement by GDD
(IEEE TRANSACTIONS
ON NUCLEAR
SCIENCE, VOL. 50, NO.
4, AUGUST 2003)
7
Can we measure it?
Simulations are fine but can we measure this important parameter?
In principle it is simple, just measured the currents on all electrodes
while irradiating the detector!
Anode
LEM Top
LEM Bottom
8 channel
Picometer
Radioactive
Source
Cathode
IBF = I_Cathode/I_Anode
8
But ...
The best would be to do it under real conditions ...
Pure argon and cryogenic temperatures! => special requirements on radioactive source!
In addition the IBF is only one factor, but famous “ions go to the liquid or not?” is another
one. Could we measure this also?
Possibly with two different measurements in the same setup.
Measurement 1:
Anode
LEM Top
LEM Bottom
8 channel
Picometer
Side electrode
Cathode
Radioactive
Source
LAr Level
9
But ...
The best would be to do it under real conditions ...
Pure argon and cryogenic temperatures! => special requirements on radioactive source!
In addition the IBF is only one factor, but famous “ions go to the liquid or not?” is another
one. Could we measure this also?
Possibly with two different measurements in the same setup.
Measurement 2:
Anode
LEM Top
LEM Bottom
8 channel
Picometer
Side electrode
Cathode
Radioactive
Source
LAr Level
10
Possible Implications
• Measurements in gas will provide IBF of LEM under close to real conditions
• Measurements with LAr will provide “ion gas to liquid transition probability” (IGLTP)
This is no “Fun R&D” (like testing other MPGDs to get a gain of 22 instead of 20) but
could have serious impact on the design of WA105!
Case:
1. IBF < 10-20%: Safe and one continue as until now
2. IBF >> 10% && IGLTP low: ions are transported by convection or recombination
with later arriving electrons => Do we will need electrodes for the ions around each
LEM?
3. IBF >> 10%&&IGLTP high: ions go into the liquid and the field distortions will be
enormous => Do we need a gate? How could it be realized?
11
Conclusions
• Two studies from ETH and CIEMAT indicate serious problems due to
the IBF
• Currently used maximum IBF for single stage MPGDs seems not to
be realistic
• Worst/realistic case could be between 0.5 and 1 and should be
simulated
• Higher value could have important impact on WA105
design/performance
• Measurements of the IBF and IGLTP should be considered
• Setup would not be very complicated but one would have to choose
well the radioactive sources
• Results of 3x1x1 m3 might be too late to react for WA105 design
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