Rate and Gain Measurements of the 1-m long GEM detector

Aiwu Zhang2015-07-06

EIC tracking R&D weekly meeting

Detector setup

Column, from 1 to 10

row, from

1 to 3

• There are 10 columns by 3 rows holes on the drift (not through holes of course), where the thickness of the board is ~ 2 mm less than at other places. The holes are named as colN-rowN (eg., 6-2), this will be used later.

• Gain is measured at the drift hole number 6-2.• Readout board is the CMS one, with 384 straight strips for each of the 8 eta-sectors. Preamp is connected to

bottom of 3rd GEM foil. Current is readout from one Panasonic which connects to 128 strips together close to the position where drift hole number 6-2 locates.

• To reduce noise, a HV filter (300 kOhm and 4.4 nF) is used; a solid Al plate is put right after the readout board (~2 cm).

• Detector under Ar/CO2 (70:30), with gaps 3/1/2/1 mm.

Drift hole 6-2

2

Background rate and current

Consider detector area: (22+45)*100/2=3350 cm^2, cosmic ray rate is expected to be: 3350*1~2 /min = 55 ~ 112 Hz.The measurement here is reasonable because threshold is a little lower than noise level (when HV is applied noise level is around 80 mV).

Current in strips vs. time

Current through readout strips is stable for all tested voltages;And current due to cosmic signal is clearly indicated.

Applied V_d=3560V, I=808uA

3

Gain and rate for the detector – at lower X ray energies and w/o collimator

• Measured at drift hole 6-2, source distance to detector is roughly 0 cm.• Current through strips are all stable (see examples in backup slide-11).• Since can not see Fe-55 signals, energies of X ray is not calibrated. Using

previous measurement on the 10 cm GEM detector, gain can be still calculated. (Energy has an impact on primary ionization numbers)

• For different X ray energies (at different voltage settings), a same primary ionization number is used. The difference on gain due to this aspect should be small. So, we can conclude that we measure almost same gain at different X ray energy settings. 4

Gain and rate for the detector – at high X ray energies and with 1 mm collimator

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Gain and rate for the detector – impact of the drift board

X ray impinges drift on the hole 6-2

X ray impinges drift at a position ~5 mm to the right of the hole 6-2

The impact of drift board to the rate and gain is small.

6

Impact of the source distance to the rate

• X ray impinges the whole drift board (at the position ~5 mm right to the hole 6-2).• Rate plateau disappears at large distance between source and the detector, and rate is

reduced by ~90% from 5 cm to 15 cm (corresponds to a 1.7 times beam spot increase).• The absorption and scattering of the material contribute due to distance change is estimated

to be about 2% change on rate. So the only explanation of this big rate change is due to gain variation in the beam area: beam spot is bigger at farther distance with more gain variation. (but, within 10 mm size on the foil, gain varies by 10%?)

Beam spot: 6 mm Beam spot: 11 mm Beam spot: 16 mm

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Impact of the impinge position to the rate

The rate change is big when moving the source from hitting the holes 7-2, 6-2, 5-2, and 4-2 on the drift board.

8

X ray spectra of the measurements (at drift hole 6-2)

Energy resolution: ~ 12%

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~5 mm off the drift hole 6-2

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Summary

• Detector gain for the GEM detector in middle sector 5 is measured, it reaches 3*10^4.

• Good rate plateau is obtained when X ray source is close to the detector (so that the beam spot is small.)

• Strange rate behaviors at different detector positions and at different source-detector distances may be due to gain non-uniformity.

X ray 40kV/5uA at 3850VX ray 20kV/5uA at 3850V

Back up – currents measured on the strips at different conditions

Usually, at highest tested detector voltage (this is also the first voltage that is tested in one series measurement), current is not stable at the beginning, but it goes to a stable level after a few minutes.For all other conditions, current trends to be stable (flat). 11