Triple-GEM detector operation for high-rate particle triggering

W. Bonivento, G. Bencivenni, A. Cardini, C. Deplano, P. de Simone, F. Murtas, D. Pinci, M. Poli-Lener and D. Raspino

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

The Gas Electron Multiplier

A Gas Electron Multiplier is made by 50 m thick kapton foil, copper clad on each side and perforated by an high surface-density of bi-conical channels;

By applying a potential difference between the two copper sides an electric field as high as 100 kV/cm is produced in the holes acting as multiplication channels;

The gain of a single GEM is of the order 20100.

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

The triple-GEM detector Multiple GEM structures allow to reach high gain in safe

operating conditions, resulting very useful for ionizing particle detention;

Several 3-GEM detector prototypes have been built and tested in last two years by our group;

3-GEM detector layout together with the labeling of different geometrical parameters

We propose this kind of detector for equipping the central region (R1) of the first station (M1) of the LHCb system.

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Detector construction

10x10 cm2 GEM stretched and glued on

frames

Prototype before closing

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Rate capability The rate capability was measured with an X-ray (5.9 keV) tube; The detector was supplied with an Ar/CO2/CF4 (60/20/20)

mixture resulting in a gain of about 2x104;

A very good gain stability was

found up to a photon flux of about 5x107 Hz/cm2

LHCb R1M1 maximum rate

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Ageing test With a gain of 2x104 a total integrated charge of 13 C/cm2

is expected in 10 years of operation in R1M1;

By irradiating a 3-GEM chamber with an flux of 50 MHz/cm2 X-rays, in 10 days a total charge of 20 C/cm2 was integrated;

Less than 5% change in the chamber behavior

LHCb R1M1 maximum integrated charge

P and T variations were monitored by using a low

irradiated 3-GEM chamber

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

The gas mixtures In the beam tests we studied 4 different gas mixtures:

1. Ar/CO2 70/30;

2. Ar/CO2/CF4 60/20/20;

3. Ar/CF4/C4H10 65/28/7;

4. Ar/CO2/CF4 45/15/40;

Drift field 3 kV/cm

Given n: the number of

clusters per unit length; v: the electron drift

velocity in the drift gap;

The 1/nv term is the main contribution to the time resolution of this kind of detector.

The Ar/CO2/CF4 45/15/40 gas mixture should give the same time performance as the Ar/CO2/C4H10 65/28/7.

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

The gas mixture gains

The triple-GEM detector gain was measured by using X-rays for the different gas mixtures;

The detector gain resulted essentially an exponential function of the sum of the 3 GEM voltage supplies:

G = A e(Vgem1+Vgem2+Vgem3)

A and depend on the gas mixture.

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

The time performanceBest TDC spectra for the gas mixtures used

RMS 9.7 ns RMS 5.3 ns

RMS 4.5 ns RMS 4.5 ns

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

The efficiency in 25 ns A very important requirement for triggering in LHC

experiments is to ensure an high efficiency in a 25 ns time window for a correct bunch crossing identification;

Fast mixtures give an 25 of 98% also at moderate gain values

Slow mixture 25 less than 88% also for high gain values

Chamber efficiency in 25 ns

David

e P

inci

Univ

ers

ità d

i C

aglia

riDischarge studies

In this case a discharge occurs;

A discharge is seen: On the GEM electrodes as a

sudden increase of the current needed for the recharge;

On the pads as a momentary drop of current because of the drop of the detector gain.

Because of the Landau’s distribution tail, sometime a large amount of pairs is created in the gas; After the multiplication the charge can exceed the Raether limit giving raise to a streamer formation in the GEM holes;

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Discharge probability The discharge probability per incident particle was

measured by testing 3 chambers at the Paul Sherrer Institute with a particle rate of about 300 MHz;

No ageing or other damages observed on the 3 detectors after about 5000 discharges integrated;

Up to this value the detector operation seems completely safe;

Because of the particle rate in R1M1 (0.5 MHz/cm2) in order to have less than 5000 discharges/detector in 10 years

discharge probability per incident particle < 10-12

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Combined results

The results of the discharge probability per incident particle are compared with the efficiency in 25 ns;

In LHCb R1M1 the 3-GEM chambers have to ensure a wide working region where:

The discharge probability is less than 10-12;

The efficiency in 25 ns time window of 2 OR-ed chambers is greater than 99%;

The pad cluster size is less than 1.2 with 1x1.25 cm2 pad;

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Results: Ar/CO2/CF4 60/20/20

Discharge probability < 10-

12

25 ns time window of two

chambers in OR > 99%

Very narrow working region (if any…)

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Results: Ar/CF4/C4H10 65/28/7

Discharge probability < 10-

12

1030 V 1075 V

45 V wide working region

25 ns time window of two

chambers in OR > 99%

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Results: Ar/CO2/CF4 45/15/40

Discharge probability < 10-

12

1250 V 1315 V

65 V wide working region

25 ns time window of two

chambers in OR > 99%

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

1.22 1.161.17

The pad cluster size

The pad cluster size values at the end of the working regions are reported in plot;

David

e P

inci

Univ

ers

ità d

i C

aglia

ri

Summary

After two years and several prototypes built and tested we may conclude:

Very high rate capability (up to 5x107 Hz/cm2); Very good ageing performance (good stability up to 20

C/cm2); Ar/CO2/CF4 60/20/20 narrow working region 10 V;

Ar/CF4/C4H10 65/28/7 wide working region 45 V;

Ar/CO2/CF4 45/15/40 wide working region 65 V with a cluster size lower than 1.17;

The triple-GEM detector with Ar/CO2/CF4 (45/15/40) mixture fulfills all the requirements to be used in

R1M1