RF test of the GEM detector prototipe

• Detector description• RF test area (LINAC3)

• detector working conditions

• RF field measurement

• Detector measurements

• Conclusions

F. Ambrosino,C. D’Addio, U. Gastaldi, E. Gschwendner, E. Radicioni, G. Saracino

Triple GEM Detector

• Triple GEM prototipe designed and assembled at LNF (G. Bencivenni et al.)

• GEM 10X10 cm2 standard geometry CERN

• PAD readout: 40 2.5x1cm2

• 50 m kapton cathode + 5 m copper

• 20 m aluminized mylar gas window

• HARP preamplifier

3

1

2

1

Distance

mmcathode

GEM1

GEM2

GEM3

PAD

mylar gas window

10

Drift

T1

T2

Induction

The detector is inside a 2 mm brass shielding with preamplifier and H.V. distributor boards

The RF test areaThe detector was placed near the two 200 MHz accelerator tanks IA2 and IA3 of the heavy ions accelerator Linac3 at CERN

GEM DETECTOR

RF power supply

202.56 MHz

Power (kW)

Emax

(MV/m)Len.( m)

IA2 250 15 1.5

IA3 285 11.5 2.2

detector window faced the RF tanks at distance ~30 cm

The back was ~1 m distant frome the RF power supply

GEM DETECTORGEM DETECTOR

H.V. power supplyH.V. power supply L.V. power supplyL.V. power supply

RF power supply headsRF power supply heads

detector configuration

• Gas: Ar:CO2 80:20 open mode

• fields and gain :– Drift field: 3 kV/cm– T1=T2 field: 3 kV/cm– Induction field: 5 kV/cm VG1= VG2= VG3=315 V– Total gain ~5x103

• 55Fe source (5.9 keV x)

3

1

2

1

Distance

mmcathode

GEM1

GEM2

GEM3

PAD

10

Drift

T1

T2

Induction

RF field measurement

We used a Agilent-HP 11955A biconical antenna to measure the RF field close to the detector area

The RF pulse is 0.6 ms long with a period of ~1.2 s

Using the known antenna factor AF and the signal VO of the RF from the antenna measured by the oscilloscope we calculated the em field E:

AF= E(Volt/m)/ VO(Volt)

20 log10 E(Vm-1) = 20log10VO (V) + AF(dBm-1)

E(Vm-1) = 10(logVo + AF/20) = VO10AF/20

AF(200MHz) = 16.7 dBm-1

E-field calculation

VO = 3 V E= 20 V/m

Detector response to the noise

After shielding and grounding the noise response of the detector was~20 mV peak to peak outside the RF pulse~ 80 mV peak to peak inside the RF pulse

Detector response to x rayUsing a 55Fe source we measured the response of the detector to 5.9 keV x rays. A spectrum was acquired using a MCA showing the 5.9 peak and the 3 keV typical “escape” peak.A background spectrum wasacquired in the same condition but without the source.Working voltage 3x315 VMost of the events are out of coincidence with respect to the RF pulse.

55Fe x in coincidence with the RF Using the RF signal from the antenna to trigger the scope we looked for x inside the .6 ms window of RF pulse

VGEM=315x3 V.

X out of coincidenceX in coincidence

Signal zoom

ConclusionsWe tested a GEM based detector, with cables and grounding not optimized for RF immunity, in the vicinity of the CERN LINAC3 accelerator.

The detector worked properly close to 2 RF accelerator tanks of 200 MHz and their power supply of ~ 250 KW. The signals from a 5.9 keV x are clearly visible above the RF background.

GEMs don’t seem to suffer particulary from the presence of RF noise.