1. THGEM report – january, 22nd 2009
Elena Rocco

2. Outline
This campaign of measurements is important for understanding which is the most appropriate gas for THGEM as photon detector. During the last meeting Jarda already presented a complete report about the back scattering. In this short summary the results about the following topics will be shown: 1.Gas gain and ion feedback (Double THGEM + 55Fe); 2.Photon electron collection efficiency; 3.Light detection with the double structure; 4.Single photon spectrum. Conclusions & planning
UV light

3. Setup: 55Fe with the double THGEM used as single structure
Geometrical and electrical scheme for a Double THGEM used as a single structure.
THGEM: C7
d=0.4 mm
pitch =0.8 mm
t=0.4 mm
rim=0.01 mm
picoammeters
55Fe
Double
THGEM C7
4.5 mm
1.6 mm
THGEM1
THGEM2
anode
Drift - wires PS
The lamp was switched on some hours before starting the measurement for stability reasons ;
The voltage applied on the THGEM during the drift scan isn’t the maximum attainable.

4. Drift scan measurements for different gas mixtures (1/2)
The information coming from the drift scan is important also for the currents behaviour in all electrodes: since the final goal is using the CsI it’s important to have a look into the ions flow behaviour.

5. DRIFT SCANS FOR DIFFERENT GAS MIXTURES – ALL CURRENTS

6. Delta V scan – Rough gain estimation
As a single layer...
There is not the same code colour, but there’s a correspondence one to one of the order of the curves.
Even if the rate is not too precise from this plot there is a hint about the gain behaviour for the different gases.
As a single layer...
THGEM: C7
d=0.4 mm
pitch =0.8 mm
t=0.4 mm
rim=0.01 mm
Kapton window

7. Double Thgem with the iron source
Gas Ei
(kV/cm) Et Ed
Ar/CO2 4 2
0.67
Ar/CH4 3
3.6
0.5
The configuration of the double THGEM with the Ar/CH4 wasn’t optimized and a gain about 2.3 * 104 was reached for the highest voltage applied.

8. Setup for measurement with the UV light
4.5 mm
1.6 mm
THGEM1
THGEM2
anode
Drift - wires PS
Quartz window of the double THGEM chamber (type C7)
Hidden N2 exhaust PS PS
Oriel Lamp
N2 inlet
Up to now the Ar/CO2 mixture was the only mixture used for these kind of measurements
THGEM: C7
d=0.4 mm
pitch =0.8 mm
t=0.4 mm
rim=0.01 mm
Filter 3.0 OD
Setup for UV light meas. with the double THGEM structure
Filter 0.5 OD
Three positions of the shutter:
1.completly open,
2.closed (some internal reflections),
3.Open + filter 3.0 OD
Narrow Filter

9. Oriel lamp stability and the reproducibility of the whole system
The voltage on the drift (wires) was applied and the current on the CsI surface has been recorded.

10. ΔV scan for the double THGEM operating as the single one
Here a neutral filter with a factor 103 of attenuation was put in front of lamp for avoiding a big current on the CsI surface.
zoom
As for the previous characterization with the Iron source the THGEM close to the anode was shortcut into the electrodes and was labelled as “anode”.

11. From Amos’ presentation
Relative
Drift scan
Total currents involved in the Drift Scan at different voltage applied to the THGEM

12. Double THGEm meas. with uv light
ΔV SCANS
Meas. performed with the shutter closed  internal reflections!
Assumption to be in the single photon mode  to be checked!!!
From the calculation of the average amplitude, the rate estimated is 40 kHz.
DRIFT SCAN

13. Conclusions – Planning for the future
Taking into account all the results obtained up to now, it seems that the CH4 or a mixture of it with Ar is the most favourable gas for photon detection with the THGEM;
2 points (at least) are remaining to be measured/tested:
1.the gas gain for a triple or more structure in Ar/CH4 or just CH4 atmosphere;
2.the fields configuration for an optimized photon electron collection efficiency (shall we change the geometry of the THGEM?);
The simulations are helping in the understanding which parameters are fundamental for a optimized collection efficiency from the CsI;
From the lab activity point of view, a new set of measurements using the UV light source with the double THGEM are needed with several gas mixtures;
Before starting a new meas. campaign, the single photon spectrum has to be redone verifying the reliability of the single photon configuration.