1. Helsinki T2 status
Risto Orava/ on behalf of Kari Kurvinen
the 1st production GEM under tests (operational tests)
the 2nd GEM not accepted due to persistent leakage current problems in two foils (all other parts finished) - wait for components for replacement (and for further GEMs)
spare parts needed in future to avoid delays in the future

2. Characterisation of GEM foils:
leakage current measurements
visual inspection
optical scanning

3. Leakage current measurements of GEM foils
the four segments (A,B,C & D, see Fig.) of the GEM foils are measured separately by Picoamp /Voltage source device (Keithley 487).
leakage currents are measured three times during the assembly (the foil before and after framing, then after gluing into a stack).
3 x 3 foils x 4
36 measurements / detector
current limited by 100 MW resistor (corresponding to the max current of 5 mA at 500V).
a foil is approved if the current stays at < 0.5 nA for 5+
so far 2 /12 foils found to be bad. These were observed to be temporarily short circuited during the second measurement! Both exhibited instabilities during the first measurement, still passed the criteria.
due to these observations, a graphical display (Labview) was added to the measurement system to detect & document instabilities as early as possible (the display was not yet available for the first production GEM).

4. A bad GEM foil A-segment is short circuited (60 – 150 kW)
noise
0.5 nA
A-segment is short circuited (60 – 150 kW)
B-segment
0 min
10 min
TIF
HV ramp up
B,C & D segments OK
0.5 nA
(Note: the time scale in the plots must be multiplied by two)
noise
C-segment
6 min
0.5 nA
noise
D-segment
6 min

5. A ”good” GEM foil ? > 10 min in air A-segment A-segment > 5 min
HV ramp up
spark!
N2 flush
started
> 10 min
in air
0.5 nA
N2 flush
stopped
0.5 nA
A-segment
A-segment
14 min
20 min
in N2
spark!
in N2
> 5 min
C-segment
12 min
B-segment
14 min
in N2
(Note: the time scale in the plots must be multiplied by 2)
D-segment
noise
10 min

6. Visual inspection General observation:
defect in the mask
 0.5 mm
only the largest visible defects are observable
coordinates are recorded approximately
serious defects due to dust, stains or scratches are
easy to see
number of recorded defects in the six latest foils:
GEM 1 18, 12, 10 (top, middle, bottom)
GEM 2 10, 31, 40
defect in the mask
General observation:
the quality of GEM foils is good
(i.e. is significantly improved from the past)

8. Optical scanning of GEM foils
scanning in mixed mode (transparent +
reflective)
blue diffuser produces a colour
contrast between the holes and copper
surfaces
1.8 million GEM holes
scanned pictures can be analysed by using fitting algorithms to count anomalies and to register their coordinates
AIM:
a quantitative measure of the quality of GEM foils
defects, dust, scratches,etc
The worst foil so far: Blue curve shows all the seen objects; the red one only the ”roughly round ones” (e.g.scratches are filtered out). The number of defects (red ones > 80) was 240, while only 40 were found under visual inspection (see previous slide).