1. Laser Geiger cell (update)
UCL
Laser Geiger cell (update)
Robert L. Flack
March 2008
Cell review meeting, MSSL

2. Cell review meeting, MSSL
Introduction
Investigated the drift speed of the electrons to the anode after ionisation in a 3cm and 5cm diameter cell.
The 3cm diameter cell was constructed with the help of the University of Manchester.
The 5cm cell was constructed at UCL (thanks to Brian and Derek).
A nitrogen laser, λ = 337nm, is used to simulate the ionisation of the gas by a charged particle (double photon capture by impurities from the pump oil in the gas).
March 2008
Cell review meeting, MSSL

3. Schematic of the test cell
March 2008
Cell review meeting, MSSL

4. Cell review meeting, MSSLtf t0
← 45μs → tf
March 2008
Cell review meeting, MSSL

5. DAQ (Thanks to Gianfranco Sciacca)
Use a Camac TDC.
Event triggered by PMT.
Anode pulse stops the event, if it appears within a pre-specified time window of 340ns.
If outside of the window it is assumed to be a cosmic.
March 2008
Cell review meeting, MSSL

6. Investigation of the drift speed of electrons
to the anode after ionisation
March 2008
Cell review meeting, MSSL

7. Laser 4 mm under the anode
Interpretation of the distribution is that it is due to the geometry of the laser beam.
The peak is due to the increase density of photons at the focal region.
Peak ~150nS
The tail is due to the less dense region of the laser beam.
Drift speed ~ 1mm/37.5nS
Time nS
March 2008
Cell review meeting, MSSL

8. Ionisation times v Vertical distance
March 2008
Cell review meeting, MSSL

9. Combined drift times and speeds
3cm in green
5cm in red
March 2008
Cell review meeting, MSSL

10. Cell review meeting, MSSL
Efficiency
The strategy is as follows. The ionisation process, double photon capture, is the same in the 3 and 5cm cell.
Hence an estimate for the relative efficiency can be found for this process.
The absolute efficiency for the totally different process, ionisation of the He gas by a charged particle, in a 3cm cell is known from NEMO3.
Assuming the efficiency for both processes scale identically then an estimate of the efficiency for the 5cm cell can be calculated.
Specifically the number of ionisations per mm.
March 2008
Cell review meeting, MSSL

11. Observation inline with the anode
Tail is constant across the cell
March 2008
Cell review meeting, MSSL

12. Relative efficiency @ 2mm
Using data 2mm from the anode:
3cm; events in 13.6mm = 71/mm
5cm; events in 13.6mm = 88/mm
The time taken to take the data is 4 times faster for the 3cm cell.
Rel eff = 4 x 71/88 = 3.2
March 2008
Cell review meeting, MSSL

13. Relative efficiency @ 8mm
Using data 8mm from the anode:
3cm; events in 17.8mm = 107/mm
5cm; events in 17.8mm = 117/mm
The time taken to take the data is 2 times faster for the 3cm cell.
Rel eff = 2 x 107/117 = 1.8
March 2008
Cell review meeting, MSSL

14. Cell review meeting, MSSL
Conclusion
The 5cm cell appears to work satisfactorily.
The distribution of speeds in the 3 and 5cm cells are consistent with NEMO3 out to 23mm radius.
The relative efficiency is 3.2 at 2mm and 1.8 at 8mm.
March 2008
Cell review meeting, MSSL

15. Cell review meeting, MSSL
Backup slides
March 2008
Cell review meeting, MSSL

16. Test cell at Manchester
March 2008
Cell review meeting, MSSL

17. Cell review meeting, MSSL
Cathode ring + wires
March 2008
Cell review meeting, MSSL

18. Cell review meeting, MSSL
Test cell at UCL
March 2008
Cell review meeting, MSSL

19. Cell review meeting, MSSL
Stage for laser
UV-Laser
Diverging
lens
Convex
lens
CCD
camera
March 2008
Cell review meeting, MSSL

20. Cell review meeting, MSSL
Diverging
lens
Converging
lens
Pulse rate = 20 Hz
Pulse width = 3ns
Power = 5 mW
Laser
light
Focal
point
Pinhole
March 2008
Cell review meeting, MSSL

21. Cell review meeting, MSSL
A typical profile
of the laser beam.
Fitted with a 2D polynomial
March 2008
Cell review meeting, MSSL