UCL Laser Geiger cell update Robert L Flack

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

Introduction Investigated the drift speed of the electrons to the anode after ionisation in a 3 cm and 5 cm diameter cell. The 3 cm diameter cell was constructed with the help of the University of Manchester. The 5 cm cell was constructed at UCL (thanks to Brian and Derek). A nitrogen laser, λ = 337 nm, 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 2

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

t 0 tf t 0 March 2008 Cell review meeting, MSSL ← 45μs → tf 4

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 340 ns. • If outside of the window it is assumed to be a cosmic. March 2008 Cell review meeting, MSSL 5

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

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 ~150 n. S Drift speed ~ 1 mm/37. 5 n. S The tail is due to the less dense region of the laser beam. Time n. S March 2008 Cell review meeting, MSSL 7

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

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

Efficiency • The strategy is as follows. The ionisation process, double photon capture, is the same in the 3 and 5 cm 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 3 cm cell is known from NEMO 3. • Assuming the efficiency for both processes scale identically then an estimate of the efficiency for the 5 cm cell can be calculated. • Specifically the number of ionisations per mm. March 2008 Cell review meeting, MSSL 10

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

Relative efficiency @ 2 mm • Using data 2 mm from the anode: – 3 cm; 958 events in 13. 6 mm = 71/mm – 5 cm; 1198 events in 13. 6 mm = 88/mm – The time taken to take the data is 4 times faster for the 3 cm cell. • Rel eff = 4 x 71/88 = 3. 2 March 2008 Cell review meeting, MSSL 12

Relative efficiency @ 8 mm • Using data 8 mm from the anode: – 3 cm; 1900 events in 17. 8 mm = 107/mm – 5 cm; 2083 events in 17. 8 mm = 117/mm – The time taken to take the data is 2 times faster for the 3 cm cell. • Rel eff = 2 x 107/117 = 1. 8 March 2008 Cell review meeting, MSSL 13

Conclusion • The 5 cm cell appears to work satisfactorily. • The distribution of speeds in the 3 and 5 cm cells are consistent with NEMO 3 out to 23 mm radius. • The relative efficiency is 3. 2 at 2 mm and 1. 8 at 8 mm. March 2008 Cell review meeting, MSSL 14

Backup slides March 2008 Cell review meeting, MSSL 15

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

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

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

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

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

pical profile he laser beam. March 2008 Fitted with a 2 D polynomial Cell review meeting, MSSL