Robert Hollingworth - University of Sheffield
Transcript of Robert Hollingworth - University of Sheffield
IDM 2004Robert Hollingworth
Low Temperature Tests of Photomultipliers For
Use in Liquid Xenon Experiments
Robert Hollingworth
IDM 2004Robert Hollingworth
Overview
Tube Description
Experimental Procedure
Tube Results
R8778 Tube and Results
Conclusions
IDM 2004Robert Hollingworth
Reasons For Testing
The tubes are to be used in ZEPLIN II, an experiment in the UK Dark Matter Collaboration using liquid xenon as a target medium.
The tubes needed to detect 175nm scintillation light, have a fast time response and a low dark rate.
Since the tubes had only been tested at room temperature, it could not be assumed that they would function similarly at LXe temperatures,~163K(110°C).
IDM 2004Robert Hollingworth
Tube Description
Tube properties:12.8 cm diameter
Hemispherical bulb
Quartz windowed
Bialkali photocathode
Large detection area
Not low background
Tubes are developmental tubes fromElectron Tubes (D742QKFLB).
IDM 2004Robert Hollingworth
Initial Box Arrangement
Tube under test placed into a lighttight, thermally insulated cooling box.
Cooling performed using cold nitrogen vapour from an external liquid nitrogen dewar, equipped with a 10W heater.
Photomultiplier
Light prism
Thermometer
460nm light transferred to the box by plastic optical fibre. LED outside of cooling box.
Light projected onto tube by 45°plastic prism.
IDM 2004Robert Hollingworth
Final Box ArrangementDuring the tests, two more light sources were added. A 370nm source and another 460nm source.
370nm light used to test a wavelength closer to the operating wavelength.
Second 460nm source used to test for any effects of photocathode resistivity changes.
Previously run tubes kept in box for temperature cycling.
Another thermometer was strapped to the tube for more accurate temperature measurements.
New thermometer Near blue light
UV light Far blue light
IDM 2004Robert Hollingworth
Description of Method
Cooling box
LN2 Dewar
Oscilloscope
All tubes powered to 1200V.
Tubes left for 24h in the dark, to allow photoluminesence in the glass to decay.
Tubes cooled using nitrogen vapour.
Cooling rate kept at 2030Kh1 to maintain thermal equilibrium between inside and outside of tube.
Response data taken for each LED at 10K intervals.
The light from each LED was pulsed at 10kHz. Light pulses were ~4ns duration.
Signals from tube recorded on an oscilloscope.
IDM 2004Robert Hollingworth
Description of Method cont.
The oscilloscope integrated the area under the (V, t) signal pulses and histogrammed them, to give the response of the tube.
The response was taken to be the amount of charge produced in the tube from a single input LED pulse; a combination of the quantum efficiency of the photocathode and the dynode gain.
The near blue light only illuminated a small central section of the photocathode. The far blue light illuminated the whole photocathode.
The effect of any changes in the resistivity of the photocathode would cause changes in the ratio of the response of the two blue lights.
The dark rate of the tubes was also measured at each temperature value.
IDM 2004Robert Hollingworth
Typical Response Results
Temp, °C
Nor
mal
ised
Res
pons
e, A
rb
Nor
mal
ised
Res
pons
e, A
rb
Nor
mal
ised
Res
pons
e, A
rb
Temp, °CTemp, °C
Blue Far, Blue Near, UV• ••
These results show that the response of the tubes did not diminish at low temperatures, but mainly increased.
It is unknown why the response to the UV light increased more significantly than that to the blue light.
IDM 2004Robert Hollingworth
Response Ratio Plots
The ratio of the far to near responses for blue light does not change dramatically. The maximum difference is less than 10%.
Thus the effect of the changing photocathode resistivity shouldn't have a large effect on the signals from the tubes.
IDM 2004Robert Hollingworth
Dark Rate Plots
Temp, °CTemp, °C Temp, °C
Dar
k ra
te, s
1
The dark rate of the tubes does fall rapidly with temperature, but does rise again at low temperatures, as expected. Unfortunately the operating temperature of the tubes is not within the plateau region of the dark rate.
Dar
k ra
te, s
1
Dar
k ra
te, s
1
IDM 2004Robert Hollingworth
R8778 Tube
A Hamamatsu R8778 was also tested.
It is a new low background tube, that could be used in the next generation of xenon based dark matter experiments.
It is an all metal tube with a quartz window.
Without the need for a graded seal the background in the tube is greatly reduced.
The same tests were performed on this tube as on the others.
IDM 2004Robert Hollingworth
R8778 Results
Temp, °C
Nor
mal
ised
Res
pons
e, A
rb
Temp, °C
•••
460nm Far460nm Near370nm
The response to the Blue light falls slightly at low temperature.
The response to the UV light still has a large increase.
The Blue light response ratio is better than before.
The dark rate is still good.D
ark
rate
, s1
IDM 2004Robert Hollingworth
Conclusions
Low temperature test have been performed on eleven 12.8cm hemispherical photomultipliers.
The results show no fatal effects that would cause the tubes to be rejected.
The tests of the R8778 showed that it too operated satisfactorily at low temperature.