1. Transition Radiation Studies with Xe GlueX Collaboration Meeting
Sergey Furletov,
Lubomir Pentchev
GlueX Collaboration Meeting
May
Jefferson Lab 1 1 1

2. e e e Setup Configuration 15 cm radiator 10 cm radiator 5 mm Xe
4 m Copper
50 m Kapton
5 mm Ar
Color represents the deposited charge per track
50 m Mylar 2 2 2

3. Why Xenon?
Xe vs Ar deposited charge for the first vs second chamber with/without radiator (electrons coming from right of the plot)
Strongest TR effect for Xe 1st chamber, but also 2nd chamber
TR visble with Ar works only with the first chamber 3 3 3

4. Track reconstruction
We require track – hits in both chambers – perpendicular to the package
Correlations with the PS left arm and also between the two PS arms
For some runs these correlations are destroyed – first interpreted as background, later found event mixing 4 4 4 4

5. Ar in Upstream Chamber
Due to the thin window of the upstream chamber we see significant effect with Ar gas mixture 5 5 5

6. Xe in Downstream Chamber
Xe in the second chamber absorbs more energetic (>8-20 keV) transition radiation photons
Low energy photons have stopped mostly in the material between the chambers 6 6 6

7. Xe in Upstream Chamber
Significant loss of charge for big drift times at very low (50V/cm) field – normally ~ V/cm 7 7 7

8. Deposited Charge vs Drift Time - Summary
Radiator
Longer drift times (factor of 2) and loss of charge in first chamber due to:
chamber thicker by 15% - works as square, distance and field ~30% 8 8 8 8

9. Why drift times in first chamber so long?
Longer drift times and loss of charge in first chamber due to:
bulging (<5mm) of the Mylar entrance may add no more than another 20% (top right)
charge accumulation on the Mylar (?) -could explain much shorter drift time close to the edge of the chamber 9 9 9

10. Reducing drift time - solution
Solution: replacing end window (aluminized Mylar) with end cathode (Kapton + 2mm Copper) at negative HV; and probably use shorter spacer. 0V 0V
PROPOSED
2.5 msec
NOW 18msec
-1400V 0V 10 10 10

11. TR effects for Ar and Xe – Results vs Geant11 11 11

12. Lessons Learned
When using Xe mixtures need higher drift field to avoid charge losses
Additional loss of charge for tracks perpendicular to wires – space charge effects within the same tracks (plot below)
Need to keep Xe/CO2 bottle above critical temperature (>200C) to have correct mixture
The information can be used to study the PS performance, background in this area (in progress)
DAQ, trigger, fADC125 performance tested for extremely late (32 msec) events, and fADC125 at lower (31.25 MHz) clock frequency 12 12 12

13. Summary and Outlook
Tests performed with Xe/Ar gas mixtures in the two upstream chambers of the spare package, positioned behind the left arm of the PS
Transition radiation effects in good agreement with Geant simulations when using Ar in front chamber and Xe in second chamber.
In case of Xe in the front chamber, we see effects only for small drift times. This is due to significant loss of electrons for long drift times as a result of the very weak (50V/cm) drift field.
What’s different in the front chamber (compared to the second one) so that Xe doesn’t work there:
bigger drift distance (~30% effect)
bulging of the entrance window (<20% effect)
charge accumulation on the entrance window – aluminized (0.1m) Mylar
Replacing the window with a better material (Kapton+2m copper) cathode and applying negative potential to it will solve these problems:
fields up to 700V/cm – small recombination cross-section
short drift times < 3msec – can easily be integrated in the standard trigger and DAQ system 13 13 13 13