# Presentation: 808424

## Cube Measurements

- Tent Crew

## Scintillation Measurement @ BNL

- Scint. Light
- Poisson Distr.

- PMT signal

- SBD signal

## Pulse Height Spectrum of Light from GEMstack

## Typical Spectra: Two-fold

- “Light” Signal is acquired by triggering the scope on the SPD signal for Alpha.
- “Pedestal” signal is obtained by recording the pulse-height-spectrum 100 sec PRIOR to the alpha particle.
- shows narrow Gaussian plus “accidentals” which include overlaps of other signals including 55Fe and other alphas.
- The “Net Signal” under any circumstance is <light>-<pedestal>

- 100 sec prior to Alpha

- Coincident to Alpha

## Test Procedure

- Measure the 55Fe signals from the pad by finding the pulse-height-peak at large pulse heights.
- Measure the Light yield through mean of spectrum taken in coincidence with the Si detector signal.
- Measure the mean signal from “accidentals” via the pulse height spectrum 100 sec PRIOR to the alpha particle.
- Compare the pulse height of the 55Fe to the net signal from light.

## ConceptsConcerningOperation

- Too much reverse pulls electrons to mesh.
- Too small dV does not make p.e(small extraction voltage makes yield lower than vacuum)
- Too small transfer puts some electrons on bottom of GEM

- 1.

- 2.

- 3.

## 55Fe & Light Signal vs dV

- The size of the 55Fe signal is exponential in dV and shows a rather typical gain curve.
- Gain=27,000 @ 508 V
- The size of the light signal is also exponential in dV with a very similar slope.
- These can be compared by forming the ratio and calculating photo-electrons.

- dVmesh=+18 V; standard chain for GEMs

## How to Get Light Yield

**How to Get Light Yield**

- Expectation:
- Measurement

## Containment

- Size of hole in cube is rather small compared to pad.
- Simple procedure to measure containment:
- Measure the Light yield on the pad in question.
- Measure the light yield on all 6 neighbors.
- Determine the fraction on the pad of interest.
- Answer:
- Containment is better than 99.6%, conservative estimate 99.2%.

## Region 2: dV Across the GEM

- There is NOT a significant change in p.e. yield as the dV of the GEMs is changed.
- The light signal tracks almost precisely the size of the 55Fe signal.
- Where is the effect of increasing QE with increasing extraction field?
- ANSWER:  This effect MUST be very small with a lever arm of only 5-10% in dV.

**5% change**

- dVmesh=+18 V; standard chain for GEMs

- NOTE:  We will discuss #p.e. later...

## Why so small dV effect?

- The top plot shows normalized photocurrent vs extraction voltage in CF4 measured last summer.
- The curve has an ever-decreasing slope.
- One can calculate the fractional change in photo-current for a 5% change in extraction field at various places on the curve.
- The net effect is that the fractional change in photo-current is always less than the fractional change in the extraction field (simple property of saturating exponentials).
- We should never expect (and do not observe) any strong dependence of the collection efficiency on the dV of the GEM within the acceptable operating range.

## Region 1.  Mesh Voltage

- Photo-electrons vs Mesh Voltage
- There IS a significant loss at -30 Volts
- This measurement is quite consistent with the NIM publication.
- We could expect at most 20% gain in light w/ better dVmesh

- dVGEM=495 V; standard chain.

## Region 3:What’s the effect of the transfer gap voltage?

- Physical Gain = (Retained+Lost)/Input
- Effective Gain = Retained/Input
- Varying the transfer field modifies the fraction of retained change.
- NOTE:  Retaining more charge is “Gain for Free”.

- 1.

- 2.

- 3.

- RetainedCharge

- Lost

- Lost

- Input

## How to make the measurement

- Batteries set the Vmesh very accurately.
- Using 3 HV connections we can vary the voltage across the first transfer GAP.
- NOTE:  The HV power module we have has three bad channels, so we cannot play this trick with all the voltages!  We would like to trade this in for a better one from BNL.

## Region 3: Transfer Field

- Increasing the transfer gap increases the effective gain for both 55Fe and Light.
- The ratio shows whether photo-electrons were recovered.
- These can be compared to the nominal settings to measure the “free gain” achieved by the transfer gap.

## Photo-electrons

- Well, there is no gain in photo-electrons from this.
- However, the effective gain IS increased.
- This measurement verifies that all photo-electrons, once avalanched, couple their charge into the transfer gap.

- dVMesh=+18 V; dVGEM=495 V

## Free Gain

- Compared to the nominal chain there is an available 25% gain for free by increasing the transfer gap voltage a little bit.
- If we do this on the top and middle GEMs, we can gain 1.6X overall gain without increasing dV on the individual GEMs.
- If we only do this on the top GEM, we improve Hadron Blindness by 25%.
- This is a good move any way you look at it.

- Nice!

## Why 5 instead of 6.2?

- 5.0/6.3 = 80%
- According to BNL measurements, 80% of the vacuum yield is expected.
- The WIS measurements concluded 100% of the vacuum yield is seen in CF4.
- NOTE:  80% of 36 p.e. is 29 p.e.  I believe the latter is the correct maximum yield of the HBD.

- BNL

- WIS

- Above 100%

- 80%

- Theory

## How well does HBD work?

- Let’s assume that we lose a few more photons to transmission etc...
- Take an expectation of 25 p.e.
- Now that we have a TRUE response to a mean of 5 p.e., we can sample the distribution 5 times, and sum them to get an HBD response prediction histogram...
- Looks pretty good to me!!!

## Summary

- Mesh voltage gives about 20% more light.
- dV(GEM) has no effect.
- This means that running at lower voltages DOES NOT reduce the light yield.
- We can run at minimal V to get excellent S/N.
- dV(gap) has no effect.
- We can get “Free Gain” by raising Vgap a bit.
- This further improves stability; no light change.
- Actual yield of p.e. is 80% of expectation.
- If we accept BNL results this is explained simply by the fact the QE is lowered in presence of gas.
- This p.e. yield is sufficient for physics.
- HBD will work in Run 9.