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Cube Measurements

Cube Measurements. Tent Crew. Scintillation Measurement @ BNL. PMT signal. Spectralon Diffuse UV Reflector. SBD signal. 241 Am Semi-collimated. Scint. Light Poisson Distr. SBD a -Trigger. a. Pulse Height Spectrum of Light from GEMstack. Typical Spectra: Two-fold.

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Cube Measurements

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  1. Cube Measurements Tent Crew

  2. Scintillation Measurement @ BNL PMT signal Spectralon Diffuse UV Reflector SBD signal 241Am Semi-collimated Scint. Light Poisson Distr. SBD a-Trigger a

  3. Pulse Height Spectrum of Light from GEMstack

  4. 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 msec 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> Coincident to Alpha 100 msec prior to Alpha

  5. 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 msec PRIOR to the alpha particle. • Compare the pulse height of the 55Fe to the net signal from light.

  6. ConceptsConcerningOperation 1. • 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 2. 3.

  7. 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

  8. How to Get Light Yield • Expectation: • Measurement

  9. 6 5 1 0 4 2 3 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%.

  10. 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…

  11. 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.

  12. 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.

  13. Region 3:What’s the effect of the transfer gap voltage? 1. Input • 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”. 2. Lost Lost RetainedCharge 3.

  14. Battery HV3 GEM HV2 Divider GEM HV1 GEM 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.

  15. 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.

  16. 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

  17. 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!

  18. Why 5 instead of 6.2? Above 100% 80% • 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 Theory

  19. 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!!!

  20. 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.

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