Cube Measurements Tent Crew

Scintillation BNL 241 Am Semi- collimated  Spectralon Diffuse UV Reflector SBD  -Trigger 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 55 Fe and other alphas.  The “Net Signal” under any circumstance is -  The “Net Signal” under any circumstance is  sec prior to Alpha Coincident to Alpha

Test Procedure  Measure the 55 Fe 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 55 Fe to the net signal from light.

Concepts Concerning Operation 1.Too much reverse pulls electrons to mesh. 2.Too small dV does not make p.e (small extraction voltage makes yield lower than vacuum) 3.Too small transfer puts some electrons on bottom of GEM

55 Fe & Light Signal vs dV  The size of the 55 Fe signal is exponential in dV and shows a rather typical gain curve.  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. dV mesh =+18 V; standard chain for GEMs

How to Get Light Yield  Expectation:  Measurement

Containment  Size of hole in cube is rather small compared to pad.  Simple procedure to measure containment: 1.Measure the Light yield on the pad in question. 2.Measure the light yield on all 6 neighbors. 3.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 55 Fe 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 dV mesh =+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 CF 4 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 dV mesh dV GEM =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” Retained Charge Lost Input

How to make the measurement  Batteries set the V mesh 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. HV3 HV2 GEM Battery Divider HV1

Region 3: Transfer Field  Increasing the transfer gap increases the effective gain for both 55 Fe 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. dV Mesh =+18 V; dV GEM =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 CF 4.  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.