Cube Cube MeasurementsMeasurementsTent CrewTent Crew

Scintillation Measurement @ BNLScintillation Measurement @ BNL

241AmSemi-collimated

Spectralon Diffuse UV Reflector

SB

D

-T

rigger

Scint. Light Poisson Distr.

PMT signal

SBD signal

Pulse Height Spectrum of Pulse Height Spectrum of Light from GEMstackLight from GEMstack

Typical Spectra: Two-foldTypical Spectra: Two-fold

““Light” Signal is acquired by triggering the scope on the SPD Light” Signal is acquired by triggering the scope on the SPD signal for Alpha.signal for Alpha.

““Pedestal” signal is obtained by recording the pulse-height-Pedestal” signal is obtained by recording the pulse-height-spectrum 100 spectrum 100 sec PRIOR to the alpha particle.sec PRIOR to the alpha particle. shows narrow Gaussian plus “accidentals” which include overlaps of shows narrow Gaussian plus “accidentals” which include overlaps of

other signals including other signals including 5555Fe and other alphas.Fe and other alphas. The “Net Signal” under any circumstance is <light>-<pedestal>The “Net Signal” under any circumstance is <light>-<pedestal>

100 sec prior to Alpha Coincident to Alpha

Test ProcedureTest Procedure

Measure the Measure the 5555Fe signals from the pad by Fe signals from the pad by finding the pulse-height-peak at large pulse finding the pulse-height-peak at large pulse heights.heights.

Measure the Light yield through mean of Measure the Light yield through mean of spectrum taken in coincidence with the Si spectrum taken in coincidence with the Si detector signal.detector signal.

Measure the mean signal from “accidentals” via Measure the mean signal from “accidentals” via the pulse height spectrum 100 the pulse height spectrum 100 sec PRIOR to sec PRIOR to the alpha particle.the alpha particle.

Compare the pulse height of the Compare the pulse height of the 5555Fe to the net Fe to the net signal from light.signal from light.

ConceptsConceptsConcerninConcerninggOperationOperation

1.1. Too much reverse pulls electrons to mesh.Too much reverse pulls electrons to mesh.2.2. Too small dV does not make p.eToo small dV does not make p.e

(small extraction voltage makes yield lower than vacuum)(small extraction voltage makes yield lower than vacuum)3.3. Too small transfer puts some electrons on bottom of GEMToo small transfer puts some electrons on bottom of GEM

1.

2.

3.

5555Fe & Light Signal vs dVFe & Light Signal vs dV

The size of the The size of the 5555Fe signal is Fe signal is exponential in dV and shows a exponential in dV and shows a rather typical gain curve.rather typical gain curve. Gain=27,000 @ 508 VGain=27,000 @ 508 V

The size of the light signal is also The size of the light signal is also exponential in dV with a exponential in dV with a very very similar slopesimilar slope..

These can be compared by These can be compared by forming the ratio and calculating forming the ratio and calculating photo-electrons.photo-electrons.

y = 2E-09e0.0596x

y = 4E-11e0.0585x

y = 2E-09e0.0585x

10

100

1000

10000

100000

460 470 480 490 500 510 520

dV

Gai

n

Gain via 55-Fe

Light Signal (mV)

Signal*50

Expon. (Gain via 55-Fe)

Expon. (Light Signal (mV))

Expon. (Signal*50)

dVmesh=+18 V; standard chain for GEMs

How to Get Light YieldHow to Get Light Yield

Expectation:Expectation:

MeasurementMeasurement

3.60.1*0.16.5

41.16.5*82.0*80.0*

%2.8

%5.27*14.3

*)(6.5

)(6.5***

)(

)(*..

tContainmen

BNLEMeV

SBUEMeV

PMTQE

GEMQEep

Si

siGEMmeshPMT

)()()(

10955

accidentalVlightVFeV

ContainmentContainment

Size of hole in cube is rather small compared to pad.Size of hole in cube is rather small compared to pad. Simple procedure to measure containment:Simple procedure to measure containment:

1.1. Measure the Light yield on the pad in question.Measure the Light yield on the pad in question.2.2. Measure the light yield on all 6 neighbors.Measure the light yield on all 6 neighbors.3.3. Determine the fraction on the pad of interest.Determine the fraction on the pad of interest.

Answer:Answer: Containment is better than 99.6%, conservative estimate 99.2%.Containment is better than 99.6%, conservative estimate 99.2%.

0

2

3

4

1

6

5

7

0

0

iiSig

SigtContainmen

Region 2: dV Across the Region 2: dV Across the GEMGEM

There is NOT a significant change in p.e. yield as the dV of the GEMs is changed.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 The light signal tracks almost precisely the size of the 5555Fe signal.Fe signal. Where is the effect of increasing QE with increasing extraction field?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.ANSWER: This effect MUST be very small with a lever arm of only 5-10% in dV.

Photo Electrons vs dV

0

1

2

3

4

5

6

460 470 480 490 500 510 520

dV (GEM)

p.e

.

Series1

5% change

dVmesh=+18 V; standard chain for GEMs NOTE: We will discuss #p.e. later…

Why so small dV effect?Why so small dV effect? The top plot shows normalized The top plot shows normalized

photocurrent vs extraction photocurrent vs extraction voltage in CFvoltage in CF44 measured last measured last summer.summer.

The curve has an ever-The curve has an ever-decreasing slope.decreasing slope.

One can calculate the fractional One can calculate the fractional change in photo-current for a 5% change in photo-current for a 5% change in extraction field at change in extraction field at various places on the curve.various places on the curve.

The net effect is that the The net effect is that the fractional change in photo-current fractional change in photo-current is always less than the fractional is always less than the fractional change in the extraction field change in the extraction field (simple property of saturating (simple property of saturating exponentials).exponentials).

We should never expect (and do We should never expect (and do not observe) any strong not observe) any strong dependence of the collection dependence of the collection efficiency on the dV of the GEM efficiency on the dV of the GEM within the acceptable operating within the acceptable operating range.range.

Normalized Yield of Light

0

0.2

0.4

0.6

0.8

1

1.2

0 100 200 300 400 500 600

dV (mesh)

Rel

ativ

e p

ho

tou

rren

t yi

eld

in

CF

4Change of Light Yield for 5% change in dV

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0.05

0 100 200 300 400 500 600

dV (mesh)

Fra

ctio

nal

ph

oto

urr

ent

chan

ge

Region 1. Region 1. Mesh Voltage Mesh Voltage

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

Photo-Electrons vs Mesh Bias

0

1

2

3

4

5

6

-150 -100 -50 0 50 100 150

dV (Mesh)

Np

e

Nearest Fe

Average Fe

Median Fe

dVGEM=495 V; standard chain.

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

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

1.

2.

3.RetainedCharge

Lost Lost

Input

How to make the How to make the measurementmeasurement

Batteries set the VBatteries set the Vmeshmesh very accurately. very accurately. Using 3 HV connections we can vary the voltage across the first Using 3 HV connections we can vary the voltage across the first

transfer GAP.transfer GAP. NOTE: The HV power module we have has three bad channels, 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 so we cannot play this trick with all the voltages! We would like to trade this in for a better one from BNL.trade this in for a better one from BNL.

HV3

HV2

GEM

GEM

GEM

Battery

DividerHV1

Region 3: Transfer FieldRegion 3: Transfer Field

Increasing the transfer gap increases the effective gain for Increasing the transfer gap increases the effective gain for both both 5555Fe and Light.Fe and Light.

The ratio shows whether photo-electrons were recovered.The ratio shows whether photo-electrons were recovered. These can be compared to the nominal settings to measure These can be compared to the nominal settings to measure

the “free gain” achieved by the transfer gap.the “free gain” achieved by the transfer gap.

Signals while varying Transfer Gap

1

10

100

1000

10000

0 200 400 600 800 1000 1200

dV

(mV

) 55Fe (mV)

Light (mV)

Light*20 (mV)

Photo-electronsPhoto-electrons

Well, there is no gain in photo-electrons from this.Well, there is no gain in photo-electrons from this. However, the effective gain IS increased.However, the effective gain IS increased. This measurement verifies that all photo-electrons, once This measurement verifies that all photo-electrons, once

avalanched, couple their charge into the transfer gap.avalanched, couple their charge into the transfer gap.

Photo-Electrons vs Transfer Gap Voltage

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

dV (transfer Gap)

Np

e

dVMesh=+18 V; dVGEM=495 V

Free GainFree Gain

Compared to the nominal chain there is an available 25% gain for free by Compared to the nominal chain there is an available 25% gain for free by increasing the transfer gap voltage a little bit.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 If we do this on the top and middle GEMs, we can gain 1.6X overall gain without increasing dV on the individual GEMs.without increasing dV on the individual GEMs.

If we only do this on the top GEM, we improve Hadron Blindness by 25%.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.This is a good move any way you look at it.

Relative Gain

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 200 400 600 800 1000 1200

dV (transfer gap)

Gai

n/s

tan

dar

d c

hai

n

Nice!

Why 5 instead of 6.2?Why 5 instead of 6.2?

5.0/6.3 = 80%5.0/6.3 = 80% According to BNL measurements, According to BNL measurements,

80% of the vacuum yield is expected.80% of the vacuum yield is expected. The WIS measurements concluded The WIS measurements concluded

100% of the vacuum yield is seen in 100% of the vacuum yield is seen in CFCF44..

NOTE: 80% of 36 p.e. is 29 p.e. I NOTE: 80% of 36 p.e. is 29 p.e. I believe the latter is the correct believe the latter is the correct maximum yield of the HBD.maximum yield of the HBD.

Photoelectron Collection Efficincy Vs Applied Collection Field@1608 Angstroms

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5

Applied Field [kV/cm]

Co

llec

tio

n E

ffic

ien

cy [

%]

CF4

Vacuum

BNL

0 100 200 300 4000

50

100

150

200

250

300

350

Vacuum CF

4

Iphc,

pA

Vmesh, V

WIS

Above 100%80%

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40

E/N (Td)

f

6.8

9.8

9.28.8

8.2

7.6

7.2

E ph

(eV) Ne Ar

CF4

a)

Theory

How well does HBD work?How well does HBD work?

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

Looks pretty good to me!!!Looks pretty good to me!!!

SummarySummary Mesh voltage gives about 20% more light.Mesh voltage gives about 20% more light. dV(GEM) has no effect.dV(GEM) has no effect.

This means that running at lower voltages DOES NOT This means that running at lower voltages DOES NOT reduce the light yield.reduce the light yield.

We can run at minimal V to get excellent S/N.We can run at minimal V to get excellent S/N. dV(gap) has no effect.dV(gap) has no effect.

We can get “Free Gain” by raising Vgap a bit.We can get “Free Gain” by raising Vgap a bit. This further improves stability; no light change.This further improves stability; no light change.

Actual yield of p.e. is 80% of expectation.Actual yield of p.e. is 80% of expectation. If we accept BNL results this is explained simply by the If we accept BNL results this is explained simply by the

fact the QE is lowered in presence of gas.fact the QE is lowered in presence of gas. This p.e. yield is sufficient for physics.This p.e. yield is sufficient for physics. HBD will work in Run 9.HBD will work in Run 9.