Chamber parameters that we can modify and that affect the rising time

Number of ionisation clusters produced in the drift gap: Poisson Distribution

Probability distribution of the distance x of the ionisation cluster NEAREST to the first GEM :

P (x )=n ・ e( - nx )

With:

Space Probability Distribution of the first GEM NEAREST Cluster(x)=1 /n where n is the average number of ionisation clusters per unit

length

Time Probability Distribution of the first GEM NEAREST Cluster

(t)= 1 /( n ・ v drif t )

Intrinsic Resolution:

Single cluster/electron Signal Formation: Each electron produce a current pulse for a time t = dinduction gap/

vdrift

Parameter that we can change:vdrift Induction Field - Gas

n Gas

Drift Velocity vs Electric Field in Ar/CO2 70/30magboltz

Ar-CO2 (70-30)

If we apply -4kV, it means that we will have 4kV/5.5M~727A

In the induction region we will have 727A x 1M=727V across 2mm, i.e. ~3.6kV/cmIn the plot we can find that at ~3.6kV/cm we have ~7cm/s.

If we decided to use 1.5M, it means that we will have ~5.5kV/cmWith this field the drift velocity will be ~8cm/s.

In the actual configuration we have

1M+0.55M+1M+0.5M+1M+0.45M+1M

I~3.6kV/cm

T2~3.6kV/cm

T1~3.6kV/cm

D~2.4kV/cm

Changing only the induction field: effects on the signal induced

Each electrons cloud produced by a cluster will produce a current pulse for a time

t = dinduction gap/ vdrift

The current induced will be approximately

I ~~ (N(ionization)x Gem GAIN x e) x (vdrift/ dinduction gap)

This mean that we will have a factor 1.15 (in the current induced by each

clusters), if we change the drift velocity from 7cm/s to 8cm/s

Changing the gas

Two effects

Changing the gas: Effects on the GAIN

Changing the gas: effects on the signal induced

Each electrons cloud produced by a cluster will produce a current pulse for a time

t = dinduction gap/ vdrift

The current induced will be approximately

I ~ (N(ionization)x Gem GAIN x e) x (vdrift/ dinduction gap)

This mean that, if we change the drift velocity adding CF4 to the gas mixture,

we will have a factor 1.5/1.3 (in intensity of the signal induced by each

clusters).

One Example: LHCb GEMs

A factor of ~1.5/2 is possible respect with Ar/CO2

For LHCb the induction gap is only 1mm

Efficiency in 20ns time window

H.V. Board Resistors Modification (in Ar/CO2).

If we want to maintain the same fields in the other gaps and the same V across the

GEM(*)

We will need:

V(ind) = 5kV/cm x 0.2cm = 1kV

R(ind) = 1kV / 728A ~ 1.38M [728A in order to have VG1+VG2+VG3 = 1090V

as now]

Resistors Std value:

field[kV/cm] Power [W] V[V] respect with 1M

@ 1mA(**) @ 0.728mA

1.2M ~4.4 1.2 ~150V

1.5M ~5.5 1.5 ~370V

(*) if we change the gas we need to do also other consideration about the gain

(**) this is the maximum current from the module (to be extremely conservative).

H.V. Board Resistors Modification (in Ar/CO2).

Power Supply Module: -5kV, 1mA (maximum possible power 5W)

HV Divider : (let me say) 200k(filter)+5.5M(actual version)+R(Induction Gap)

If I consider in total 6.2M (i.e. with 1.5M on induction), the max current available will be ~800A

It means that respect with 728uA we could eventually increase the voltage of ~400V in total

and 100V on GEM.

In terms of gain this is ~~ a factor 5-10.

If I consider in total 5.9M (i.e. with 1.2M on induction), the max current available will be ~845A

It means that respect with 728uA we could eventually increase the voltage of ~700V in total

and ~175V on GEM.

In terms of gain this is ~~ a factor 30-60.

TripleGEM timing properties from H8 test Beam: MSPL Considerations…

H.V. on TripleGEM ~ -4.1kV

The needed MSPL with the actual configuration

The first MSPL that guarantees that all the signals are collected for a time period bigger than 1 clk cycle is 4CLK . We need “for a time period bigger than 1 clk

cycle” because we can change the latency in steps of 1clk.

Threshold Scan: Acquisition Mode (40k triggers/point)

MSPL:2clk and 4clk

No differences. OK

Timing Studies:

• MSPL tests revealed the possibility of using more clk cycles without degrading the level of the noise.

• To increase the drift velocity we can replace the induction gap resistor in the H.V. divider. A good value could be 1.2/1.5M.

• In order to have a bigger improvement of the rising time we could use Ar/CO2/CF4 that increase not only the drift velocity but also the number of primary clusters in the drift gap (more effective than changing only the induction field).(We have to be careful because using CF4, the gain of each GEM foil will be reduced.)

• In order to have margin in the increase of the gain, taking into account the HV power supply output range, a value of 1.2/1.3M for the induction gap resistors could be a good compromise between the increase of the drift velocity and the possibility to increase the voltage across the GEM foil if we will change the gas.

• For the system set-up in H8: – relief available– we will check the additive connectors needed and we will have to install

them.– we will have to clean the rotameter in order to have all the gas lines

needed to flux 10 chambers.– we will have to check the functionality of the mass flow-meter for the

mixing unit .