MEASUREMENT OF THE ADP GAIN

F. Cavallari, M.M.Obertino

MEASUREMENT OF THE APD GAIN

The gain curve (M vs V) is measured

ü for each APD by Hamamatsu at ~25OC (V50 propagated at 18OC)

ü for each capsule in Lyon lab. at ~18OC

üfor each capsule after it is glued to a crystal in Rome and Cern lab. at ~18OC

In each point is calculated :

Example of gain curve measured in Lyon

dVdM

MV1

=α

V50

5050 ]1

[ == MdVdM

Mα

In the database are stored V50 and α50 18OC for each set of measurements

The nominal bias voltage VN is chosen as the mean value of the V50 (from Lyon) of all the capsules supplied by the same HV channel

Determination of αLAB

Lyon measurement:

Lyon recalculated:

+=∆

−∆+= αα

VVMVVM

VMV)()(

)(1

2−+ +

=αα

αV

APD gain cannot be directly measured at H4 ( is difficultto measure the signal for very low bias voltage )

BUT

according to CMS IN 2001/33 we can•determine αV•find a relation between αV and the APD gain •under the hypothesis that the relation between αV and M is universal and valid for all APDs, use this relation to calculate the APD gain

MEASUREMENT OF THE APD GAIN

1

1

11

1

1

)()()()(

1)()()()(

1)()(

1

−

−

−+

+

+ −−

⋅+

+−−

⋅+

==NN

NN

NNNN

NN

NN

N

NV VV

VSVSVSVSVV

VSVSVSVSdV

VdMVM

α

PNAPD

VS N =)(where at the voltage VN

MEASUREMENT OF THE APD GAIN

We perfomed a ’mini’ HV scan around the working point ( VN ) increasing and decreasing VN of a few volts (VN-1 ; VN ; VN+1)

αV was calculated as:

2 HV scan performed in 2003 TB:−∆VN +/-2.5V−∆VN +/-1.2 V and +/-2.5 V

MEASUREMENT OF THE APD GAIN

HV1:I. 380.6VII. 378.1VIII. 375.6V

HV0:I. 360.6VII. 358.1VIII. 355.6V Data taken with blue and IR laser

50)](1[ 5050 ⋅−+= VVGain Nα

αTB – α50

Lyon

Hamamatsu

MEASUREMENT OF THE APD GAIN

ü Not very good agreement between Hamamatsu and Lyon measurement of the gain curve: GH(VN)~47

üαTB are sistematicallybigger then α50

MEASUREMENT OF THE APD GAIN

Active areaM>1

Area with M=1

Difference between Hamamatsu andLyon data can be explained by the fact that during the Lyon measurements only the active area was “illuminated” while (for this lot of APD) Hamamatsu illuminates the full area of the APD

But why always αTB > αLAB ?

Hamamatsu

Lyon

Cern-Capucinette

αLAB

αTB

MEASUREMENT OF THE APD GAIN

The gain were recalculated assuming αTB correct and using the Lyon gain curve

MTB = 53.19 +/- 0.13σ = 1.00 +/- 0.13

What seems to be universal is the relation between (αTB – α50 ) and M

(αTB – α50)

50)](1[_ 5050 ⋅−+= VVnHamGain α

(αTB – α50 )=A+B Gain_Ham

From this formula we can calculate the G_tb for each APD :

BA

G MTB

−−= 50αα

Testbeam

Ham.

2002 data: APD Gain at 18OC

2002 data: APD Gain at 20OC

αLAB from Lyon measurements. αLAB from Ham. measurements.

MEASUREMENT OF THE APD GAIN

With the IR laser we obtain αTB ~0.028 which corresponds to a gain ~34

IR laser

Blue laser