Linearity Tests

The laser induced high noise pulses on the APD rendering it useless.

We tried several methods to shield and filter the noise.

With a stabilized version of filtering used the APD.

Comparing pulses from PMT and APD for different voltages, different filters, we noticed inconsistent results.

Corrected the results for non linearities in order to get consistent results.

Linearity Tests

•APD Hamamatsu C5331-11, Φ 1mm, extended blue sensitivity.

•PMT BURLE 8575.

•Blue LED 470 nm.

•Pulsed at 2 Hz, same rate as the laser.

•The supply voltage charges a capacitor which discharges through theLED.

Devices

APD output

y = 38,941x - 504,01

0

50

100

150

200

250

300

350

400

10 15 20 25

LED Suppply V

AP

D m

V

APD

Γραμμική (APD)

Calculation of N phot from APD

0

1000

2000

3000

4000

5000

10 15 20 25

LED suplly

Nu

mb

er

of

Ph

oto

ns

Calculation of N phot fromAPD

Nonlinear above

300 mV

Limited by noise below 50 mV

We can extend the linearity if we correct for the broadening of the pulse

APD plots

The APD module is linear but has a limited dynamic range.

We can possible extend the lower range if we can reduce the noise from the laser.

APD Range

Laser ShieldThe laser is enclosed in cage of copper sheet 0.5

mm thick. All gaps are covered with copper tape with conductive adhesive.

The APD module is closed in a cast aluminum box, gaps covered with copper tape.

Power is delivered through RG58 cable, with filter inside the box.

Signal cable passes through a ferrite core, to reduce induced currents on feedback.

The lowest we can get are spikes of 10-15 mV.

PMT LinearityTube Output

y = 528,85x - 6666,6

y = 331,52x - 4322,4

y = 133,67x - 1757,9

0

500

1000

1500

2000

2500

10 11 12 13 14 15 16 17 18 19 20 21 22

LED Supply V

Pu

lse

mV PMT 1200

PMT 1300

PMT 1100

Calculation for N phot from tube

0,00E+00

2,00E+04

4,00E+04

6,00E+04

8,00E+04

1,00E+05

1,20E+05

1,40E+05

1,60E+05

10 12 14 16 18 20 22

Led supply

# o

f P

ho

ton

s

1200 v

1300 V

1100 V

Linearity for big pulses.

Looks better if we use number of Photons (charge). We have used the measured width.

Notice change of slope below 500 mV.

PMT Range• Tubes have a wide dynamic range but there not linear in all this range. We have to chose operating range.

• What we see in the first plot is “overlinearity” , because we use big pulses. The gain with high light input is bigger than with low. This is before the region of “sagging”.

• Correction for width has to be applied.

Repeat previous test.

•A fiber is attached to each PMT and APD.

•Recording the output pulse high from PMT and APD. Calculate Charge and then number of Ph-el.

•Compare Center and Bottom positions.

First option: Laser + fibers

Common Source: A low power UV laser (by courtesy of IU)

Diffuser : A 10*10*10 cm3 plastic scintillator excited by the laser

Box with holes for theblue light output

Scintillator

Distribution: bunch of 15 plastic fibers, 4m long, 100μm diameter

Input,diameter ~0.1mm

outputs x15

Fiber Bundle, PMT

APD box

Light yield from the scintillator block

1460

1550

5110

450

N ph-el PMT

N ph-el APD

Ratio C/B =3.3

Ratio C/B=3.2

Summary

•APD has a good linearity within its range.

•PMT has to be corrected for width. Use charge rather than voltage for pulse high.

•The light pulse coming out of the lower hole is wider by 2-3 ns compared to the one from the center. Contribution of internally reflected light.

Last improvement.We were able to reduce the pick-up noise on the

APD using a double shield on the output cable. The noise level was reduced down to 2 mV !