Particle incident angle study with Mimosa 17

C.Dritsa, J.Baudot

Outline• Motivation• Digitiser• Analysis• Summary

11th CBM collaboration meeting GSI Darmstadt

Motivation

Open charm feasibility studies are of major importance for CBMThe MVD is the key detector for open charm measurements.

Background & Signal Generation

Transport

(GEANT)

Simulate detector’s response

For MVD:gaussian smearing

The actual model using gaussian smearing is not adapted for studying event pile-up and delta electrons.

Can this model be improved ?

The implementation of a more realistic MAPS response ( digitisation model ) will allow us studying the impact of the above points especially on the open charm reconstruction. This work is under progress at IPHC, Strasbourg.

Geometry (thickness, stations’ position)

Simplistic illustration of the digitisation model

Digitisation model for non depleted detector (MAPS detector):

1. Particle trajectory divided in segments inside the sensitive volume.2. Energy deposited in each segment is translated into charge.3. Charge spread in the sensitive volume within a defined cone.

No Electric Field:

Electrons are diffusing sensitive volume

θ

Advantage of digitiser: possibility to study particles with inclined tracks.Need to adapt the model’s parameters in order to reproduce experimental data.

Beam Test

•Beam test performed in November 2007 at CERN with a 120 GeV pion beam.

•The chip under test was a MIMOSA 17 :30 μm pitch - 14μm epi

MIMOSA 17

MAPS response to tracks with large incident angle was not studied in detail yet.

• The two external planes (reference planes) are used for track reconstruction.• The middle plane is the Detector Under Test (DUT): the tracks extrapolated in the DUT are matched with the hit located closest to the reconstructed track position.• Measurements were taken for several angles:• θ: 0°, 15°, 30°, 45°, 60°, 75° and 80°

Analysis steps

Top view of the setup

Reference plane:MIMOSA 17

Reference plane:MIMOSA 17

DUT

θ: 080!

Charge collected

in each pixel

PitchSize

tN pix

tan

How is the cluster shape affected when track is inclined ?

L

,PitchSizeNU pix

Lt

L: length of the particle trajectory in the epitaxial layert: epitaxial layer thicknessθ: angle of incidence with respect to the vertical

cos

80 tQ tot

tantU

cos

tL ,)/(80 LmeQ tot

• 7x7 cluster

• Each square represents one pixel

•Color scale normalised

Average Cluster Shape

Q on seed (MPV, e-)

250

249

285

348

506

823

964

Angle (°)

0

15

30

45

60

75

80

0° 15° 30°

45° 60° 75°

80°

Differences in the average cluster shape are obvious for large angles (>60o)

0° 15° 30°

45° 60° 80°

Cha

rge

colle

cted

(el

ectr

ons)

Collected charge

Signal to Noise Ratio on seed

• For small angles (<30o) the SNR is almost constant

• For bigger angles (>60o) the SNR varies significantly

Residuals

U

Vσ of the distribution of track-hit distance.

Can the inclination of the track be derived from cluster properties?

The number of significant pixels (above given threshold) is higher for 80° than for 0°.The aspect ratio is different.

Allow to identify and suppress hits from delta electrons?

U

V 0° 80°

Comparison of properties of average clusters for two angles:

....

...)3()2()1(

3,32,21,1

23,3

22,2

21,1

2

uvuvuv

uvuvuv

QQQ

vQvQvQ

Q

VQWidth defined by :

- 0°- 80°

V cluster width

U c

lust

er

wid

th

V cluster width

Can the inclination of the track be derived from cluster properties?

80°v=

u

Beam test (pions, 120GeV) on inclined tracks up to 80° was performed.

For small angles (<30o) the SNR of the seed, the charge of the seed and the charge of the full cluster are constant.

For large angles (>60o) the SNR of the seed , the charge of the seed and the charge of the full cluster increase significantly.

The residuals along the V direction (along the rotation axis) remain constant but the residuals along the U direction increase (up to 4 times for 80 degrees, but algorithm not optimised)

It seems possible to identify hits created from tracks with large incident angle. Investigate possible applications of this identification.

Next steps:Implementation of the digitiserPerform simulation study to evaluate the fraction of particles with large incident angle.

Summary and results

MotivationImplement MAPS digitiser.Study MAPS response to inclined tracks.

1 1

1 1 1 1

1 1 1

Can the inclination of the track using information from the cluster be derived ?

Q(Vi,Ui) Q(1,1)

Q(0,0)

Q(-1,1)

....

...)1()1()0(

1,11,10,0

21,1

21,1

20,0

2

QQQ

QQQ

Q

VQ

U

V

1 Digital3 CoG 3x35 CoG 5x57 Eta 4var9 Eta 3x311 Eta 2x213 Eta 5x5

0 degrees 80 degrees

Residuals 7x7 cluster: Different methods

The residuals obtained from eta 3x3 are the best even for 80 degrees rotation

Res

idu

als

(μm

)

Charge on Seed

250 249 285348

506

823

964

0

200

400

600

800

1000

1200

0 10 20 30 40 50 60 70 80 90

rotation anlge (degrees)

elec

tro

ns

Charge on seedMIMOSA 17 ( 14microns epi )

Charge 49 pixelsMIMOSA 17 ( 14microns epi )

Charge on 49 pixels

886 895 10051258

1847

3755

5048

0

1000

2000

3000

4000

5000

6000

0 10 20 30 40 50 60 70 80 90

rotation angle

char

ge

on

49

pix

els

(e-)

Noise on seed

14,8 14,8

14,7

14,8

15

14,9

14,5

14,4

14,5

14,6

14,7

14,8

14,9

15

15,1

0 10 20 30 40 50 60 70 80 90

rotation angle (degrees)

elec

trons

Noise on seedMIMOSA 17 ( 14microns epi )

Charge on 49 pixels

886 895 10051258

1847

3755

5048

0

1000

2000

3000

4000

5000

6000

0 10 20 30 40 50 60 70 80 90

rotation angle

char

ge

on

49

pix

els

(e-)

cos

80 tQ tot

constQtot cos

if CCE is const and clustering is correct:

Qtot*cos(theta)

750

800

850

900

950

1000

-20 0 20 40 60 80 100

angle (degrees)

Q(4

9pix

els)

*co

s(th

eta)

~10%

Error bars correspond to 1o

error in defining the rotation

angle

With a 10% precision CCE can be approximated as constant wrt the particle incident angle

What about charge collection efficiency at large angles?

Charge on 49 pixels (Qtot)

0 degrees 80 degrees

Residuals 7x7 cluster

μm μm

0 15 30

45 60 75

80

SNR on seed (MPV in e-)

250

249

285

348

506

823

964

Angle (degrees)

0

15

30

45

60

75

80

0 15 30

45 60 75

80

• 7x7 cluster

• Each square represents one pixel

•Color scale normalised

Mean Cluster Form (3D)