Villa Clara. Cuba - http://biomed.uninet.edu/2010/n1/index.html Rev Electron Biomed / Electron

The charge collection efficiency is one of the most important parameters to analyze.

After high irradiation fluences it is important to ensure that the sensors are going to measure a total charge above a noise threshold, stablished by the electronics. If not, due to the ATLAS binary system, some strips during the clusters formation would be lost. This could lead to a wrong position of the transversing particle hit that could compromise the tracking. Therefore, a comparison before and after irradiation of the collected charge is vital.

The collected charge is measured as a function of the bias voltage using the ALiBaVa system as explained in chapter 3. A

β

source (⁹⁰Sr) is placed above the sensor and a photomultiplier (below the sensor) will provide the trigger. The electrons registered at the readout electrodes form a signal which will be analized to extract the charge as a function of the applied voltage (see sections3.4.3and3.4.5for more details).

The active thickness of ATLAS12A sensors is (302

±

¹²⁾

µm

. From this thickness

(t) the expected charge can be estimated by [104]:

N = t

3.7 [190 + 16.3ln(t)]

(4.15) Therefore, for a not irradiated sensor the total collected charge above full depletion voltage will be

N = 23105

electrons, commonly expressed as 23.105

ke

⁻. The ITk-Strips Collaboration agreed that normalizations will be used for all charge collection measurements. For that purpose all the institutes used the same type of not irradiated miniature barrel sensor as reference.

The collected charge registered at IFIC for this sensor can be seen in figure4.54.

Figure 4.54: Charge Collection of a not irradiated barrel sensor. The curve is fitted to a function defined by4.16. Parametersp0andp1indicate the full depletion voltage and the collected charge on average respectively.

The function used to fit the charge is defined as:

Q(V ) =

From4.16the collected charge above the plateau (above 400 V) is

(23.09 ± 0.32) ke

⁻^. The theoretical value (

23.105 ke

⁻) is compatible with the measurement error. With this sensor a normalization factor is stablished to convert the

ADC

counts registered by

ALiBaVa

ke

⁻in the rest of the sensors. This factor is given by4.17and corresponds

4.4 Microstrip silicon detectors from Hamamatsu (ATLAS12A) 165

to the relation between the average of the collected charge in

ADC

counts and the corresponding

ke

⁻scaled to 23.105

ke

⁻.

f

norm

= Calculated Charge (23.105 ke

⁻

)

Average o f ADC counts (above the plateau) = 158.6 (ke

⁻

/ADC)

(4.17) The error in the charge (in electrons) is given by:

e(Q) = q

(0.4)

+ (0.035 · Q)

² ^(4.18) The uncertainty on the measurements due to the sensor variation in the Landau fit is included in the errors calculation. This value corresponds to 400

ke

⁻ and comes from the standard deviation of the measurements below the full collection. An error of 3.5% in charge collection is also added in quadrature. This is a relative error that comes from normalization.

Besides the barrel reference sensor two more not irradiated sensors (End-cap type,

EC

) were also measured. All the collected charge curves are depicted in figure 4.55.

Figure 4.55: Collected charge resistered for the three not irradiated sensors. The full depletion voltage is around 370 V which is close to the value extracted from theC−V curve. All the sensors collected a charge above 21ke⁻at full depletion.

EC

LargePitch

C

P19

had unexpected low charge collection (2

ke

⁻ below the reference sensor). In the

CCE

curve it seems that its behaviour above full depletion was not very stable, having fluctuations of about 0,4

ke

⁻between measurements.

Moreover, looking at the

I −V

curve on figure4.46a small jump in the leakage current can be appreciated. The visual inspection on this sensor was repeated looking for scratches or dust deposition. However, no important issues were found. These sensors are extremely sensitive and handling or transportation can affect their behaviour.

In the case of the irradiated sensors the setup is placed inside a vertical freezer (at -35^◦C) as explained in3. The difference in the charge collection with the reference sensor in these cases is more evident. Figure4.56shows the collected charge of all the measured sensors. Proton, neutron and gamma irradiated sensors are included.

For bias voltages above 450 V sensor

W626

BZ3C

P02

presented microdischarges.

These can be a consequence of high electric field regions around the implants. The microdischarges are identified by inestability in the leakage current with sudden increase and decrease in its amplitude. The measurements with this sensor were stopped to protect it from possible permanent damage.

From4.56it is clear that the higher the dose of radiation the less collected charge.

The

I −V

^{curves at}^4.4.1reflected that the damage induced by neutron irradiation is worse than the caused by proton irradiation. This effect is also revealed in the case of the charge collection. As explained in chapter3radiation causes the displacement of silicon atoms from its substitution site to an interstitial site to form a Frenkel pair. This radiation induced damage is mainly in the form of deep level single defects and defect clusters (extended damage region). For neutron radiation, the dominant damage is in the form of defect clusters while for gamma and electron radiations, it is mainly due to deep level single defects. For charge particles (protons, pions, etc.), it is the mixture of the two. For the same fluence the sensors irradiated with neutrons collected around

4.4 Microstrip silicon detectors from Hamamatsu (ATLAS12A) 167

Figure 4.56: Collected charge measured for the irradiated sensors (irradiated with proton, neutron and gamma particles). The not irradiated sensor results are also included as reference.

30% less charge than the ones with proton irradiation. Moreover, for the sensors irradiated with protons a charge plateau is intuited around 800 V. This is compatible with the

C −V

measurements where three proton irradiated sensors reached the full depletion voltage between 500 and 800 V. The plateau is not observed in sensors with neutron irradiation.

Regarding the sensors with gamma irradiation the behaviour of the charge colletion is similar to the not irradiated reference sensor presenting a plateau above 350 V. The total collected charge is quite close to 23

ke

⁻ which is the expected value for a not irradiated sensor. Gamma particles do not damage the silicon bulk in excess. The major damage contribution is located at the sensor surface so the influence is not as strong as the caused by neutrons or protons.

Representing the average of the collected charge (for fixed bias voltage) as a function of the fluence the deterioration of the sensor performance is more evident.

Figures 4.57 and 4.58 show this effect for neutron and proton irradiated sensors respectively.

Figure 4.57: Charge collection as a function of the fluence for neutron irradiated sensors.

Figure 4.58: Charge collection as a function of the fluence for proton irradiated sensors.

Table4.19summarizes the results obtained for collected charge of the measured sensors selecting a bias voltage close to the HL-LHC voltage of operation (

∼

^{600 V).}

The not irradiated reference sensor result is also included for a directly comparison.

These tests were checked against the results from the other institutes involved. A summary of all the results of the collaboration is represented in figures4.59and4.60 with the measurements obtained for neutron and proton irradiation respectively.

4.4 Microstrip silicon detectors from Hamamatsu (ATLAS12A) 169

Charge Colection - Irradiated sensors (at 600V)

The charge collection achieved by all the institutes for each radiation and dose is

similar and proved to be consistent.

Figure 4.60: Summary of the collected charge measurements for the proton irradiated ATLAS12A sensors by the seven groups involved. These results were published at [105].

In document http://biomed.uninet.edu/2010/n1/index.html Rev Electron Biomed / Electron J Biomed 2010;1:1 (página 58-61)