Nuclear Instruments and Methods in Physics Research A - F9

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300L. Casagrande et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 299–3031. IntroductionFinely segmented silicon detectors are the mostcommon choice for close to interaction pointtracking in high energy physics experiments. Dueto the harsh environment, the effect of radiationinduced damage in silicon represents a key issue inview of their long-term operation. Moderatecooling is applied to irradiated silicon detectorsto lower the leakage current and to inhibit reverseannealing [1]. However, this kind of operationdoes not prevent the changes in effective dopingconcentration, implying a dramatic increase of thedepletion voltage. When the radiation fluenceapproaches 10 15 n=cm 2 ; standard detectors becomeunusable [2]. Sophisticated multi-guard-ring designscan be used to increase the maximum biasvoltage which can be applied to a detector. Silicondetectors based on oxygenated bulk show asmaller increase of the depletion voltage whenirradiated with charged particles [3].An alternative way to improve the radiationhardness of silicon detectors is their operation atcryogenic temperatures. Indeed, it has been shown[4] that heavily irradiated silicon detectors, nolonger functional at room temperature, recovertheir performance when operated at temperaturesbelow 130 K. This experimental observation isusually referred to as the ‘‘Lazarus effect’’.At very low temperatures, the de-trapping rateof charged carriers from radiation induced deeplevels is strongly affected by the reduced thermalenergy, leading to the situation in which animportant fraction of traps remains filled, henceinactive. This prevents further trapping of carriersgenerated by particles traversing the detector and,therefore, the signal is not reduced. Moreover, thecharged traps reduce the effective doping concentrationthus reducing the full depletion voltage ofthe detector. This qualitative interpretation is inagreement with the available observations [5].Furthermore, at cryogenic temperatures the leakagecurrent becomes negligible. This facilitates thedesign of the front-end readout electronics interms of noise requirements, and a simple readoutscheme (like DC coupling) can be used. Inthis paper we present data obtained on silicondiodes irradiated at room temperature to fluencesof 5 10 14 and 2 10 15 1 MeV eq. neutrons=cm 2 :We also present preliminary results on theirradiation of a silicon pad detector with450 GeV protons during operation at 80 K.2. Results on samples irradiated at roomtemperatureWe have investigated the performance of twosilicon (Al/p þ /n/n þ /Al) diodes irradiated at roomtemperature with neutrons to fluences of 5 10 14and 1 10 15 n=cm 2 ; respectively. The 400 mmthick smaples has a sensitive area of 5 5mm 2surrounded by a guard-ring. Their resistivitybefore irradiation was around 4.5 kO cm.The measurements were performed in a cryostatdescribed elsewhere [6]. The samples were exposedto a radioactive source ( 90 Sr) and a trigger diodewas used to select minimum ionising particles(MIPs). The detector signal was read out by acharge amplifier, the first stage of which was aGaAs FET operational down to 1 K mountedinside the cryostat. The noise of the amplifier wasabout 1500 electrons FWHM. The signal wasshaped (1 ms shaping time) and sent to a multichannelanalyser. The recorded charge spectrawere fitted with a Landau distribution. The chargecollection efficiency (CCE) was determined as themost probable value from the fit, normalised to thesame value obtained for a non-irradiated detectorof the same thickness operated above full depletion.The errors shown in the figures include thesystematic errors due to the uncertainty on thedetector thickness.The current–voltage characteristic of the irradiatedsamples at 77 K showed a negligible leakagecurrent (o1 nA) even when operated in forwardbias up to 250 V. Fig. 1 shows the CCE of thesamples as a function of the temperature. Themeasurements were only performed whenthe leakage current was reduced to such a valuethat the noise was below 2000 electrons FWHM.In the case of the sample irradiated with5 10 14 n=cm 2 operated in forward bias, this wasonly achieved for temperatures below B140 K. Inreverse bias, the CCE improves with decreasingtemperature until it reaches a maximum around
L. Casagrande et al. / Nuclear Instruments and Methods in Physics Research A 477 (2002) 299–303 301Fig. 1. CCE as a function of the temperature in reverse and forward bias operation after fluences of (a) 5 10 14 n=cm 2 and (b)1 10 15 n/cm 2 :figures correspond to measurements in stableconditions. In fact, it has been observed [6] thatCCE in reverse bias decreases with time after thebias voltage is set, until it reaches a stable value. Inconventional reverse bias operation, for the sampleirradiated with the higherst fluence, an MIP signalof about 6500 electrons can be obtained. Inforward bias there is no time dependence of theCCE and B100% CCE can be achieved with200 V after a fluence of 5 10 14 n=cm 2 : The sameresult can be obtained in reverse bias in presence ofshort wavelength light [6].3. Results from a sample irradiated at 80 KFig. 2. CCE at T=77 K as a function of the bias voltage for thediodes irradiated at room temperature.130 K. In forward bias, 2–3 times higher values ofthe CCE can be achieved also at higher temperatures,as long as the leakage current does notexceed tolerable values.Fig. 2 shows the voltage dependence of the CCEin reverse and forward bias. The data shown in theA3 3 silicon pad detector matrix was placedinside a cryostat in the CERN-SPS 450 GeVproton beam line. Each pad had a sensitive areaof 1:5 1:5mm 2 : The sensor thickness was400 mm. Two movable scintillators were used fortriggering single protons for the CCE measurements.We used a liquid nitrogen continuous flowcryostat. The sample holder was kept in anultrahigh vacuum chamber and was connected toa cold copper finger. The sample was then cooledby thermal conduction. The cryostat had two
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