low energy x-ray spectroscopic applications of very large area silicon drift detector

Andrea Vacchi Infn Trieste

Low energy X-Ray SpectroscopicLow energy X-Ray SpectroscopicApplications of Very Large AreaApplications of Very Large Area

Silicon Drift DetectorSilicon Drift Detector

ABSTRACTABSTRACT

The spectroscopic performances of a very large sensitive area SDD (about 53 cm2) at the

energies of 241Am and 55Fe sources shows that the goal of a high energy resolution

combined with large sensitive areas can be achieved. This result has moved a large

interest and the present project proposes a further development of this detectors in the

direction of high performances x ray spectroscopy.


INFN Trieste laboratories Silicon Drift Detectors from device modeling to mass production - practical experience

INFN Trieste laboratories Silicon Drift Detectors from device modeling to mass production - practical experience

From specs to detector

–HV divider & stability

–Injectors & drift speed variations

–NTD fluctuations

–Safe design & yield

– Radiation damage

–Mass production

From specs to detector

–HV divider & stability

–Injectors & drift speed variations

–NTD fluctuations

–Safe design & yield

– Radiation damage

–Mass production

SDD first proposed by E. Gatti P. Rehak 1984


Silicon drift detectors (SDD)

• due to their collection electrode geometry, SDD have excellent noise performance and are well suited for low-energy X-ray spectroscopy applications.

n-side

p-side

divider

anodes

oxide (SiO2)

metal

depletionpotential-U

driftpotential

anode drift distance

By means of a series of cathodes (biased with a scaling potential) a highly-linear drift field is established from the center of the detector towards two arrays of anodes placed at opposite sides of the SDD.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.


This design of this SDD originates from the primary need to perform unambiguous measurement of

the impact point of charged particles along both the X and Y axis in presence of high multiplicity,

together with a large sensitive area.

The ionizing radiation create electron-hole pairs: the electron

cloud is focused at the center of the silicon bulk and drifts to

the anodes, while the holes are collected by the cathodes.

When the electrons reach the end of the drift zone, they are

abruptly pushed towards the anodes by an electrode (placed

beneath them) in order to minimize the collecting time.

The X coordinate is measured along the anodes where the

charge is collected, the Y coordinate corresponds to the

travel time of the electrons.anode

electrons

The silicon drift detector (SDD)The silicon drift detector (SDD)

Simulation of the potential distribution in the SDD

Ionising radiation


256 readout anodes256 readout anodes


driftdrift

Wafer type:Wafer type: 5” Neutron Transmutation Doped

<111> 3 k.cm, 300 m thickArea:Area:

sensitive: 7.02 7.53 cm2, divided into two drift regions

total: 7.25 8.76 cm2, (ratio = 0.83)

Each drift region:Each drift region: 35 mm long 291 cathodes driven by built-in voltage

divider 256 anodes – 294 m pitch 3 rows of 33 MOS charge injectors (for

the drift velocity calibration)Guard regions:Guard regions:

independent built-in voltage dividers

Typical operating parameters:Typical operating parameters: Drift bias voltage: -2.4 kV, 8V/cathode E=670V/cm Maximum drift time : 4.3 s, vd =8 m/ns

Power dissipation on board:Power dissipation on board: 0.95 W0.95 W equivalent Rtot of all drift + guard dividers 4781 kΩ total current in all dividers ~0.40 mA

The SDD was selected to equip the 3rd and 4th layers of the Inner Tracking System (ITS) of the ALICE experiment at LHC. The finger print of this silicon drift detector is:

256 readout anodes256 readout anodes 256 readout anodes256 readout anodes

driftdrift



driftdrift



driftdrift



guard cathodes (32 guard cathodes (32 m pitch)m pitch)

292 drift cathodes (120 292 drift cathodes (120 m pitch)m pitch)

implanted HV voltage dividersimplanted HV voltage dividers

256 collection anodes (294 256 collection anodes (294 m pitch)m pitch)

collection zone close-up

injector lines close-up

injector line bonding pad injector line bonding pad

MOS injector (every 8MOS injector (every 8thth anode) anode)

Detector design featuresintegrated dividers generate the biases for all of the drift cathodes and the guard region which used to scale down gradually the negative high-voltage to ground at the edge of the SDD.sets of MOS injectors placed at different distances from the anodes are used for the drift velocity calibration.


Drift distance (mm)

Res

olu

tion

(m

) anode axis (Z)

drift-time axis (R-)

About 600 detectors have been produced using a dedicated double sided process.

Due to the redundant design and production technology the yield was grater than 60% .

260 detectors (84 in layer 3 and 176 in layer 4) have been mounted on the barrel of the ALICE-ITS

now in the commissioning phase acquiring cosmic muons data.

Beam-test dataBeam-test data

reconstruct the impact position of the ionizing particles with a resolution

better than 30µm in both X and Y axis.


suitable for low-energy X-ray spectroscopy applications:

+ anode capacitance is about 50 fF,

+ the leakage current at the anode measured at room temperature is very low

This allows a very-low noise contribution from the front-end electronics. Considering its sensitive area, more than an order of magnitude larger than that of the standard SDDs, this detector could open the way to application areas of the X-ray spectroscopy that require wide surface coverage.

Each anode sees a 10 mm2 surface - the reported current values correspond to 4 anodes read-out together


The measurement setupThe measurement setup Climatic chamber

The detector and the front-end electronics are placed in a climatic chamber able to control the temperature.

This also acts as shield detector and electronics from irradiated noise.

Bias board


The last 6 drift cathodes near the anodes are decoupled directly on the detector board to minimize noise coupling.

An aluminum block is used to collimate the radioactive source. This minimizes the partial charge collection.

The collimator, 4cm high, has a thin slit (about 0.5mm) that allows to illuminate almost the whole drift length of a set of anodes. It sits 5mm above the detector.

Bias board

RADIOACTIVESOURCE

COLLIMATOR

BIAS CONNECTOR

TO PREAMPLIFIERS

JFETs

Detector board

A low capacitance JFET (CGS = 0.4pF) is connected to a

group of 4 anodes (CDet,tot = 0.2pF) and read out by a

multi-channel preamplifier board.

The JFET integrates a feedback capacitor CF = 50fF,

and a transistor to discharge the capacitor realizing the

reset of the preamplifier.

The total capacitance measured at the input of the

preamplifier (the gate of the JFET) is about 1pF


560 eV FWHM @ 13.95keV 4%

Co

un

ts

ADC channels

241Am spectrum measured at +16°C with a trapezoidal shaping having a rise/fall time tR = 0.4µs and a flat top of 0.2µs. The high voltage bias of the detector is -1600V. The leakage current of the four anodes is ~ 5pA


Co

un

ts

ADC channels

386 eV FWHM 7%

Mn kα

Mn kβ

55Fe spectrum measured at 0°C with a trapezoidal shaping having a rise/fall time tR = 1.2µs and a flat top of 0.2µs. The high voltage bias of the detector is -1300V. The leakage current of the four anodes is ~ 1pA. The red line represents the fit function used to determine the energy resolution.


Next step on board first stageDetail fet F2 for




ConcludingConcluding

Largest drift detectorActive area: 52 cm2Anode pitch: 294µm512 anodes on two rowsPosition resolution: <50µmMOS injectors for calibration

Participants:- Isaf Roma

Marco Feroci - Ictp Trieste

Dipak Debnath

Debashis Bhowmick Ritabrata Sarkar Claudio Tuniz

Andres Cicuttin Mliz Crespo

- Infn TriesteGianluigi ZampaAndrea VacchiAlexandre RashevskyValter BonviciniErik Vallazza

- Iasf Bologna Labanti

- Infn BolognaGiuseppe

Baldazzi- PoliMilano

Longoni-

Targets: High-resolution low noise large area silicon detectors based on the Silicon Drift Detector principle.

o Low energy X-ray spectroscopyo Radioactivity contamination detectorso Atomic physics studieso X - ray astronomyo Medical applications

low energy x-ray spectroscopic applications of very large area silicon drift detector

Documents

drift zone

drift regionstotal

pitch292 drift cathodes

drift bias voltage

vcmmaximum drift time

drift guard dividers

silicon drift detectors

arrays of anodes