Dosimetry of an x-ray tube and Irradiation results of a DEPFET - - PowerPoint PPT Presentation

Dosimetry of an x-ray tube and Irradiation results of a DEPFET matrix

Carried out at the University of Karlsruhe IMPRS Workshop

Motivation: The KEKB accelerator and…

• High luminosity collider
• Asymetric energy for

studying

• Measure CKM matrix

elements

• CP violation

Measuring device:

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B B S Y e e    

 

) 4 (

…the Belle II detector

• The two innermost layers
• f the vertex detector will

be consisted of DEPFET matrices

• Ionizing Radiation from:
• Beam-Gas Interaction
• Synchrotron radiation

will damage those matrices

• Ca. 1 Mrad/a (=10 kGy/a)
• Simulating of these various

sources with x-ray radiation

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What are DEPFETs?

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• Depend on sideward

depletion: Internal Gate

• Generated charges will be

stored in the Internal Gate

• Modulation of Drain-

Source current

• Clear operation necessary
• Beneath Gate

metallization: Layer of silicon dioxide Sensitive to ionizing radiation

Damage mechanism

• Generataion of

electron/hole pairs in the

• xide
• Mobility of holes is weak in

comparison to electrons

• Holes will be trapped near

the interface and stay there for O(h) till O(a)

• Also: Increase of interface

state density

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Dosimetry of the x-ray facility in Karlsruhe

• To quantify the damage

done by ionizing radiation Applied Dose is necessary

• Calibration was given, 

Recheck it  Problem Challenge started …

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Recipe for succsesfull Calibration

• X-ray source
• Detector

a) Fast readout, highly sensitive, good spectral response, low noise, superb amplifier b) If not (a), then find a compromise of (a)

• Energy spectrum would be good

a) Measure one b) Simulate one

Thanks to my colleague: Oksana Brovchenko

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• 1. Determine dose rate in silicon diode
• Dose rate measurement via

depleted diode

• Measuring of reverse-current
• X-ray photons generate

electron/hole-pairs in Si- Bulk

– Every charge carrier pair represents an energy of 3.6 eV – With the x-ray generated current one gets the deposited power, with the mass of the diode  dose rate in Si

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• 2. Making use of the spectrum
• Spectrum of tungsten anode (including a 0.4 mm

Be filtering, black)

• Generate via absorption function of Zr a new

transmitted spectrum (red)

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• 3. Dose in SiO2
• Dose measurement

(diode)  power of spectrum (Si, blue) is known

• Original spectrum

(black)  Filter (Zr, red) Absorption in SiO2 (green)

• Final dose is

determined

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New dose rates

• Every dose rate matches to a specific set of

parameters, let’s assume

• U=60 kV (max. tube voltage)
• I=33 mA (max. tube current)
• Distance is 155 mm ( )
• Dose rate in silicon (300 µm)
• Dose rate in silicon dioxide (180 nm)

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New dose rates

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1 r D  

s Gy D

µm mm mA kv Si

305 , |

300 , 155 , 33 , 60

  s Gy D

nm mm mA kV SiO

239 , |

180 , 155 , 33 , 60

2

 

Back to the DEPFET: Setup and DAQ

• Study with a 6x16 Minimatrix
• Important contacts on PCB 

easy accessibility

• Drain contact needed probe

needle

• Several irradiation and

measurement steps

• Readout duration of input

characteristic of all 96 pixels

• ~ 6…7h

– min. 4 days of room temperature annealing

• DAQ via LabVIEW: Sweep of

Gate voltage, Drain current is measured

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Results of input characteristics √IDS(UGS)

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Change of threshold voltage

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Change of gain gm

• Input characteristic curve

fitted with

– I=aU²+bU+c – Gain gm=mU+b via dI/dU m=2a no numeric deviation – Gain evaluated at Drain current = 50 µA

• Maybe effect is part of

setup and readout process needs to be rechecked to find out if effect still

• ccurs

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Outlook & Conclusions

• Dosimetry

– A good agreement between simulated spectrum and dose measurement with silicon diode has been found – Further investigations are under way:

• Spectrum has been measured (matches fine to the simulated one, except low energy photons

and L-Lines of tungsten)

• Dosimetry with RadFETs have been done
• DEPFET

– Shift of threshold voltage matches to previous single pixel measurements (strongly dependent on gate voltage) – Spreading of threshold voltage after irradiation increased

• threshold voltage shift ∆ = ±0,12 V

unirradiated

• threshold voltage shift ∆ = ± 0,20 V

at 142 krad (1.42 kGy)

• threshold voltage shift ∆ = ±0,22 V

at 2.37 Mrad (23.65 kGy)

– Impact on Belle II

• Steering chips (Switcher) won‘t have problems with the spreading
• Change of gm interferes on gq (via gate oxide capacitance). Variations in Drain-Source current

may affect the current readout chip.

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Backup Slides

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Change of threshold voltage vs. Dose

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