Trace alpha-particle detection system for water networks: from direct detection in liquid phase to element identification J. De Sanoit, M. Pomorski, P. Bergonzo 1
We want CONTXT In emergency situations : how to identify and probe water contamination from alpha emitters? A reliable and reusable device Cleanable nabling spectroscopy at high resolution Fast (max a few 10s of minutes) nabling quantification of actinides Available for water, but also drinks Low energy, a field trial device. Several actinides are solely alpha emiters APPROACH lectrochemistry on top of a silicon detector!"#$%&'&())&*+ 51A*6B0. </-=3&20.6* C205+/ &!&/52A1?0. 76&.89.,25,0&'&:))&;+ >?&'&@))&*+,-&./01,2-+0,23&1456* Patented WO 2012045872 (2010) 2
* 1/ Preconcentration of actinides 2/ Counting time.! Typ 30Bq/l detected in 10 min Devices BDD diamond is grown on Si from a Canberra PIPS '""" *!#& 1) The detection limits only depends from the acquisition time Identification of the traces possible Cleaning of the sensor is automatic at the end of the measurement ( 9-;7!""" &""" *+,-*./*012345* *+,-*.6789:876/!'@ A-!## () "!"" #"" $"" %"" <-)6=9*./*>,:,?! Thank you for your attention Little time to discuss signal issues in polyx 3
Probing the transient response of poly-x diamond to improve the stability of diamond devices under pulsed periodic excitation. P. Bergonzo, H. Hamrita, D. Tromson, C. Descamps, C. Mer, M. Nesladek N. Tranchant, F. Plion CA, LIST, Centre d tudes de Saclay, Gif-sur-Yvette, France Requirements - OK SC CVD should be close to ideal, (but unfortunately not defect free!) - But! what about large area availability very thin layers e.g. for alpha detection or membranes for synchrotrons cost Autonomy for production! Still a chance for polyx materials?! ssentially very uniform over large areas! 4
The role of defects " CVD materials, as well as high quality IIa type diamonds inherently exhibit defect levels TSC (A) 0.01 1-4 1-6 1-8 1-10 Natural SA1 Natural A03 CVD-polyX TL (A) 0.01 1-4 1-6 1-8 1-10 1-12 700650600550 500 450 400 350 300 Temperature (K) 1-12 The role of defects " CVD materials, as well as high quality IIa type diamonds inherently exhibit defect levels Lot of literature available! Impurities, Dislocations, Vacancies etc - of interest for TL dosimetry - stable at RT TSC (A) 0.01 1-4 1-6 Natural SA1 Natural A03 CVD-polyX TL (A) 0.01 1-4 1-6 - highly detrimental for IC - # unstability 1-8 1-10 1-12 700650600550 500 450 400 350 300 Temperature (K) 1-8 1-10 1-12 5
1,8 polyx materials characteristics xperiments performed under 6 MeV photon beams (medical accelerator) Comparing a diamond device to a reference gas ionisation chamber Typ. dose rate is = 3Gy/min NetCurrent (na) 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0 100 200 300 400 Time (s) Ref gas IC CVD @ 50V Basics of signal formation X, β, γ 6
X, β, γ X, β, γ 7
X, β, γ =0 8
X, β, γ Shallow traps (for holes)! Progressively emptied 9
X, β, γ X, β, γ +Q Shallow traps (for holes)! Progressively emptied 10
=0 Q +Q Shallow traps (for holes)! Progressively emptied X, β, γ Q Q +Q Shallow traps (for holes)! Progressively emptied 11
=0 Q Q +Q Shallow traps (for holes)! Progressively emptied =0 Q Shallow traps (for holes)! Progressively emptied 12
X, β, γ V-V Q "overshoot" Q +Q Shallow traps (for holes)! Progressively emptied Same discussion on electrons! =0 -Q Q Q +Q Shallow traps (for holes)! Progressively emptied 13
It comes : - The ON state is related to an equilibrium between carrier trapping and de-trapping : - Sensitivity is strongly affected by transiting charges! Thus to fluency! - quilibrium also varies with dose levels! Non linearities! - Stability is strongly varying with the device temperature :! This is one way to improve the signal stability : work at temperatures at which shallow levels are emptied.! OK but not always applicable Illustrative case Probing the response under a medical accelerator : - 6 MeV photon beam - Polycrystalline diamond (very defective!) - Dose rate is 3Gy/min 14
xplaining the poor device characteristics? xperiments performed under 6 MeV photon beams. Comparing a diamond device to a reference gas ionisation chamber Typ. dose rate is = 3Gy/min 1,8 NetCurrent (na) 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0 100 200 300 400 Time (s) Ref gas IC CVD @ 50V And sub-linearities? Q is only created by the carriers transiting through the device! Therefore the greater the signal the greater Q Normalised response Photocurrent a gas device (a.u.) 1.0 350 Fluency (in monitor units) 300 250 200 0.5 150 100 50 400 200 100 50 0 0.0 0 50 100 150 200 250 300 350 400 450 0 600 1200 Fluency (Monitor Units) Time (s)! The lower the fluency, the lower the signal :! the lower the trapped charge,! higher signals after stabilisation ( Q vanishes) Q +Q $ 15
ffect of the field At low fields, the overshoot is predominant, and vanishes at high fields NetCurrent (na) 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 IC CVDA2-50V 50V NetCurrent (na) 4,0 3,5 3,0 2,5 2,0 1,5 1,0 0,5 100V IC CVDA2-100V 0,0 0,0-0,2 0 200 400 600 800 1000 Time (s) -0,5 0 200 400 600 800 1000 1200 Time (s) 16 NetCurrent (na) 6 3 IC CVDA2-200V 200V Current (na) 14 12 10 8 6 4 800V Injection! 0 0 30 60 Time (s) 2 0 0 5 10 15 20 25 Time (s) X, β, γ e- h+! Photoconductive gain!! Blocking contact needed See A. Rose, Concepts in Photoconductivity (1963) 16
Response to a pulsed excitation Tests on the SAPHIR accelerator (Saclay) Pulsed X-rays, 17MV accelerator Pulses are 2µs long, at 25 Hz Bias Tee PA Multi Channel Analyser Diamond Detector Linac Pulsed height spectra under DC bias (= Histograms of the voltage pulses ) 80 1224 Bias is 0.5 V/µm Number of events 60 40 20 1224 1224 1224 1217 1213 1168 NetCurrent (na) 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 Ref gas IC CVD @ 50V 0 100 200 300 400 Time (s) 980 0 0 500 1000 1500 2000 Channel number ach 5 sec 17
Pulsed height spectra under DC bias (= Histograms of the voltage pulses ) 80 1224 priming Bias is 0.5 V/µm Number of events 60 40 20 1224 1224 1224 1217 1213 1168 NetCurrent (na) 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 Ref gas IC CVD @ 50V 0 100 200 300 400 Time (s) 980 0 0 500 1000 1500 2000 Channel number ach 5 sec Pulsed height spectra under DC bias Bias is 1 V/µm 80 1727 Overshoot Number of events 60 40 20 1727 1744 1760 1777 1793 1808 NetCurrent (na) 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 Ref gas IC CVD @ 50V 0 100 200 300 400 Time (s) 1800 0 0 500 1000 1500 2000 Channel number ach 5 sec 18
mptying defects at 0V bias Pulsing the bias! Using one "lost" pulse to further empty traps Mode 1/2 mptying defects at 0V bias 1/2 Photocurrent (na) 5 0-5 1/1 DC [!] 1/2 @ 0V -20 0 20 Time (µs) 19
It comes :! Unstable effects are very commonly observed in polyx devices! The most determinant influence of shallow defect levels (typ 0.3 to 0.6 ev : the ones emptied close to RT)! They affect all materials! (from 18 to 113 defects/cm 3)! Solutions proposed : - Heat the device (Patent WO2006024661) - Use light to detrap shallow carriers between events - 0V-bias the device and irradiate between two events (most convincing) (Patent FR243507 D25440 AV)! Thank you for your attention (I stop now ) 20