Characterization of primed state of CVD diamond by light and alpha particles C. Manfredotti Experimental Physics Department University of Torino INFN- Sezione di Torino

Previous alpha particle measurements

Investigation of the primed state with light: ALPHA PARTICLE DETECTION Experimental set up Erasing previous sample history Priming Time sequence of measurements

Alpha particle detection can discriminate between hole and electron contribution to Charge Collection Efficiency (CCE) by changing bias polarity and by profiting of the short range of 5.5 MeV alpha particles in diamond (13  m)  particle Bias voltage (positive) and signal Electrons cover the longest part of the sample thickness and give the largest contribution to charge signal. e-e- Ramo’s theorem: CCE (for one carrier)=  q/q =  x/L L xx Holes almost do not contribute: the range of a particle is too short. h+h+ + -

Charge collection efficiency as a function of priming and of light sensitization X-ray priming enhances holes response Green, red and infrared light are not able to affect the primed state Remember TL? Blue light bleaching is the same and affects holes Blue light enhances electron response and lowers hole response

IBIC ( Ion Beam Induced Charge) measurements Effect of light on maps of Charge Collection Efficiency ( CCE )

IBIC maps on De Beers detector primed ( 10 Gy) Bias – 600 VBias V electrons holes Response is still better for holes, but counting rate is very low (counting efficiency is 5 – 10%) and detector polarizes completely (no counts) after 1 – 2 hours of irradiation

IBIC maps after priming and during blue light (450 nm) illumination Bias – 600 VBias V Counting efficiency improves dramatically, particularly for electrons Response and uniformity for electrons are much better than for holes. Charge collection distance (CCD) for electrons can reach  m !

Effects of priming and light ( 400 nm ) on alpha spectra Time behaviour of alpha spectra Discrimination between electrons and holes Preliminary data

Alpha spectra – Different primings Holes

Alpha spectra – Electrons Priming – Priming + light From these spectra, time evolution of both centroid and total counts ( integral ) has been derived

Decay of primed state - Short term Priming and light Holes

Decay of primed state –Short term Holes – Low doses priming

Decay of primed state – Short term Electrons

Decay of primed state – Different amounts of priming Short term Electrons

Decay of primed state – Long term Electrons and holes

Effect of white light on electron collection Tungsten lamp, no interf. filter

Conclusions Only blue light ( at least below 500 nm ) affects the primed state Holes : blue light reduces the average CCE of the primed state Electrons : blue light improves the average CCE After a short transient ( 1 hr ) both X-ray primed state hole contribution to CCE and blue light continuously primed state electron contribution to CCE seem stable in time ( 30 hr ) Blue light priming is different from sample to sample and it is independent of amount of previous X-ray priming – even better with no previous priming Hole reponse ( PC, alpha spectra ) is sensitive to low amounts of priming ( and it is linear with doses up to few tens of mGy )

Priming effects on photoconductivity PPC Persistent PhotoConductivity

Dependence of PC on dose priming dose Increasing photon energy Deeper levels are involved BGPC increases More detrapping Increasing number of incident photons BGPC decreases Priming fills hole traps. After annealing at 360°C (restoring of the unprimed state), the band at 2.4 eV disappears. It disappears also if the BGPC measurements are carried out starting from higher energy values. Why a maximum at 2.4 eV? Sample A

The sample response depends on its “history”. Annealing: heating at 360 °C for 60 seconds for three times, performed in order to restore the starting conditions. PC peak position and height depend on time and on illumination intensity Factors affecting the BGPC value:  Surface exposed to  -rays (growth or substrate).  Undesired exposure to room light.  Time elapsed before starting the measurements.  Total number of incident photons (exposure time at each wavelength).

Transient photoconductivity of primed and unprimed states

Decay of the primed state under illumination For energies of the incident photons between 1.76 and 4.80 eV, the BGPC signal after the priming decreases with a decay that can be fitted using the expression: exp[(-t/τ) β ] with β < 1. Effect of the priming:  filling hole traps. Decay of the primed state:  optical detrapping of holes

Gain factor Ratio S primed / S virgin The photoconductive gain around eV with respect to unprimed case is very high, more than 300 in the case of sample B. This was observed also for other samples, with no evidence of dependence on their electronic quality.

Applications in dosimetry Advantages both large-area detectors or miniaturised detectors high sensitivity good spatial resolution Moreover, diamond is to be considered as a tissue equivalent material since its atomic number is close to the effective atomic number of soft tissue (5.92 for fat and 7.4 for muscle). Linearity range Diamond can be used as a passive solid state dosimeter for bio-medical applications. Its attractiveness essentially stems from its radiation hardness, chemical stability against all the body fluids and its absolute nontoxicity.