Quartz resonator based low-energy ionizing radiation detection
J.-M Friedt1,2, C. Mavon1, S. Ballandras2, V. Blondeau-Patissier2, M. Fromm1
(1) Laboratoire de Microanalyses Nucl´eaires A. Chambaudet UMR CEA E4, Univ. de Franche-Comte, Besan¸con, FRANCE (2) Institut FEMTO-ST UMR CNRS 6174, D´epartement LPMO, 32 avenue de l’Observatoire, Besan¸con, FRANCE
Objectives
• Analyze the influence of soft X-rays & electronsonquartz acoustic resonators
• Analyze the usability of quartz oscillators forquantitativeionizing radiationdose measurements
Cold-cathode X-ray generator
• a high voltage generator (0-12 kV, 0-10 mA) accelerates electrons and creates a plasma (pressure=5.10−2 mbars)
• the accelerated electrons hit a grounded 11µm Al electrode
• Bremsstrahlung/ionization X-rays leak through the Al sheet (spectrum=
Bremsstrahlung+absorption of low energy Xrays)
• peak energy defined by the energy of the incoming electrons (=voltage)
• these electrons propagate either in vacuum (5.10−2 mbars) or air, but strong absorption in air due to low energy
• calibration of the X-ray generated dose using an ionization chamber (re- quires the area above the target to be at room pressure)
• alternative to X-rays: accelerated electrons as ionizing radiations (re- quires the area above the target to be at primary vacuum pressure '3−5×10−2 torrs).
0 50 100 150 200 250 300 350
0 1 2 3 4 5 6
number of hits
E (keV)
20 um thick Al target, air, Amptek XR100 detector close to the target, with RTD
A
µ target Al 11 m
cathod chamber ionization d ( m)µ
5.0 1.02.0 10.0
3.01.4 263.036.0 E (kV)
glass glass
6−1E−3 mbars electrons X rays colimator (brass) Atmospheric pressure
VHV vacuumprimary
(data: NIST) in glass depth (1/e incoming)X ray penetration
µ In all cases (E<10 keV) the electron mean free path in glass is less than 1 m
Schematic of the cold-cathode Xray generator and measured X-ray spec- trum: the peak is the result of absorption of the low energy X rays by the Al target and air, and the upper limit is defined by the energy of the incoming electrons.
Ionizing radiation detection
0 200 400 600 800 1000 1200 1400 1600 1800 2000
0 2 4 6
x 104
cold cathod generator params.
pressure current voltage
0 200 400 600 800 1000 1200 1400 1600 1800 2000
1.1 1.2 1.3 1.4 1.5
SAW phase
0 200 400 600 800 1000 1200 1400 1600 1800 2000
2200 2300 2400 2500 2600 2700
SAW resistor (∝T)
time (s)
V=cst V=cst
I=cst V too low
φ insensitive to pressure
Experimental measurement of all relevant parameters of the electron gener- ator (cathode voltage and current, vacuum chamber pressure), the phase at constant frequency ('125 MHz) of the Love-mode delay line and AT-quartz substrate temperature through a thermistor deposited next to the interdigi- tated transducers.
• monitoring of the phase of a Love-mode SAW (2.2µm SiO2 on AT-cut quartz), λ = 40 µm, centrale frequency '125 MHz, as well as a thin (180 nm-think) Al strip on the quartz substrate acting as thermistor (3.5 Ω/K, 2280 Ω at room temperature).
• no visible effect of soft X-rays on the behaviour of acoustic sensors: in- sufficient dose ?
• strong temperature increase of our sensor (in-situ measurement), up to 90 K, upon irradiation by accelerated electrons
• threshold voltage below which no effect of electron irradiation is observed (first↓): absorption of electrons by remaining air in chamber ?
• heating correlated with strong phase shift during electron irradiation (2nd and 4th↓)
• little effect of pressure on the behaviour of the delay line (3rd↓)
• observation compatible with STW (505 MHz) and BAW (12 MHz) res- onators
Conclusion and perspectives
• substrate heating has been identified as the cause of phase/frequency shift of our delay line/resonator used as radiation detector
• quartz based resonators/delay lines are thus used as sensitive calorimeters (heating associated with energy transfer from ionizing radiation)
• this mechanism can be enhanced to improve the sensitiviry by using Z-cut quartz as substrate (strong temperature coefficient)
• accurate calibration temperature dependence of the phase/frequency is needed to separate heating from other possible disturbances associated with ionizing radiations
• wireless interrogation is to be enhanced to be compatible with range/medium
required fortherapeuticuses of ionizing radiations Experimental setup of the delay-line irradiated by accelerated electrons
We are greatful to J.-M Bordy (BNM-LNHB, Fontenay-aux-Roses, France) for fruitful discussions.
