Measurement system for detection of Radon in building

Transcription

1 Measurement system for detection of Radon in building

2 Outline Properties of Radon Detection of ionised particles Measurement system for radon

3 Properties of Radon Radon is a gaseous isotope ( 222 Rn) with a half-life of ~4 days Its relative long decay time makes it possible to be released from the ground and contaminate the air in houses. Radon can also be released from building material, i.e. blue light concrete or gas concrete made of alum shale Inhaled Radon in lungs decomposes into 218 Po and 214 Po, which decay products, can cause lung-cancer. It is especially dangerous for smoker since radioactive particles can be stuck to particles in the smoke. Highest allowed concentration of Radon in buildings is 200 Bq/m 3 (Sweden). WHO have suggested a decreased value to 100 Bq/m 3 from the international standard of 1000 Bq/m 3 Bq= one decay/s

4 Properties of Radon 222 Rn (not charged) 5.49 MeV 218 Po (charged) 6.003MeV 214 Po (charged) 7.686MeV

5 Properties of Radon Correspond to 135 packs of cigarettes a day

6 Properties of Radon

7 Properties of Radon Ra C 214 Po Ra A 218 Po

8 Detection of ionised particles

9 Detection of ionised particles How can a simple detector look like! What minimum requirement is needed? α Chamber A Elec. 1 Filled with gas, Ar, He + U Elec 2

10 Detection of ionised particles Problem with gas filled detector Low density, long track to stop the irradiation Sealed chamber Detector window may not be transparent for irradiation To restore the detector only refill of gas is necessary

11 Detection of ionised particles So to improve, we need a detector material with a high density, thin detector window and mounted if necessary in a non sealed capsule The answer to this request, is a solid material! The solid material should only conduct when irradiated! A metallic solid conduct all the time! An insulator does of course not conduct, when irradiated, the conducting properties does not change. It is still an non conducting material, i.e. the produced charges has no mobility

12 Detection of ionised particles Directly after collision After de-excitation process A good candidate is therefore the semiconductor, which's conductivity can change during irradiation. The produced charges in the material must accordingly have a good mobility. This means that, electrons must be free to move in the conducting band, holes must be free to move in the valence band.

13 Detection of ionised particles Produce to high leakage current, which result in current induced noise! Solution! Blocking Contact

14 Detection of ionised particles PN-diod (a,b,c) Schottky contact (d,e,f) Hetero-structure (two different semiconductor)

15 Detection of ionised particles Planar processing of detectors Passivated, silicon planar diode detector Almost operated with reverse bias voltage, (except photodiodes normally operated with zero bias voltage) J. Kemmer, Nucl. Instr. and Meth. 226, 45, (1984)

16 Detection of ionised particles Energy resolution is necessary for measurement of ionised particles. High internal electric field over depleted detector Low detector capacitance High SNR Low ohmic contacts Low series-noise Low leakage current Low leakage current is necessary for high resolution, but the recombination life time must also be high (processing depended) Thin detector window (for low energetic particle) Thin metallic contact Shallow contact doping The detector must have thickness enough to stop the particle (for high energetic particle)

17 Detection of ionised particles

18 Counts Detection of ionised particles tunn U232 framsida kev/ch FWHM(8.785 MeV)= 31.6 kev Channel A

19 Measurement system for radon reset Particle Sensor Charge Sensitive Amplifier Shaping Amplifier Amplitude measurement u.proc. Presentation Storage of data Chamber 200V Temp. Moisture

20 Measurement system for radon Particle Sensor Chamber 200V Only radon is passing through the filter Radon daughters are charged (positive), which attracted them to the negative potential of the detector

21 Measurement system for radon Charge Sensitive Amplifier

22 Measurement system for radon R C Shaping Amplifier C Gain R More information in Article

23 Measurement system for radon Amplitude measurement Fast AD converter Peak detector (A particle of 5 MeV should give a signal of 1V) Reset function must be included, discharge the capacitor C2 The reset is done by the u-proc.

24 Measurement system for radon Presentation: Radon konc/m 3 u.proc. Presentation Storage of data Counts (particles) Measured time Temperature Temp. Moisture Moisture Energy (MeV) File should contain: particle number,energy, time, temp, moisture. The communication with PC is done by using USB or wireless interface The program language used is processing

25 Project task 3 groups, regular meeting, one/ week Individual simulation of analog electronics and peak detector Individual report on embedded programming Group report regarding PC-program Demonstration and oral presentation of the project 1. Connection of the radon platform to mbed. Conversion from 5V to 3.3V Handling reset fkn and counting of pulses Fkn check of high voltage and detector bias

26 Project task 2. Temp and moisture sensor (SHT10) Connection of sensor to digital input Programming to read temp and moisture from sensor 3. Calculation and presentation of data Calculation of radon content based on pulses/ time unit Presentation of data on LCD-display Logging and presentation of data on PC as fkn of time *4. Controlling of ventilation to keep the radon content on a safe level (for grade A)

27 Measurement system for radon