Radiation at accelerator laboratories

Transcription

1 Radiation at accelerator laboratories Prompt radiation from the particle beam Beam induced radiation Neutrons Gammas Synchrotron radiation Radiation from activated material Activation of air Beam Neutrons

2 Continued Activation of cooling water Radiation protection Shielding Monitoring Safety precautions Access restrictions Interlocks

3 Regulations Radiation workers (class A) 20 msv/a (5-year floating average) 12.5 µsv/h (1600 h/a) 50 msv/a for one year At JYFL: general limit is 0.5 µsv/h Rooms with free access

4 Dose Effect or limit Sv/a Limit for dose from food (long term) Sv Typical dose from Tchernobyl in Finland during 50 years Sv/a Typical dose level in Finland. In some areas radon may increase this much Sv/a Limit for food (short term) 0.02 Sv/a Dose limit for a radiation worker (5-year floating average) 0.05 Sv/a Maximum dose for one year (radiation worker) 1.2 Sv Single dose may give radiation illness symptoms. Long term dose: No symptoms 3 4 Sv Single dose: 50 % die within a few weeks, if extensive medical aid is not given Sv Single dose: Nervous system and digestive organs will be damaged quickly

5 Units Absorbed dose D: 1 gray =1 Gy = 1 J/kg Absorbed dose rate D : Gy/s Dose equivalent H = N Q D Quality factor Q Other biological effects: N (=1) H: 1 Sievert = 1 Sv

6 Radiation X-ray- and gamma radiation 1 Beta and electron radiation 1 Thermal neutrons 2.3 Protons and other ions, whose charge is 1e and rest mass larger than 1u and whose energy is not known Fast neutrons 20 Effective quality factor 10 Alpha particles and other particles, whose charge is larger than 1e (or unknown) and whose energy is unknown 20

7 Dose equivalent rate H = dh dt ( Sv ) h Effective dose equivalent H E = wt HT w T weight for a specific tissue

8 Proton beam Primary beam

9 Helium

10 Gamma dose Chinese hamster ovary (CHO) cells

11 Primary beam Never apply to living organisms Except by purpose Radiation therapy Sterilization

12 Secondary radiation Neutrons and gammas from the beam hitting material Dose rate decreases immediately when the beam is switched off

13 Residual radiation Beam hits the accelerator, beam tube or other devices Protons produce most activities (p,xn) Co-isotopes from Fe Secondary neutrons activate material through neutron capture E.g. 63 Cu + n = 64 Cu (12.7 h) Fe isotopes from Fe

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15 Targets for thermal neutrons 63 Cu in natural Copper Sodium in concrete Argon in air Zinc in copper Manganese and cobalt in iron or steel Antimony in lead Trace quantities of manganese, cobalt, cesium and europium in earth and concrete Possibly tungsten-186 in natural tungsten

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17 Other aspects Minimize the amount of material that can be activated E.g. inside the accelerator Ta collimator in the spiral inflector (JYFL) Proper choice of materials Depends on accelerated ions and their energies Cross-sections for nuclear reactions E.g. 30 Mev (or more) protons induce 22 Na from aluminum

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19 Shielding As close to the source as possible Distance helps: 1/r 2 Use chicanes (corridors with corners/bends) Lower pressure in rooms where air may be activated (ventilation) Use separate water cooling circuit for water that can be activated Proper choice of materials and the order of materials Thermalization of fast neutrons first Material choice depends on neutron energy ( 10 B, Fe, plastics, )

20 Shielding Assume the worst case for dimensioning the wall, floor and ceiling thickness/material The whole beam is stopped Which beam? Light ion High energy High intensity The allowed dose rate (µsv/h) outside the radiating room sets limits for the radiation attenuation

21 Monitoring Already according to the Safety License you have to monitor radiation levels (dose rates) Gamma and neutron monitoring Alarms Feed-backs Interlocks Personal dose monitoring (class A and B radiation workers) 4 week intervals (A) 12 week intervals (B) For work with a clear risk of dose Plan the work well Measure the dose on-line

22 Permissions You always need a Safety License by STUK/Radiation and Nuclear Safety Authority for your operation if it may produce radiation!