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

Continued…

• Activation of cooling water • Radiation protection

– Shielding – …

• Monitoring • Safety precautions

– Access restrictions – Interlocks – …

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

Dose Effect or limit 0.001 Sv/a Limit for dose from food (long term) 0.002 Sv Typical dose from Tchernobyl in Finland during 50 years

0.004 Sv/a Typical dose level in Finland. In some areas radon may increase this much

0.005 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

40 – 60 Sv Single dose: Nervous system and digestive organs will be damaged quickly

• Absorbed dose – D: 1 gray =1 Gy = 1 J/kg

• Absorbed dose rate – : Gy/s

• Dose equivalent – – Quality factor Q – Other biological effects: N (=1) – H: 1 Sievert = 1 Sv

DQNH ⋅⋅=

Units

D

Radiation Effective quality factor

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

10

Fast neutrons 20

Alpha particles and other particles, whose charge is larger than 1e (or unknown) and whose energy is unknown

20

• Dose equivalent rate

• Effective dose equivalent – – wT weight for a specific tissue

( )hSv

dtdHH =

TTE HwH ⋅=

Primary beam • Proton beam

•Helium

Chinese hamster ovary (CHO) cells

Gamma dose

Primary beam

• Never apply to living organisms – Except by purpose

• Radiation therapy • Sterilization • …

Secondary radiation

• Neutrons and gammas from the beam hitting material – Dose rate decreases immediately when the

beam is switched off

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. 63Cu + n = 64Cu (12.7 h) – Fe isotopes from Fe

“Targets” for thermal neutrons

• 63Cu 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

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 22Na from

aluminum

Shielding • As close to the source as possible • Distance helps: 1/r2

• 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 (10B, Fe, plastics,…)

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

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

Permissions

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