adiation protection at high energy proton accelerators cernradprot/index_htm_files/erice...

Marco Silari, CERN RP at High Energy Proton Accelerators 1

RADIATION PROTECTION AT HIGHENERGY PROTON ACCELERATORS

Marco Silari and Graham R. StevensonCERN

1211 Geneva 23, Switzerland

International School of Radiation Damage and Protection10th Course: Accelerator Radiation Protection


Summary of the presentation

• Characteristics of hadron cascades• Particularity of high-energy hadron accelerators

– accelerators, targets areas, experimental areas,superconducting RF cavities

• Prompt radiation• Muons• Potential exposure to secondary beams• Hazard of heavy ion beams• Radiation protection at future accelerators


Hadron cascade

• The way in which the radiological problems associated with a proton accelerator vary with energy depends on two parameters:– the multiplicity of the

production of secondary particles which increases as the proton energy increases, and

– the increase in average energy of these secondarieswhich makes them capable of producing further inelastic interactions.


Secondary particle production (1)

The fluence ofhadrons with energy greater than 40 MeV at 1 metre per proton interacting an a 5 cm long copper target at proton energies of 7 (*), 23 (+), 225 ( ) and 400 GeV (∇)



This increase in the fluence of secondaryhadrons will have as a direct consequence an increase in the induced radioactivity in an object installed close to a loss point such as an extraction septum, target or vacuum chamber for a given number of lost protons.

The length of the activated regions downstream of such an interaction point also increases dramatically with the energy of the proton beam.



FLUKA simulation of the star density distribution per interacting proton in a 10 cm radius iron cylinder, 0.5 cm thick, placed around a thin copper target struck by protons of different energies



FLUKA simulations of cascades in ironshowing contours of star density(10-3 stars cm-3) per interacting proton in a dump struck by protons of different energies⇒ This behaviour governs the thickness of lateral shielding required for proton beam-dumps


Lateral shielding requirementsLateral shield thickness in metres required to achieve 10 μSv h-1

alongside a beam dump for a proton beam intensity of 1012 s-1. N.B. 0.5 m of concrete must be added to all iron thicknesses.

Proton Energy Concrete (ρ = 2.4 g cm-3) Iron (ρ = 7.2 g cm-3)

3 GeV 6.56 2.78

10 GeV 7.02 2.97

30 GeV 7.44 3.15

100 GeV 7.91 3.35

300 GeV 8.34 3.53

1 TeV 8.81 3.72

(Data from Fassò et al., Shielding against high-energy radiation, Landolt-Börnstein,1990).


Energy dependence of hadronic activity (1)

Hadronic activity is e.g. the total number of stars produced in a cascade or the number of neutrons produced having energies between 1 and 10 MeV.

Let N(E) be one such measure of activity and consider the activity N(nE) produced by a hadron of energy nE, where n is a multiplier roughly identified with the average multiplicity of high-energysecondaries (charged and neutral) produced in the first collision. Unless it is a π0, a secondary with energy Ei produces a hadronicactivity N(Ei) , and

∑=i iENnEN )()(

)()1()( 0 EnNfnENπ

−≈

fraction of the energy lost to the hadronic sector through π0 in a single interaction


Energy dependence of hadronic activity (2)

mKEEN =)(

( )n

fm

ln)1/(1ln

1 0π−

=−

If n and fπ0 can be regarded as constants independent of energy, a solution to the iterative equation above is a power law:

with

In the energy range from several GeV to 1 TeV, fπ0 = 0.25 - 0.33 and n = 5 - 10.

n = 5, fπ0 = 0.25 ⇒ m = 0.82

fπ0 = 0.33 ⇒ m = 0.75. A suitable average value of m ≈ 0.83

n = 10, fπ0 = 0.25 ⇒ m = 0.87


Radiation areas in the SPS

H > 2 mSv/h

100 μSv/h < H < 2 mSv/h

7.5 μSv/h < H < 100 μSv/h

Peculiarities:• Spatial separation of problems• Induced activity includes a lot of spallation products ⇒ relevant for the production of radioactive waste• With increasing energy the extent of regions with an induced activity hazard increases


Classification of radiation areas AREA Dose rate limit (μSv/h) Consigne

Average Maximum

Nondesignated ≤ 0.15 ≤ 0.5 • No film badge required

• Public exposure < 1 mSv/year Supervised

≤ 2.5 ≤ 7.5 • No film badge required• Employees exposure < 1 mSv/year

Simplecontrolled ≤ 25 ≤ 100

• Film badge required• Employees exposure cannot exceed 15 mSv/year

Limited stay ≤ 2 mSv/h • Film badge and personal dosimeter required

• Work needs authorisation of RP or RSO High

radiation> 2mSv/h

but≤ 100 mSv/h

• Film badge and personal dosimeter required• Strict access control enforced• Access needs authorisation of RP or RSO

Prohibited

≥ 100 mSv/h• Access protected by machine interlocks• Access needs authorisation of division

leader, Medical Service and RP group• Access monitored by RP group


CERN area monitors

Several types of ionisation chambers(air-, hydrogen or argon-filled) and rem counters are used to monitor the radiation fields in the accelerator tunnels, in the experimental areas and in the environment.


CERN RP central data acquisition system

All installed radiation monitors can be read remotely. Data are stored in a database for future retrieval.

Monitor parameters such as alarm threshold can only be modified by authorised personnel.


Controlled access to accelerator areas (1)

Access to primary beam areas is supervised by the Accelerator Control Room

Access is granted via a film badge reader. Upon check that the person is authorised to access the area, the operator frees a key and gives access


Controlled access to accelerator areas (2)

Areas in the SPS can either be under closed, supervisedor free access


Ring survey in the SPS

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

200 204 208 212 216 220 224 228 232

Position

Dos

e eq

uiva

lent

rat

e (µ

Sv/

h)

ProtonsIons

LSS2

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

400 404 408 412 416 420 424 428 432

Position

Dose

equ

ival

ent r

ate

(µSv

/h)

ProtonsIons

LSS4

Horizontal dispersion in meters

-1

0

1

2

3

4

5

Position


Electron emission in superconducting cavities

HIGH INTENSITY, LOW ENERGY(~ 0.5 MeV) ELECTRONS

LOCATION OF MAXIMUM ELECTRIC FIELD (IRIS)

LOW INTENSITY, HIGH ENERGY ELECTRONS

50 cm


Stray radiation from SC RF cavities

0 5 10 15 200

20

40

60

80

100

120

140

Conditioning time (h)

Gam

ma

dose

rate

(mG

y/h)

B

0

2

4

6

8

10

Elec

tric

field

(MV

/m)

0 2 4 6 8-20

0

20

40

60

80

100

120

140

160 helium processing without helium processing

Gam

ma

dose

rate

(mG

y/h)

Electric field (MV/m)

Each cavity has its own “history” and the conditioning process can vary significantly from unit to unit, as does the intensity of thebremsstrahlung radiation

Sharp increase in the radiation emission when the electric field ⇒is raised above a given threshold


Induced radioactivity in LEP cavities

• The dose rate decreases by about a factor of 10 in 40 minutes, due to the decay of short-lived radionuclides, followed by a much slower decrease (another factor of 10 in about 48 hours).• Stainless steel

• Short-lived:50Cr(γ,n)49Cr (half-life 42.1 min), 54Fe(γ,n)53Fe (8.51 min), 54Fe(γ,n)53mFe (2.6 min), 92Mo(γ,n)91mMo (1.09 min) and 92Mo(γ,n)91Mo (15.49 min)• Long-lived: 48V, 51Cr, 52Mn, 54Mn, 56Ni, 57Ni, 56Co, 57Co, 58Co, 60Co, 88Y, 92mNb, 95Nb, 99Mo

• Copper:• Short-lived:63Cu(γ,n)62Cu (half-life 9.74 min) and 63Cu(γ,3n)60Cu (23.2 min)• Long-lived: 51Cr, 54Mn, 56Co, 57Co, 58Co, 60Co, 65Zn, 72Se, 75Se, 74As, 120Sb


Radiation fields around proton accelerators: neutrons

Neutron spectral fluence outside a 80 cm thick concrete shield and a 40 cm thick iron shield

NeutronsProtonsCharged particles

– muons (leptons, m=105 MeV, τ=2.2 10-6 s)

– protons

– ...


Neutrons outside shielding of high-energy proton accelerators

Fraction of ambient dose equivalent below a given energy, as a function of energy, for the neutron spectral fluences outside a 80 cm thick concrete shield (0) and a 40 cm thick iron shield (*)


Radiation fields around proton accelerators: muons (1)

Muons arise from the decay of pions and kaons, either in particle beams or in cascade induced by high energy hadrons. They can also be produced in high-energy hadron-nucleus interactions

Decay lengths from pions and kaons are 55.9 m and 7.51 m times the momentum (in GeV/c) of the parent, respectively

Muons are weakly interacting particles → they can only be stopped by “ranging them out”. Muons mainly lose energy by ionisation, as their cross-section for nuclear interaction is very low.

Usually muon shielding is only important at accelerators above 10 GeV. At lower energy the shielding necessary to reduce radiation levels arising from nuclear cascade processes is in excess of the ionisation range of muons that could contribute to the radiation problem.


Radiation fields around proton accelerators: muons (2)

Muons from pion decay have a momentum spectrum that extends from 57% of the momentum of the parent pion to the pion momentum itself. Secondary pion beams generally have dumps of longitudinal depth of 1-2 m Fe → decay muons will penetrate the dumps for pion beams with momentum > 2-3 GeV/c

Muon shielding is therefore limited to the forward direction. Typical thickness of hadron dumps at high energy proton accelerators is a few metres of iron

A beam of 107 pions per pulse with momentum of 20 GeV/c travelling over a distance of 50 m ⇒ ~ 5 x105 muons per pulse (5% of the parent beam) ⇒ for a pulse repetition period of 2 s, taking an approximate fluence to dose equivalent conversion factor equal to 40 fSv m2 and assuming that the muon beam is averaged over a typical area for the human torso of 700 cm2 ⇒ 500 µSv/h


Effect of straggling on the range of muons

Shield

Material

Momentum

(GeV/c)

Most

muons

stop at:

(m)

10% go

beyond:

(m)

1% go

beyond:

(m)

0.1% go

beyond:

(m)

Range

determined

from

(dE/dx)total

(m)

Range

determined

from

(dE/dx)ionisation

(m)

Iron 200 110 120 132 140 105 132

ρ = 7.2

g cm-3

400 190 205 220 228 175 260

Earth 50 110 120 130 135 105 110

ρ = 2.0

g cm-3

100 210 220 235 245 205 210

200 390 410 430 445 380 410

400 710 740 780 800 670 815

500 870 890 930 950 800 1010


Muon shielding

Comparison of the longitudinal thickness in metres of iron shielding required to achieve 10 μSv h-1 due to the hadron and muon components of the cascade for a proton beam intensity of 1012 s-1.

Proton Energy Hadron shield Muon shield

5 GeV 3.4

10 GeV 4.6 6.0

30 GeV 14.0

100 GeV 8.4 36.0

300 GeV 77.0

1 TeV 10.2 170.0


Experimental areas

Vertical longitudinal cut through the beam lines of the CERN SPS North Experimental Areas


Narrow beam dosimetry

Since in the case of partial irradiation effective dose is not an adequate risk indicator as it is unable to take into account the incidence of deterministic effects, both effective dose and organ dose in the exposed tissue or organ have to be considered. The absorbed dose in an organ is an estimator for deterministic effects should the threshold for such effects be reached. Where this threshold is not reached, the effective dose can be used to estimate the probability of stochastic effects.


Effects of partial-body irradiation

50 Gy for most organs will cause an effect in 1-5% of persons irradiated

70 Gy for most organs will cause an effect in 25-50% of persons irradiated

Tissue and Effect Threshold(Gy)

Annual Limit (Sv)Alone Whole-body

TestesTemporary sterility 0.15 0.2 0.05Permanent sterility 3.5 0.2 0.05

OvariesSterility 2.5-6.0 0.2 0.05

Lens of the eyeDetectable opacities 0.5-2.0 0.15 0.05Cataract 5.0 0.15 0.05

Bone MarrowDepression of hematopoeises 0.5 0.4 0.05Fatal aplasia 1.5 0.4 0.05


Beam loss

Beam Beam intensity to cause Death Temporary Sterility (5 Gy) (0.15 Gy)

20 GeV protons 5 X 1013 1.5 X 1011

450 GeV protons 2 X 1012 1 X 1010

7 TeV protons 1 X 1011 3 X 108

50 GeV electrons 1 X 1012 1.5 X 1010

An SPS beam of several hundred GeV (250 machine pulses per hour) and 108 particles per pulse can give rise to a dose rate

at 1 metre of approximately 50 mGy/h or 250 mSv/h


In-beam exposure (1)

FLUKA calculations of dose in a 1 mm radius cylinder around proton beams of different energies in tissue-equivalent material: * 20 GeV,

100 GeV, + 500 GeV, 2 TeV and × 7 TeV

Dose at the surface ≈ 10-8

Gy per incident particle


In-beam exposure (2)

FLUKA calculations of dose in a 1 mm radius cylinder around electron beams of different energies in tissue-equivalent material: * 20 GeV, 50 GeV, + 100 GeV, 200 GeV and × 500 GeV

Dose at the surface ≈ 10-8

Gy per incident particle

Secondary electron beams can be created at proton accelerators


Minimum-ionizing particles• A minimum-ionizing particle loses energy at a rate of about

2 MeV/(g cm-2)

• For a uniform flux and without any cascading, and assuming that the beam corresponds to an area of 2 X 2 mm2,

this corresponds to:

2 X 1.6 X 10-13 (J/MeV) X 1000 (g/kg)/4 X 10-2 cm2 = 8 X 10-9 Gy

• This corresponds well to the FLUKA calculations at the surface

• This means that we shall cause a detectable opacity in the lens of the eye with a single pulse of 108 particles


Organ doses - Summary

We can now determine the number of pulses which will cause damage for a beam intensity of 108

particles per machine pulse.

Damage RequiredDose (Gy)

Machinepulses

Testes – Temporary sterility 0.15 2Bone marrow 1 15Testes or Ovaries – Permanentsterility 4 60

General organ damage 50 700


Organ dose and effective dose for protons

0 100 200 300 4001E-12

1E-11

1E-10

1E-9

Org

an d

ose

(Gy

per p

roto

n)

Proton Energy (GeV)

Eye Thymus Thyroid Breast Lung

DT = AD + BD log kE = AE + BE log k

(Gy or per primary particle)

k = particle energy in GeV

0 100 200 300 400

1E-12

1E-11

Effe

ctiv

e D

ose

(Sv

per p

roto

n)

Proton Energy (GeV)

Thyroid Breast Eye Thymus Lung

ORGAN AD BD AE BE

Right eye 3.24 10-10 2.38 10-11 4.63 10-13 8.77 10-13

Thyroid 5.82 10-11 1.20 10-11 1.47 10-11 3.76 10-12

Thymus 3.80 10-11 8.19 10-12 4.88 10-12 2.40 10-12

Breast 7.89 10-12 1.22 10-12 2.52 10-12 2.00 10-12

Lung 1.45 10-12 1.82 10-12 1.01 10-12 1.15 10-12

fitt

ing

para

met

ers


Organ dose and effective dose for electrons

0 100 200 300 4001E-12

1E-11

1E-10

1E-9

Org

an d

ose

(Gy

per p

rimar

y el

ectro

n)

Electron Energy (GeV)

Eye Thymus Lung Thyroid Breast

0 100 200 300 400

1E-13

1E-12

Effe

ctiv

e D

ose

(Sv

per e

lect

ron)


Thyroid Breast Eye Thymus Lung


Organs contributing to effective dose

0 100 200 300 400

0.1

1

Frac

tion

of th

e ef

fect

ive

dose

due

to n

on-ta

rget

org

ans

Proton Energy (GeV)

Breast Lung Thymus Thyroid

0 100 200 300 4001E-3

0.01

0.1

1

Frac

tion

of th

e ef

fect

ive

dose

due

to n

on-ta

rget

org

ans


Breast Thyroid Thymus Lung

Fraction of effective dose due to non-target organs for protons, for four of the five target organs investigated (the eye has no associated wT-value)

Same for electrons


Lead-ion beams (1): thick target

As for proton beams, the transverse shielding of high-energy heavy ion beams mainly involves the attenuation of the secondary neutrons generated in the hadronic cascade. Most of the available Monte-Carlo codes cannot be employeddirectly because they do not transport ions with masses larger than one atomic mass unit.

There is also a general lack of knowledge about the source terms for neutron production from high-energy heavy ions.

Recent measurements at CERN have shown that the spectralfluence of the secondary neutrons outside a thick shield is similar for light (protons) and heavy (lead) ions stopped in a thick target.


Lead-ion beams (2): thick target

The approach of considering a high energy lead ion as an independent grouping of free protons is sufficiently accurate for the purpose of evaluating the ambient dose equivalent of secondary neutrons outside thick shielding.

The neutron yield from lead beams dumped in a thick target appears to depend on energy as

8.0PbEY ∝

where EPb is the energy per nucleon of the lead ion beams.

The yield also appears to scale linearly with the mass number of the projectile.


Lead-ion beams (3): thin target

Beam FLUKA

(sr-1)

Experimental(FLUKA guess)

(sr-1)

Scaling factor(A=208)

40 GeV/c protons + π+ (3.199 ± 0.003) x 10-1 3.499 x 10-1

40 GeV/c lead ions (3.666 ± 0.003) x 10-1 (a) 26.6 A0.80

158 GeV/c lead ions (4.566 ± 0.003) x 10-1 (a) 41.1 A0.84

90° neutron yield from high energy protons and lead ions on a thin lead target (neutron per primary particle per steradian)

(a) The simulation results refer to a proton beam of the same energy per nucleon.


Lead-ion beams (4)• As far as we know, the stray radiation caused by realistic heavy ions rises no faster than with the number of nucleons in the projectile nucleus.Therefore the dose rates caused by a secondary SPS lead beam are about 200 times higher than a secondary proton beam of the same particle intensity.

• Thus a lead beam containing 106 ions is equivalent to a “normal” beam of 2 X 108

particles.

BUT

• The Bethe-Bloch formulation for ionization energy loss tells us that the rate of energy loss varies as the square of the charge of the projectile nucleus.

• Thus the “minimum” energy loss rate of 2 MeV/g cm-2 becomes for a lead nucleus a loss rate of

13.4 GeV/g cm-2

• And this is not all distributed in “small” events as can be seen from emulsion photo-micrographs of cosmic-rays tracks.


Relativistic lead-ions in emulsions


Relativistic lead-ions in plastics


Damage caused by lead-ions

• The original line of dislocation damage in plastics has a diameter of about 100 Angstroms, or 0.01 microns. This can be etched to give a visible cone of about 35 microns depth/diameter.

• There is complete physical destruction of the structure of the plastic over an area of 10-12 cm2 or for a beam of 106 lead particles the surface area destroyed is 10-6 cm2.

• The lead tracks in emulsions have a core of about 20 microns in radius.Thus each lead track leaves a solid line of developed silver grains of 10-5 cm2 cross-sectional area.

• So a beam of 106 particles spread out uniformly can turn an emulsion of 10 cm2

cross-sectional area black.

• In a beam of 4 mm2 cross-section there is the power to physically destroy a fraction of 2.5 X 10-5 of its cross-sectional area or to put 250 times more energy into an emulsion than is necessary to make developable silver grains.


PB - Damage to tissue

• Multiplying the biological damage factors determined before simply by Z2

Pb = 6700 we might cause a detectable opacity in the lens of the eye with a single pulse of 1.5 X 104 lead of ions.

• The organ dose is at least 1.5 X 10-6 Gy per beam particle. Thus:

Damage RequiredDose (Gy)

Pb Intensity

Testes – Temporary sterility 0.15 3X104

Bone marrow 1 2X105

Testes or Ovaries – Permanent sterility 4 8X105

General organ damage 50 1.5X107

The conclusion is that it is more than prudent to keep out of a lead beam of 106 ions per burst!


CERN Neutrinos to Gran Sasso


Radiation hazard from neutrinos (1)

Annual dose equivalent (µSv)Solar neutrinos (Eν ~ 1 - 10 MeV) 10-7

Atmospheric neutrinos (Eν ~ 100 MeV - 2 GeV) 2 x 10-9

Neutrino experiments (Eν ~ 10 – 100 GeV): Fermilab (NuMI) SBL, 1 km distance 10

LBL, 730 km distance 8.5 x 10-6

CERN/Gran Sasso SBL 10Gran Sasso 5 x 10-5

Expected annual dose equivalent from natural and accelerator neutrino sources (Short and Long Baseline neutrino experiments)


CERN Neutrino Factory



b

R

d

hs

z

θ∼1/γθ∼1/γ

φ

a

ν

s2 = 2 R d - d2 θ ∼ 1/γθ ∼ 1/γ

sin φ = s / R a ≅ 2 θ θ ss

h ≅ z tan φ b ≅ a / φ

E CoM d (m) s (km) φ z (km) h (m) θ a (m) b (m)0.5 TeV 100 35 5.6 10-3 10 56 424 10-6 30 53004.0 TeV 500 80 12.5 10-3 10 125 53 10-6 8.5 680

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