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Radiation Monitoring

Vashek VyletDuke University

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Contents

• Introduction• General Aspects• Types of Detectors• Neutron spectrometrywith Bonner spheres• Examples of measured spectra with BS

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Introduction

• Goal: Measure/estimate ambient radiation fields generated by accelerators (induced activity - conventional techniques)

• What: Operational (ambient dose equivalent) or physical (Φ(E)) quantities, the latter can be translated to operational quantities using appropriate conversion factors

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Introduction

• Purpose 1: Insure radiation safety of workers & public; evaluate and monitor efficacy of specific protection measures; backup information for personnel dose investigations

• Purpose 2: Insure and demonstrate regulatory compliance (keep records!)

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Introduction

• Special to accelerators: great variations in type, energy & spatial distribution; time structure (pulsed fields) of prompt radiation

• Radiation fields depend on great number of parameters need to record details: date & time, average current, E, mode of operation, special machine setup, …etc

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Sources

• Mostly photons, neutrons and muons• Beam losses, special operation (conditioning),

auxiliary devices (klystrons)• Direct & scattered radiation near sources,

skyshine from far away• Spatial aspects: uniform field or hot spots,

leakage through cracks, ducts and sharp forward sources (brems., muons)

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Type of Measurements: Surveys

• Short/simple survey: field possibly known, looking for “ballpark” values or yes/no information - routinely done during normal operations

• Extensive surveys: at startup of new facility, after long shutdown periods or changes in configuration; normal operation and/or mis-steering and accident scenarios

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Type: Long-Term Monitoring

• Continuous long-term monitoring, using active (with recording capability) or passivedevices. Recording of sampled or integrated (preferred) signal

• Stationary active monitors may be interlocked as part of safety system

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Type: Special Measurements

• Detailed characterization of radiation fields: components, energy spectra, spatial distribution …

• Detailed characterization of detector behavior - requires additional instrumentation

• Complex measurements under special operating conditions (special measures)

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General Aspects - Calibration

• S(E) = response function; ks = spectrum weighted response per unit fluence

tots

tot

s

RdEE

dEEESk

dEEkdEEESR

Φ=

Φ

Φ=

Φ=Φ=

∫∫

∫∫Φ

).(

).().(

).().().(43421

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Calibration

• Cs = calibration factor for spectrum S, (hφ)s = spectrum weighted average fluence-to-dose equivalent conversion factor

ssss

s

khCRCH

dEEh

dEEEhh

)( .

).(

).().()(

φ

φ

φφ

=⎯→⎯=

Φ=

∫∫

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Dead Time

• Dead time τ = difference in pulse arrival times below which two incoming pulses will not be resolved

• May be determined/dominated by intrinsic properties of detector or counting electronics

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Dead Timen = true interaction ratem = recorded count rate

= system dead time

τmmn

−=

1Nonparalysable:

Paralysable: τnnem −=needs iterative approach

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Dead TimeParalysable: two solutions possible! Solution: change true rateand observe if count rate increases or decreases.

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Dead time with pulsed sources

• << T no problem• < T difficult problem

• T < < 1/f -T n = f .ln(f/(f- m)) does not deped on

m

n

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Dead time with pulsed sources

• Rule of thumb: OK if count rate is less than a tenth of rep rate - then loss is ~5%

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.2 0.4 0.6 0.8 1

measured rate/machine pulse rate

mea

sure

d/re

al c

ount

rat

e

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General Aspects - RF and mag. fields

• RF generates signal in anything acting as antena: unshielded or badly shielded cables & components attached to sensitive preamp.

high level of noise (false signal, distorted MCA spectra, …)

• Magnetic field affect moving charge: PMTs and gas detectors especially sensitive

Liu et aSLAC-PUB

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Choice of Instrument & Method

• Adapted to application: en. response, particle type, temperature sensitivity, rf, …

• Care to avoid dead time problems, paralysis, saturation & recombination in intense fields

• Interlocked monitors should be failsafe -use check source or natural bkg, although the latter may pose problems (low rates)

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Interlocked Area Monitors

Photon IC

Moderated BF3

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Choice of Instrument & Method

• Unknown fields have to be characterized (energy or LET spectra) and proper calibration factors established

• Take into account directionality of field & detector response

• Take precautions during special (mis-steer, accident simulation) measurements requiring interlock bypass, roping of areas...

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Gas Detectors

• GM, IC, PC: primary application in photon survey meters; IC & PC also for neutrons, with propper wall and gas choice

• Problems: pulsed fields, mag. & RF fields• Large ICs: stationary (interlocked) monitors

(BSOIC at SLAC, Chipmunk at Fermilab ..)• ICs operated in current mode impervious to

pulsed fields (except higher recombination)

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Gas Detectors

• GMs - bad dead time problem (paralysable)• GMs not suitable for neutrons (equally

sensitive to photons), but …• Albatros: Moderated GM wrapped in silver

- activated by neutrons (saturation within minutes) - no trouble with pulsed fields

• PCs filled with BF3 or 3He used for thermal neutron detection

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Special Gas Detectors: TEPC

• Tissue-Equivalent Proportinal Counter: TE walls and TE gas - from PHA one can determine D(y) ~D(LET) average Q

• HANDI used at CERN• Problems in pulsed fields• Not very sensitive (for 1-micron sphere)

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Recombination IC

• Takes advantage of dependence of columnar recombination on LET

• I = kVn, where n=f(Q), or• Double chamber method (one at high and

one at low voltage)• Tricky, not widely used around accelerators

(earlier used at CERN and Fermilab)

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Proton recoil PC (neutrons)

• Filled with hydrogen or hydrogenous gas, neutrons detected by proton recoils

• Neutron spectrum can be obtained by unfolding recoil proton spectrum

• Problems: sensitivity, pulsed fields• Narrow energy range for given volume and

pressure; 10 keV - few MeV requires use of several detectors in parallel (ROSPEC)

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Scintillators

• Used in photon survey meters and monitors• For muons: photon bkg eliminated by

coincidence between two detectors forming a directional telescope (with absorber)

• Organic scintillators used for neutrons (proton recoil); photon bkg can be removed by pulse-shape analysis

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Fermilab’s MERL

• Muon telescope in MERL

• Also portable version (muon “gun”)

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Passive Detectors

• Nuclear Emulsions: disappearing art• TLD, OSL and track-etch: photons &

neutrons, mostly for personnel dosimetry, but also area monitoring

• Activation detectors: neutrons• Optichromic and radiochromic detectors:

high level doses (near beamline)

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Bubble Detectors

• Superheated droplets suspended in gel• Neutron and gamma• Active or passive• Limited use life• Limited dynamic range

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Moderated Neutron Detectors

• With active or passive th. n. detectors• Fast neutrons moderated to thermal

energies detected with greater efficiency, using high x-sections for thermal neutrons (10B, 3He, 6Li)

• Surveys, prolonged monitoring, and spectrometry

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Moderated Neutron Detectors

• 5” PE cylinder with Panasonic TLD card used for area monitoring and SLAC and Duke

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Rem-Meter• Response ~ Fluence-to-Dose-Equivalent

conversion factor hφ R~H, independent of neutrons energy counts ⇔ rems

10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 1031.0×10-12

1.0×10-11

1.0×10-10

1.0×10-09

E [MeV]

h φ [S

v.cm

2 ]

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Extension of E-range

• Energy response of ordinary rem-meter falls off beyond ~10 MeV

• Lead layer: (n,2n), (n,3n), …conversion to neutrons with lower energies (Linus:Birattari et al., 1990)

• Used in astrophysics for decades to detect cosmic ray neutrons (BF3 counters, PE, Pb)

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Detection time in moderators

• Moderation, and especially thermal walk take time detection delayed from neutron entry into moderator volume

• Slow neutrons delayed in shielding• This leads to spread of detection time in

pulsed machines (to tens, even hundreds μs) and greatly alleviates dead time problems

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Arrival time distribution - 3” ball

Liu et alRPD

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Arrival time distribution - 8” ball

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Arrival time distributions

• ANL, room return (20 MeV e- linac)

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Neutron Spectrometry

• Proton recoil - PCs & scintillators - good E resolution, limited energy range, problems in pulsed fields

• Bubbles: dynamic range; statistics; cost• TOF - great resolution; complex, very low

sensitivity (not suitable at RP levels)• Activation threshold detectors: low

sensitivity, laborious readout;

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Neutron Spectrometry

• Bonner spheres: set of spherical moderators of different sizes, used with th. n. detector

• Wide energy range, high sensitivity, low E resolution (sufficient for RP)

• Energy response can be further extended by use of (n,xn) in heavy materials (Pb, W)

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Spectrometry with BSS

• Principle: measure n count rates, then unfold Φ(E):for i=1, …,m

• Typically m=5-8, n=30-100

• Under-determined system; additional info provided through “1st guess”, smoothing, and other constraints on the spectrum

∑=

Φ=n

gggii SR

1, .

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Spectrometry with BSS

• Unfolding codes - same/similar as for spectrometry with activation foils

• BUNKI, LOUHI, SAND2, SWIFT (MC), MAXED, ...

• Most important: well tuned response matrix

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Extension of E-range - Pb

10010-8 10-6 10-210-4 102 104

ENERGY [MeV]

SE

NS

ITIV

ITY

[a.u

.]

with leadno lead

1

10-1

10-2

10-3

12"

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Extension of E-range - 11C

• 11C activation (threshold ~20 MeV) in 5x5”plastic scintillator (J. B. McCaslin, 1960)

• (n,2n) cross-section added to response matrix

• Saturated activity entered in lieu of sphere count rate

12C(n,2n)11C

00.5

11.5

22.5

3

10 100 1000

E [ MeV]

N. σ

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Extension of E-range - 11C

• Typical irradiation time: ~40 min (2.T1/2)

C … net count rate ε … counting efficiency

)e)(1eε.(eC.λA

irrcdd λ.t)tλ(tλ.tsat −+−− −−=

Irradiation tirr td counting tc

11C : competing photo-activation 12C(γ,n)11C at electron machines

contamination ( plastic bags)

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Extension of E-range

0.0E+0

5.0E-2

1.0E-1

1.5E-1

2.0E-1

2.5E-1

1.E-1 1.E+0 1.E+1 1.E+2 1.E+3

E [MeV]

Sens

itivi

ty [c

m^2

] 8”

12”

12Pb

C-11

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Duke BS System

30.48

PA PortableMCAHV

A

25.420.32

14.2 8 cm

He-3counter

12.7x12.7cmscintillator (C11)

Optimized configuration of lead layer

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14.2 cm sphere with inserts

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Response Matrix (Duke system)

0.E+0

1.E-2

2.E-2

3.E-2

4.E-2

5.E-2

6.E-2

7.E-2

8.E-2

9.E-2

1.E-1

1.E-7 1.E-6 1.E-5 1.E-4 1.E-3 1.E-2 1.E-1 1.E+0 1.E+1 1.E+2 1.E+3

E [MeV]

Sens

itivi

ty [a

.u.]

bare

3.15"PE

5.6" PE

5.6"Pb

8" PE

8"Pb

10" PE

10" Pb

12" PE

12" Pb

Measured Neutron Spectra and Practical Experience with Bonner

Spheres

10” at Riken

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What to expect ...

• Proton machines:– hadronic “hard” shower– neutrons:

• “rapid” component (> 20 MeV), part of cascade• evaporation neutrons from residual nuclei (~MeV)

– most penetrating part: neutrons >150 MeV (for E>150 MeV σin reaches a constant minimum: σin = 43.1 A0.70 )

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What to expect ...

• Electron machines:– Elmag. “soft” shower– photoneutrons from

three production processes

– behind thick shields spectrum determined by high E neutrons

100

10-1

10-2

100 102 103

PHOTON ENERGY [MeV]

Giant Resonance( ,n)γ

Pions

Pseudodeuteron

/A(m

b/N

ucle

on)

σ

Photonuclear Cross Section

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What to expect ...

• Common aspect of e and p machines:– thick shields neutrons with E>150 MeV are

the determining factor– “Equilibrium” spectra: High E portion +

products of hadronic cascade emerging from outer shielding layers

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Specific Aspects

• Dead time in pulsed fields– SLAC: e- and e+ pulses from 10 ns to 1.6 μs– Moderation & thermal walk save the situation

(neutron signal spread over 100s of μs)• Variable beam intensity, beam interruptions

need a reference monitor ; more of a problem for 11C activation

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Non-equilibrium spectra - SLAC SSRL

0 1 m leadconcrete

SPEAR

Faraday cup

dipole magnetFARC

135 90 45ReferenceBF3 counter

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SLAC SSRL

• MCA Pulse-height spectra (with γ-flash)

0

50

100

150

200

250

0 200 400 600 800 1000

Channel

Cou

nts

gate offgate on

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SLAC SSRL

• Gamma flash with “non-equilibrium”spectra

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SLAC SSRL - SPEAR 135º

Energy (MeV)10 -6 10 -3 10 0 10 30

20000

40000

60000

80000Fl

uenc

e/le

thar

gy

with C-11without C-11

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SLAC SSRL - SPEAR 90º

10 -6 10 -3 10 0 10 30

100000

200000

300000

Flue

nce/

leth

argy

Energy (MeV)

with C-11without C-11

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SLAC SSRL - SPEAR 45º

10 -6 10 -3 10 0 10 30

50000

100000

150000

200000

with C-11without C-11

Flue

nce/

leth

argy

Energy (MeV)

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Measured spectra - CERN

E.Φ

(E)

ENERGY [MeV]100 10310-310-6

105

106

107

concreteiron

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Measured spectra - CERN

Iron Shielding Concrete Shielding

Φ<20 Φ>20 Φ total Φ<20 Φ>20 Φ totalFLUKA 11.44 0.66 12.10 0.83 0.62 1.45BS 16.51 0.50 17.01 1.08 0.49 1.57Ratio 1.44 0. 78 1.41 1.30 0.79 1.08

(normalized to 1E+6 counts of monitor IC)

• Comparison with FLUKA calculations:

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Measured spectra - SLAC FFTB

• Final Focus Test Beam (FFTB)

Beam Dump

Beam Line

2147 GeV e-

Muonshielding

ConcreteIron0 1 2 3 m

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Measured spectra - SLAC FFTB

ENERGY [MeV]

E.Φ

( E)

100 10310-310-6