Impact of amended 10CFR835 on neutron calculations and measurements at high energy electron accelerators

A. Fass ò

SLAC, Radiation Protection Department ASW August 12, 2008

Regulation International Recommendation

ICRP21 (1973) NCRP38 ICRP26 (1977) 10CFR835 (1998) ICRU39 (1985)

Protection Quantity

H E Effective Dose Equivalent

Operational Quantity

MADE from ICRU20-1971 MADE H* H ’ , H p introduced ICRP51 (1987) Not applied in the US ICRU43 (1988) H* H ’ , H ≥H E p ICRP60 (1991) E: Effective Dose = ∑ W T H T = ∑ W T ∑ W R D T 10CFR835 (2007) ICRU51 (1993) H* H ’ , H p E H* H ’ , H p ICRP74 (1997) (maximum 20 at >150 keV/um) Q ×

Comments

Old Q(L) ANSI N13.11 (1983) DOELAP 2 recommended for neutrons Conversion factors No Q increase for neutrons New Q(L) for H* H ’ , H p (maximum 30 at 100 keV/um) W R for E (max. 20) New Q(L) ANSI N13.11 (2001) (use old ISO) New DOELAP?

The increase in neutron Quality Factor Q (to be used only with operational quantities ) and the corresponding increase in the radiation weighting factors (affecting the new protection quantity Effective Dose ) have caused some concern within the Accelerator Radiation Protection community.

However, Q and w R are not necessarily used directly in many cases. Most often their increase will show only

indirectly

fluence-to-dose conversion coefficients . through the new ASW August 12, 2008

Some commonly accepted facts

Operational quantities are always conservative with respect to the corresponding protection quantities Operational quantities are measurable, while protection quantities are not

So say ICRU, ICRP, the DOE (Federal Register), and we hear it always repeated at conferences and in professional articles But are these facts always true for all types of radiation and energies? They certainly are for photons of energies < 10 MeV. And, admittedly, most of the doses received by personnel (also at SLAC) are due to them.

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“Challenging radiological conditions”

“DOE agrees that at high energies, such as those above 10 MeV, the biological impact of particles on human tissue may be more uncertain than at other energies and that monitoring of workplaces and individuals exposed to particles with these energies may be very challenging

.

However, other challenging radiological conditions exist in the DOE complex that are not explicitely addressed in 10 CFR part 835. Moreover, radiation fields consisting of particles greater than 10 MeV do not occur extensively within the DOE complex.

” DOE Federal Register Vol. 72, No. 110 / Friday, June 8, 2007 / Rules and Regulations In other words: the situation of high energy accelerators has not been considered in 10 CFR part 835. Do those “commonly accepted facts” apply also to them?

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Let’s not worry about radiation weighting factors

Still from the same issue of the DOE Federal Register: “ DOE notes that the purpose of radiation weighting factors is to establish dose limits, set up other dose dependent criteria for protection purposes, and plan radiological work. They are not for the purpose of measuring radiation fields and individual doses.

” Then, how do we measure radiation fields and individual doses?

Perhaps we can do indeed without the radiation weighting factors (needed to calculate the Effective Dose , which “ cannot be measured” ). But do we still need to worry about the increase in Quality Factors (needed to calibrate the instruments we use to measure Operational Quantities , which “ can be measured” )?

Let us have a closer look at those “commony accepted facts”.

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The first commonly accepted fact “Operational quantities are always conservative with respect to the protection quantity E”

.

Not true at high neutron energies, when the maximum dose does not occur within 1 cm from the surface of the body. See: Ferrari, A. and Pelliccioni, M. “Fluence to effective dose conversion coefficients for neutrons up to 10 TeV”. Radiat. Prot. Dosim. 76(4), 215-224 (1998).

Ferrari, A. and Pelliccioni, M. “Fluence to effective dose conversion data and effective quality factors for high energy neutrons”. Radiat. Prot. Dosim. 76(4), 215-224 (1998).

This fact has been dismissed sometimes on the ground that “most of the neutrons have low energies”. But this is not the case for high energy accelerators : outside thick shielding, at SLAC half of the dose is due to neutrons with energies > 20 MeV, independent of the shielding thickness (equilibrium spectrum) ASW August 12, 2008

The second commonly accepted fact “Operational quantities are measurable, while protection quantities are not”

.

Not always true for neutrons :

there are two basic types of measurements used at high energy accelerators.

1)

To measure

Absorbed Dose

with a tissue-equivalent ionization chamber, and to multiply it by an average quality factor

Q,

which must be obtained with special instruments (e.g. a recombination chamber), or by calculation.

2)

To measure

Fluence

with an instrument whose response has been designed to reproduce a fluence-to-dose conversion coefficient (e.g. Andersson-Braun rem counter, LINUS extended range rem counter). Ambient dose equivalent can be measured directly only with the first method (but not quite so directly: the

Q

is a generic value, that cannot be obtained case by case for a specific field measurement)

At SLAC, we are using the second method.

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Comparison of fluence to dose conversion coefficients recommended in the old 10CFR835 and in the new one (

Effective Dose E

, and operational quantities

H*(10)

and

H p (10)

as reported in ICRP 74 ). The curves of

E, H*(10)

and

MADE MADE

curve of ICRP 21 is also shown. The have been extended to higher energies using data calculated by Pelliccioni. The coefficients used in the SHIELD11 point-kernel code are also shown.

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To evaluate the differences between the old and the new conversion coefficients, we have folded them with the spectra of two neutron sources commonly used for calibration of area instruments and personal dosimeters: 252 Cf and Am-Be. The spectra have been taken from the International Standard ISO 8529-1. The same comparison has been made with a typical neutron equilibrium spectrum , calculated by S. Roesler for a 7 ft concrete shielding thickness at the FFTB SLAC facility. ASW August 12, 2008

S. Roesler, J.C. Liu, S.H. Rokni and S. Taniguchi,

Neutron Energy and Time-of-flight Spectra Behind the Lateral Shield of a High Energy Electron Accelerator Beam Dump, Part II: Monte Carlo Simulations

Nucl. Instrum. Meth. A503, 606-616 (2003) SLAC-PUB 9210 Points : liquid scintillator experiment SLAC/Tohoku/CERN Histograms : FLUKA calculation Note that spectra are attenuated with increasing thickness, but they keep the same shape (equilibrium spectrum) ASW August 12, 2008

Color code: green: our calculation black: official standards (old) blue: official standards (new) All values in rem/(n cm -2 ) ×10 -8

old 10CFR835 ICRP 21 MADE ISO 8529 old ICRP 74 E IAEA TRS 403 (PTB) ICRP 74 H * (10) ISO 8529-3 IAEA TRS 403 (PTB) ICRP 74 H p (10) ISO 8529-3 ANSI/HPS N13.11-2001 IAEA TRS 403 (PTB) 252 Cf 4.00

4.00

3.40

4.01

3.35

3.34

3.40

3.36

3.36

3.85

3.85

3.82

Am-Be 3.70

3.72

3.81

4.11

n.a.

3.91

3.91

n.a.

4.11

4.11

n.a.

n.a.

SLAC Equilibrium spectrum 3.67

3.11

n.a.

3.40

n.a.

3.33

n.a.

n.a

2.55

n.a.

n.a.

n.a.

The new conversion factors lead to increases for all source spectra, but not for the SLAC equilibrium spectrum:

ICRP 74 E 252 Cf

+0.3 %

Am-Be

+10.9 %

SLAC Equilibrium spectrum

–7.9 %

ICRP 74 H * (10)

+15 % +5.6 % –10.3 %

ICRP 74 H p (10)

+19.5 % +11 % (–44 %) (1) (1) It must be noted that no fluence to H p (10) conversion factors are available for energies larger than 20 MeV. The conversion factor table of the old 10CFR835 extends up to 400 MeV. The maximum energy of the SLAC equilibrium spectrum considered was 1 GeV. ASW August 12, 2008

Choice of the quantity

At SLAC, all the measurements and calculations are based on Fluence , and can be easily converted to any protection or operational quantity by applying the relevant conversion coefficient. Conversion coefficients are available for both Effective Dose and Ambient Dose Equivalent for all energies of interest. Coefficients for Personal Dose Equivalent are available only for energies < 20 MeV.

Since Effective Dose is the protection quantity for which compliance is necessary , and operational quantities don’t present in our case any of the usual advantages (i.e. to be conservative and measurable), it looks preferrable to choose Effective Dose . However, there are a few aspects to be discussed.

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Issues to be addressed at SLAC

● Calibration of field instruments ► The neutron sources used for calibration have spectra very different from those found outside accelerator shielding. Anyway, Andersson-Braun instruments are known to measure only dose of neutrons with energy < 20 MeV, which is about 50% of total neutron dose at SLAC. The LINUS extended rem counter, however, can measure dose over the entire spectrum.

► The response of an Andersson-Braun rem counter approximates equally well (or equally badly  ) the shape of all the curves shown before . The calibration with a source can establish a different absolute value depending on the quantity chosen. The same applies to the LINUS.

► A calibration in Effective Dose made with a 252 Cf source will result in an increase of 0.3% , practically negligible, especially if compared with the factor 2 which has to be applied to account for the high energy part of the neutron spectrum. In the case of the LINUS extended range rem counter, we can even calibrate it in Effective Dose without any correction factor!

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Issues to be addressed at SLAC

● Calibration of field instrument ► We have shown that H * (10) neutron field is not > E for a typical high energy ► However the calibration is done with a low energy source. ► If we do the calibration in H * (10) increase by about 15% .

the instrument response will ► Do we need to do that? Calibrating in E complies with the law and does not change the present calibration.

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Issues to be addressed at SLAC

● Personnel dosimetry ► For individual monitoring, the Federal Register recommends the use of operational quantities H p (10), H p (3) and H p (0.07) .

Apparently, this is also the choice of DOELAP, ANSI and ISO.

► For neutrons, the same objection already expressed holds: the neutron dosimeter (in our case a CR39 track detector) does not measure the neutron dose at a specified depth, but the neutron fluence . Conversion coefficients allow to convert neutron fluence equally well to H p after all E is the quantity we must comply with.

as to E : but ► Calibrations are made with neutron sources of known yield and spectrum, from which a fluence is derived, which can be convoluted with any conversion coefficient.

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Issues to be addressed at SLAC

● Personnel dosimetry Also in this case: ► We have shown that H p (10) neutron field is not > E for a typical high energy ► However the calibration is done with a low energy source. ► If we do the calibration in H p (10) increase by about 20%.

the instrument response will ► Do we need to do that? In principle no (see before), but probably DOELAP will decide for us.

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Issues to be addressed at SLAC

● Shielding calculations: Monte Carlo At SLAC, shielding design is done mainly by Monte Carlo by means of the FLUKA code. This code has a number of advantages: ► It is the same code which has been used to calculate fluence-to-dose conversion coefficients over the largest energy range for all particles present in accelerator radiation fields, not only for neutrons (gamma, electrons, pions, protons, muons, kaons) ► Such coefficients are available for both effective dose in several irradiation geometries and ambient dose equivalent ► A special subroutine, written by S. Roesler (CERN), allows to choose the quantity to be calculated (including contributions from all types of particles) Due to the lack of conservativeness of ambient dose equivalent at high energies, we prefer to calculate effective dose. For what concerns the choice of the irradiation geometry, we use the “worst” geometry, i.e. the maximum value of the conversion coefficient for all geometries at any particular energy.

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Issues to be addressed at SLAC

● Shielding calculations: analytical For fast shielding estimates, we still use the point-kernel analytical code SHIELD11 , which was the basic shielding tools in the past, when Monte Carlo codes capable to simulate photonuclear reactions did not yet exist.

It was, and still is widely used world-wide for shielding of electron accelerators.

SHIELD11 is based on experiments performed at SLAC between 1968 and 1979. It considers 3 neutron and 2 gamma groups.

Also in SHIELD11 the basic physical quantity is fluence.

Three conversion factors are applied, one per each neutron group. The conversion factor for the highest energy group (> 100 MeV) is higher than the present values of both Effective Dose and Ambient Dose Equivalent , while the other two are lower. For an equilibrium spectrum, such as is found outside thick shielding, SHIELD11 is certainly still conservative. We are investigating its performance for non-equilibrium spectra by comparing it with FLUKA calculations. If necessary, we will modify the code using new conversion factors. ASW August 12, 2008

Conclusions

SLAC, similar to most other high-energy accelerators, is well shielded and prompt neutron and gamma doses in accessible areas are kept at a negligible level.

Nearly all personnel dose recorded is due to photons from work on activated components and the only neutron dose is from calibration sources.

All our measurements are based on fluence-to-dose conversion coefficients.

Shielding : Effective Dose is our quantity of choice, calculated with the worst geometry conversion coefficient by the FLUKA code.

Instrument calibration: 10 CFR 835 requires Effective Dose . We can easily get it using the appropriate conversion coefficient. Use of Ambient Dose Equivalent would only bring in an unnecessary increase of the calibration factor by 15% Personnel dosimetry: again , Effective Dose is the quantity we must comply with, and one could do the same as above. Use of Personal Dose Equivalent would bring in an increase of the calibration factor by 20 %.

But the final word stays with DOELAP.

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