16th International Congress on Neutron Capture Therapy

June 14-19, 2014 Helsinki, Finland

Study of suitability of Fricke-gel-layer dosimeters for in-air measurements to characterise

epithermal/thermal neutron beams for NCT 

G. Gambarini1,2, E. Artuso1,2, V. Regazzoni1,2, M. Felisi1, L. Volpe1,2, L.Barcaglioni3,2, S. Agosteo3,2, L. Garlati3, F. d’Errico4,

V. Klupak6, L. Viererbl6, J. Burian6, M. Marek6  

1 Department of Physics, Università degli Studi di Milano, Milan, Italy2 INFN, Istituto Nazionale di Fisica Nucleare, Italy3 Energy Department, Politecnico di Milano, Milan, Italy4 Department of Civil and Industrial Engineering, Università degli Studi di Pisa, Italy

and Yale University, School of Medicine, New Haven, CT, USA5 Department of Neutron Physics, Research Centre Řež, Czech Republicù

Fricke-gel dosimetry methods have been improved

for IMAGING and PROFILING the absorbed dose

in tissue-equivalent (TE) phantoms exposed to an

epithermal neutron beam for NCT.

Their reliability for in-air measurements to

characterize epithermal/thermal neutron beams for

NCT is now studied.

Dosimetry in BNCT

What has to be measured? Dtotal

= DB + Dp

+ Dn

+

Dγ

“therapeutic dose”, from 10B(n, α)7Li σ = 3837 b

from 14N(n,p)14C Ep= 630 keV σ = 1.9 b

due to epithermal and fast neutron elastic scattering mainly with H nuclei

from 1H(n,γ)2H Eγ = 2.2 MeV σ = 0.33 b and reactor background

The various dose components have with different LET and different RBE !!!

Necessity of performing determinations of each dose component

The various dose components have spatial distribution depending on shape and dimension of the irradiated volume !!!

Necessity of performing determinations of the spatial distribution of each dose component for each irradiation geometry of interest.

THE METHODS PROPOSED AND STUDIED IN OUR LABORATORY FOR DOSE

MEASUREMENTS ARE BASED ON :

FRICKE-GEL DOSIMETERS

(DOSE IMAGES AND PROFILES)

THERMOLUMINESCENCE DOSIMETERS

(DOSE MAPPING)

What is a Fricke gel dosimeter ?Gel matrix containing a modified Fricke solution (mainly ferrous sulphate, in millimolar amount) and Xylenol Orange. Fe2+ Fe3+ RADIATION

Variation of visible light absorbance from irradiated and non-irradiated dosimeters:

they are radiochromic

Dosimeter geometries developed for large phantoms:

in rectangular, squared or circular

frames (with transparent polystyrene

windows 1 mm thick)

Fricke gel thickness: 3 mm

wideness: from 60 to 185 mm

Dosimeter geometries developed for small phantoms:in slim transparent plastic tubes

external diameter 3 mm maximum length 130 mm

Beforeandafter

irradiation

Properly developed software achieves images of optical density difference (Absorbance) that is proportional to the absorbed dose.

Grey level (GL) images of the transmitted light are detected by means of a CCD camera.

CCD Camerawith optical filter around 580 nm

Gel- dosimeter

Plane light source

Computer

Dosimeter analysis: optical imaging

Advantages of gel dosimeters in BNCT Good tissue-equivalence for neutrons and for

each secondary radiation. Possibility of separating dose components by

changing the matrix isotopic composition.

Advantages of gel dosimeters in form of layers or slim tubes

It is possible to change the gel isotopic composition, for dose separation purposes, without significantly changing neutron transport in T.E. phantoms.

WATER EQUIVALENCE andTISSUE EQUIVALENCE (TE)

Fricke gel is a dilute solution (millimolar amounts of solute) then the water equivalence is good.

With the addition of proper amount of Nitrogen, also tissue equivalence is good.The significant elements for TE in neutron fields are H, N, C+O

  H C N O C + O

FriXy Gel 0,111 0,005 0 0,883 0,888

Tissue male 0,105 0,256 0,027 0,602 0,858

Tissue female 0,106 0,315 0,024 0,547 0,862

Water 0,112 0 0 0,882 0,882

Zph ZCo Zpp n0 (m-31026) (gcm-3) FriXy-gel 7,70 7,58 7,21 3380 1.013 Water 7,67 7,52 7,17 3340 1 Average adult soft tissue (male) 7,47 7,20 6,81 3420 1.030 Adult brain tissue 7,74 7,45 7,05 3460 1.040

The effective atomic number for photoelectric (Zph),

Compton (ZCo) and pair production (Zpp) effects, the

electron density (n0) and the mass density () have

been evaluated.

TISSUE EQUIVALENCE IS GOODDetectors are suitable for in-phantom dosimetry

Are they suitable also for in-air measurements aimed at beam characterization?

Monte Carlo simulations and measurements have been carried out to investigate if gel dosimeters in form of layers may perturb the radiation field.

Gel dosimeter layers (3mm thick) are hold between two transparent Polystyrene sheets 1 mm thick.

total thickness = 5 mm

Measurements were carried out at two columns of LVR-15 research reactor at Řež

LVR-15 reactor

Epithermal BNCT column

BNCT column with a gel dosimeter

HK1 columnwith a gel dosimeter

Dose central profiles extraxted from the dose images (at 1 cm from collimator exit)

Dose

rate

(G

y/h

)

x (cm)

y (cm)

Photon dose

Dose

rate

(G

y/h

)

x (cm)y (cm)

Fast neutron dose

Example of resuts of in-air measurements with gel dosimeters

Measurements also with TLDs-700 were performed with two configurations

TLDs-700 between two thin polystyrene strips: thickness 1 mmwidth 10 mmlength 120 mm

TLDs-700 inserted in structures with the same geometry of gel dosimetry: circular, with 12 cm of diameter, gel layer of 3 mm and 2 polystyrene sheets 1 mm thick (total thickness: 5 mm)

Measurements with TLDs-700

BNCT column with some

HK1 columnwith some TLDs

The gamma doses were obtained by the analysis of the thermoluminesce emission from TLDs-700, with the method based on the heights of the first and of the second dosimetric peak.

The method allow to obtain the gamma dose with a formula, without perfoming other measurements to subtract thermal neutrincontribution in the dosimeter response.

Concerning the gamma dose, MC calculations cannot be utilized, because MC evaluates only the contribution of the reactions of thermal neutrons with H in the dosimeter itself

1H(n,γ)2H Eγ = 2.2 MeV

but this dose has to be added to the gamma dose from reactor background and moderating materials.

Experimental measurements were performed to investigate the consistency of the gamma dose measured with Fricke gel layers.

Gamma dose

HK1Gamma dose

BNCTcolum

nGamma dose

Gamma dose rate profiles extracted from dose images obtained with gel dosimeters and gamma dose rate values obtained from the TLD-700 GCs (exposed in two columns of the LVR-15 reactor)

BNCT column

HK1 column

Gamma dose

Experimental results have confirmed that gel

dosimeters in do not affect the gamma

dose, within the experimental error, also in

the case of in-air measurements.

Gamma dose

Not negligible effects were found for thermal neutron fluence.

MonteCarlo calculations and measurements have been performed for the epithermal BNCT beam moderated with a disk of 2 cm of polyethylene.

(Note: Thermal neutron contribution in epithermal beams is too low it is within the error of gamma dose values).

Thermal neutron fluence

Comparison between thermal neutron fluence transversal profiles calculated (with MC) and measured (with TLDs) : in a strip (thickness 1 mm, width 10 mm, length 120 mm) In the central plane of a gel dosimeter (circular, with 12 cm of

diameter and 5 mm of thickness)

In order to avoid perturbation of thermal neutron fluence, in-air measurement can be performed with Fricke gel dosimeters in slim transparent plastic tubes (external diameter : 3 mm).

With such dosimeters thermal neutron fluence profiles can be attained.

Monte Carlo calculation have been performed to investigate the increasing of thermal neutron fluence with increasing the thickness of moderating materials.

LVR-15 Epithermal beamfor BNCT

Water layers :Thickness 1 mmDiameter 120 mm

Transversal profiles of the fluence of neutrons with energy < 0.5 eV after disks of water (12 cm of diameter) of different thickness

…then after 20 mm, 21 mm, 22 mm, 23 mm, 24 mm 25 mm

Transversal profiles of the fluence of neutrons with energy < 0.5 eV after disks of water (12 cm of diameter) of different thickness

… after 20 mm, 21 mm, 22 mm, 23 mm, 24 mm 25 mm

Thermal neutron fluence is increased by the backscattered thermal neutrons. This effect can explain the obtained results.

…… WORK IN PROGRESS ……

Couples of dosimeters with different isotopic content

Standard Gel: -rays and fast neutrons (recoil-protons)

Standard-Gel added with 10B (40 ppm): -rays, fast neutrons, and 7Li particles

Gel like Standard-Gel made with heavy water: -rays and fast neutrons (recoil-deuterons)

Separation of the different dose components

We are also studying the various factors that give uncertainty to the absolute values of the various measured doses.

Standard gel dosimeter

Borated gel dosimeter (40 ppm 10B)

Boron dose evaluation

DB = [ k1Δ(OD)borated - k2Δ(OD)standard ] / 0.41

Relative sensitivity to DBoron with respect to Dγ

k1 = -ray calibration coefficient of Borated gelk2 = -ray calibration coefficient of Standard gel

Fast neutron and gamma doses separation

(OD)st = α1Dγ + α2Drp

Standard gel dosimeter + heavy water dosimeter

α 1 = st gel sensitivity to rays

α 2 = st gel sensitivity to recoil protons (rp)

α 3 = hw gel sensitivity to rays

α 4 = hw gel sensitivity to recoil deuterons (rd)

Dfast = α 3∙ (OD)st – α1∙ (OD)hw α 1 α 3 – α1 α 4 f

Dγ = (OD)st – α2 ∙ Drp α 1

f = Drd/Drp = 0.66±0.01

from calibration

from Bragg peak measurements

from Monte Carlo

(OD)hw = α3Dγ + α4Drd

TROUBLE: Dependence of Gel-dosimeter response on radiation LET

Studies to improve these factors are in progress.

RADIATION RELATIVE SENSITIVITY

and 7Li particles 0.41

Low-energy protons 0.85

Low-energy deuterons 0.55

For the relative sensitivities to high-LET radiations with respect to photons we have adopted the following values:

Measurements are in progress to verify or eventually amend the coefficient related to the dependence of dosimeter response on radiation linear energy transfer (LET).

In literature, there is lack of information about this topic.

In order to carry out experiments on this matter, measurements with neutrons, protons and carbon ions are planned.

FINALLY: Precision of the evaluation of thermal neutron fluence

Thermal neutron fluence images are obtained from boron dose images, by means of KERMA factor, evaluated for the amount of 10B introduced in the Fricke solution for the dosimeter preparation (usually 40 micrograms per gram of the final gel)

possible error in boron compound weigh

Dosimeter calibration to thermal neutron (for each preparation) could eliminate this problem, but this calibration is not always possible and, however, it is not quicky performed.

First Remark

Great care also in dosimeter calibration with photons has to be paid.

In fact, an error 10% of the calibration coefficient would give an error of about 35% of the boron dose (evaluated in the hypothesis of validity of the coefficient 0.41 for the relative sensivity to charged particles from 10B reactions.

The error of the coefficient 0.41 could increase consistently the error of boron dose determination and of the evaluated thermal neutron fluence.

Final Remark

In spite of all the described troubles, Fricke gel dosimeters in form of layers or slim tubes offer great potentiality in BNCT dosimetry, in particular for in-phantom measurements but also for in-air beam characterization.

The method deserves to be improved, principally with research of the dependence of the response on radiation LET.

THANK YOU

FOR THE ATTENTION