Development of a Thermal Neutron Source based on a Medical ...

Development of a Thermal

Neutron Source based on a

Medical Electron Linac

Valeria Monti

Second Year Seminar, XXX cycleDecember 15th, 2016

• E_LiBANS project

• Physics of the thermal photo-neutron source

• Source installation in Turin Physics Departement

• Linac setting

• Photo-converter studies

• Diagnostics and on-prototype measurements

• Looking forward

December 15th, 2016 Valeria Monti 2

Outline

Aim:

Building an intense thermal neutrons source, with high purity levels, based on a compact linear accelerator

Applications:

- Irradiation of cell specimens and tissue samples for boron neutron capture therapy (BNCT) pre-clinical

research

- Instrument calibration

- Detectors R&D


Thermal neutron sources around the world:

E_LiBANS project

Nuclear reactors D-D/D-T sealed tubes protons accelerators radioisotopes

neutrons from high energy photon beams

Medical high

energy eLINAC

produces photons

‘PHOTOCONVERTER’

produces neutrons

and moderates them

to the wanted energy

Specific

DIAGNOSTICS

qualify the produced

neutron field

compact, reliable,

safe, not expensive

Experimental cavity

permanently monitored by

E_LiBANS project


Thermal Photo-Neutron Source

Electrons

~18 MeV

Linac Target

X-ray emission

by Bremsstrahlung

W/Pb

neutrons

Aim: to maximise the thermal neutrons production avoiding fast neutrons and photons contamination

inside the experimental cavity

High Z

target

Photon SourcePhoto-converter

detectors

Linac

collimators


Fast neutron

emission by γ,n

(1-2 MeV)

Photo-Neutrons Production Mechanism

Electrons

~18 MeV

W/Pb

neutrons

Aim: to maximise the thermal neutrons production avoiding fast neutrons and photons contamination

inside the experimental cavity

High Z

target

Electron SourceElectro-photo-converter

detectors


Fast neutron

emission by γ,n

(1-2 MeV)

Photo-Neutrons Production Mechanism

• γ,n reaction envolves only photons with

E>7MeV while the linac beam has a

bremsstrahlung energy distribution

• γ,n cross section maximum 600 mbarn at 13 MeV,

<2% of the total cross section. A huge amount

of unconverted photons to be stopped.

(σ (γ,n ) / σtot ) Eγ=13.4 Mev = 0.015

(γ,n) threshold

Bremsstrahlung photon spectrum at the Linac target


Conversion probability

Gamma cross sections in Lead

Reaction Features

• γ,n reaction gives rise to fast

neutrons (mean energy:1-2 MeV).

These need to be slowed down to

thermal energy. Moderation

process implies neutrons loss by

capture and escape

• Difficult to guide the neutrons in

the wanted direction, external

shield nedeed, relevant source-

cavity distance effect

Photo-neutron energy spectrum in 3 cm thick lead target


Reaction Features

July 2016 commissioning of the machine in electron and photon mode completed

Electron source:

18 MeV , 18 MeV without scatter foil

Linac Set-up


γ source:

15 MV, 18 MV, 18 MV without flattening filter

Source Installation – Elekta Precise Linac


Gain factor without

filters: 2.2 (E> Eth)

(γ,n) threshold

Linac parameters and k factor

Calibration 100MU=1Gy, dose at build up

in standard water phantom

SSD 100 cm, Field 10x10 cm2


CROSS BEAM PROFILES

Photon beam 15 MeV with Flattening Filter

Photon beam 18 MeV without Flattening Filter

Crossline Inline

Linac Calibration

cm+10-10

cm

Project target limits

<1 mSv/year in control room and all other places classified as ‘supervised area’

150 Gy/week max work load


Control room

labyrinth

Primary barrier

Linac head

Bunker door

Measurements in rilevant point with a

Berthold Neutron Dose Rate Meter for

neutron detection, scintillators and

Ionisation Chambers for gamma dose

All values below the limits

Simulations to check dose invariance

with and without neutron conversion

structure

Radiation-Protection Measurements

MCNP6 simulation study in order to reduce contaminations

Lead target

Graphite

Heavy water

Tungsten target

Polyethylene

Air • W +Pb target production and

gamma shielding

• D2O and Graphite moderation

and reflection

• Boron carbide in polyepoxide

capture of thermal neutron going

out of the photoconverter

• Thin lead shield for capture

photons from carbon and boron Jaws collimator


Incident photon beam

Photo-Converter Beam Shaping Assembly


Similar g,n microscopic

cross section GDR model

Inhelastic scattering cross section

W activation problem

Photon absorption cross section (g,n)

Neutron absorption cross section

Neutron inhelastic scattering cross section

cros

s se

ctio

n(b

arn)

cros

s se

ctio

n(b

arn)

cros

s se

ctio

n(b

arn)

WPb

0

∞

𝜎𝑎𝑏𝑠 𝐸𝛾 𝑑𝐸 =𝜋 𝑒2 ℎ

𝑀𝑐

𝑁 𝑍

𝐴

𝜌𝑊 = 19.2 𝑔𝑟/𝑐𝑚3

𝜌𝑃𝑏 = 11.3 𝑔𝑟/𝑐𝑚3

Transparence

Slowing down

but

Production

74184𝑊 + 𝑛 → 74

185𝑊 → 75185𝑅𝑒 + 𝑒− + ν𝑒

𝑡12= 75,1 𝑑

Gamma shield+

Choice of the Target Material W-Pb

Expandable cavity for different objects exposure


Possibility to expand the cavity from 3 cm to 33 cm depth with 5 cm steps, without

changing the surrounding heavy water layer

( … )



• Cavity

(28.6*28.6*10)cm3

• Total weight c.a. 1600 kg

on a suitable movable

support

• W+Pb-Target

(30*30*20) cm3

Photon beam

direction

cavity




83%

15%

2%

Fluence rate in cavity

thermal (9.6 ± 0.1)*106 cm-2s-1 83.3%

epithermal (1.69 ± 0.02)**106 cm-2s-1 14.6%

fast (2.43 ± 0.07)*105 cm-2s-1 2.1%

gamma 1.05*106 cm-2s-1

Neutrons and Photons Energy Spectrum


𝐷𝑓

𝜑𝑡ℎ= 1,6 ∗ 10−13 𝐺𝑦 𝑐𝑚2

𝐷𝛾

𝜑𝑡ℎ= 7, 1 ∗ 10−13 𝐺𝑦 𝑐𝑚2

IAEA in air free beam parameter:

MNCP6 simulation

Standard working conditions

Assuming working rate at 400 MU/min:

Energy 18 MeV

Electron current on target 1.05x1014 e-/s

Distance linac target – photo-converter: 59 cm

Photo-Converter Expected Spectrum


Longitudinal profile (// to beam axis)

Decrease in the first cms while cavity

stretching: 25%

10 cm depthvariation 5%



Neutron Field Characterization

Average thermal fluence rate

decrease while cavity stretching:

35%


Cross profile (⊥ to beam axis)

Neutron Field Characterization

10 cm thicknessvariation 5%



9-10 September 2016 - Maesurements at

San Luigi Hospital – 18 MV Elekta Linac

Photoconverter small prototype

(ca. 600 kg)

Passive detectors

BDT BD-PND bubble detectors

Active detectors

Thermal Neutron Rate Detector TNRD

SiC sensors

Vented ionisation chambers


Photo-Converter Small Prototype Measurements

BDT BD-PND bubble detectors

Working rate 100 MU/min

Exposition with Cadmium cover to quantify the residual responce

to fast neutrons

Dosimeter Sensitivity*

Bubbles/μSv

Dose rate

μSv/s

Simulation

Expectation

μSv/s

BDT 1 Cd 0.31±0.04 9.4±1.5 9.5

BDT 2 Cd 0.43±0.06 7.3±1.2 9.5

BDT 1 1.43±0.2 10.0±2.9** 7.6

BDT 2 2.7±0.3 11.9±3.0** 7.6

BDT 3 2.6±0.5 11.0±2.6** 7.6

PND 4 fast 0.32±0.06 5.4±0.4 6.7

PND 5 fast 0.31±0.06 5.0±1.2 6.7


* Dosimeters recalibration at Esther facility in Milano

** After fast dose subtraction

1 2 3 4 5 6 7 8 9

0

2

4

6

8

10

12

14

16

simulazioni

misure

Passive Diagnostics


Vented ionisation chambers

SiC with Lithium deposit

• Calibrated at Hotnes Am-B source and at TRIGA

reactor ENEA Casaccia metrologically qualified

New ELiBANS Active Thermal Neutron Detectors

6LiF deposite on different substrates

36𝐿𝑖 + 𝑛 → 1

3𝐻 + 𝛼

TNRD

• Active

• Unbiased

• Low noise ( 100 e-)

• Photon insensitive

• Small dimension

• Able to work at high fluence rates

• Pulse mode and Current mode DAQ system

Linearity of the thermal neutrons production

with the linac dose rates (50-550 MU/min)

Uniformity of the cavity center-corner difference 2%

0

0,05

0,1

0,15

0,2

0,25

0 100 200 300 400 500 600

TNR

D r

esp

on

se(V

)

Linac dose rate (MU/min )

neutron production vs Linac electron current

9

9,1

9,2

9,3

9,4

9,5

9,6

center up-left up-right down-right

TNR

D r

esp

on

se(V

s)

position in the cavity

neutron abundance vs position


Active Detectors Measurements

electrons

Lead

Graphite

Heavy water

Tungsten

air

W target --> (r=7cm) +Pb target (Rtot=10cm)

Cavity dimension(20*20*5)cm3

channel Ø=2 cm

Electron pencil beam directly impinging the electro-photo-converter spherical target for major conversion

efficiency

Possibility to move the cavity out of the primary beam direction without disuniformity inconvenience (isotropy of

neutron emission)


Electron-Photo-Converter with e- Source

88%

11% 1%

𝐷𝑓

𝜑𝑡ℎ= 7,2 ∗ 10−14 𝐺𝑦 𝑐𝑚2

𝐷𝛾

𝜑𝑡ℎ= 6,6 ∗ 10−13 𝐺𝑦 𝑐𝑚2

Thermal neutron flux: 1.0*108 cm-2s-1


Standard working conditions

Assuming working rate at 400 MU/min:

Energy 18 MeV

Frequency 200 Hz

Period 2.4 μs

Peak current 35 mA

Electron current on target 1.05x1014 e-/s

Enhanced thermal flux and high quality field

Neutrons and Photons energy spectrum

Electron-Photo-Converter with e- Source

• My work is devoted to the construction of an intense thermal photo-neutron source

• I worked on the linac installation providing simulations for the radiation-protection

project and validating the results by measurements of the doses and I was involved

in the machine calibration

• The main part of my work has been the study of the best configuration for the

photo-converter in terms of geometry and materials

• The assembly of the final geometry is currently underway

• The development of active thermal neutron detectors has been successful in terms of

linearity response and under high intensity neutrons fluxes

• A matrix of active detector is under construction to online monitor the field in the

cavity of the photoconverter


Conclusions and Outlook

• The complete metrological characterisation of the thermal neutron field for different

cavity dimensions will be done and a comparison with the simulation will be studied

• Thermal filed characterised with the most performant active detectors among the

different types under development (including comparison with a standard

reference – golden foils)

• Fast component quantified by measurements with Cd shields

• Gamma contamination evaluated by means of TLD diagnostics

• The feasibility of an electro-converter will be studied and a demonstrator will be

developed


Future Plan

The legal end of my doctorate period will be on 21st April 2018 because of the 6 months suspension for TFA last year

• 17° International Congress on Neutron Capture Therapy , 2-7 October 2016, Columbia, Missouri

• 8° Yuong Researcher BNCT Meeting, 13 September 2015, Pavia, Italy

• E. Durisi, K. Alikaniotis, O. Borla, F. Bragato, M. Costa, G. Giannini, V. Monti, L. Visca, G. Vivaldo, A.

Zanini Design and simulation of an optimized e-linac based neutron source for BNCT

research, Applied Radiation and isotopes 106 (2015) 63-67

http://dx.doi.org/10.1016/j.apradiso.2015.07.039

• K. Alikaniotis, O. Borla, V. Monti, G. Vivaldo, A. Zanini, G. Giannini Radiotherapy dose

enhancement using BNCT in conventional LINACs high-energy treatment: simulation

and experiment Reports of practical oncology ad radiotherapy 21 (2016) 117-122

http://dx.doi.org/10.1016/j.rpor.2015.07.003

• M. Costa, E. Durisi, V. Monti, L. Visca, A. Zanini and G. Giannini Neutron sources based on medical

Linac Il Novo Cimento 38 C (2015) 180 DOI 10.1393/ncc/i2015-15180-4


Related Orals and Papers

http://dx.doi.org/10.1016/j.apradiso.2015.07.039

http://dx.doi.org/10.1016/j.rpor.2015.07.003

Thank you for your attention!

Elibans collaboration:

INFN Torino: M. Costa, N. Amapane, E. Durisi, R. Gerbaldo, V. Monti, U.Nastasi, M. Ruspa, L. Visca, A. Zanini

INFN LNF: R.Bedogni, J.M. Gomez-Ros, M. D. Sacco, M. Treccani, O. Sanchez

INFN Trieste: G.Giannini, D. Treleani, M. Vascotto, K. Alikaniotis

Politecnico Milano: A. Pola, D. Bortot, L. Garlati, A. Porta

San Luigi Hospital and San Giovanni Bosco Hospital : S. Anglesio, U. Nastasi


BNCT parametersIAEA Tec Doc 2001

Electron-mode Linac + electron-photo-converter

Photons/cm2 per source particle

Beam pipe shaped to have the maximum production in the centre of the sphere:• Tungsten radiation length 𝑋0=

0.54cm, 𝐸 = 𝐸0𝑒−𝑥

𝑋0

• Critical energy (end of bremsstrahlung domination) Ecr = 9.51 MeV

MCNP6 simulations in order to find the suitable material and radius of the spherical target

Photon with E>6.5 MeV distribution

Neutron pruduction vs W radius


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