radiation dosimetry with tamu beams

M.E. BrandanInstituto de Física UNAM

brandan@fisica.unam.mx

Radiation dosimetry Radiation dosimetry with TAMU beamswith TAMU beams

About thermoluminescence About thermoluminescence Thermoluminescence (TL) is

the emission of light -induced by heating- from a material previosuly exposed to ionizing radiation

TL materials are dielectric crystals containing activators Traps for charge carriers Luminescence centers which

emit light

TL materialsTL materials TL materials are used a dosemeters (TLD) because the emitted

light is (almost) proportional to deposited energy. A linear response is observed over a limited dose interval (few

microGy to tens of Gy).• The best known properties are those associated to their use as

X- or • LiF:Ti,Mg (TLD-100)• LiF:Mg,Cu,P (TLD-100H)• CaF2:Mn (TLD-400)• CaSO4:Dy (TLD-900)

• They come as disks, rods or chips, a few mm in size.• They are not absolute dosemeters, they require a calibration.• Their response (emitted light per imparted energy) depends on

radiation type.

Project at Project at UNAM UNAM

• 3 MV Pelletron (IFUNAM) and TAMU cyclotron (College St, TX)• TL dosimetry laboratory• Interpretation

• Track Interaction Models (MC version MC) for supralinearity• Track Structure Models for efficiency

Systematic study of the TLD response to low- and intermediate- energy ions

• Efficiency (as a function of the ion’s everything)• Linearity (as a function of fluence)

TLD-100 glow curve TLD-100 glow curve (emitted light as function of heating)(emitted light as function of heating)

0 50 100 150 200 250 300 350 4000.0

5.0x100

1.0x101

1.5x101

90 mGy

Inte

nsid

ad T

L (u

.a.)

Temperatura (°C)0 50 100 150 200 250 300 350 400

0.0

5.0x100

1.0x101

1.5x101

2.0x101

2.5x101

83.6 mGy

Inte

nsid

ad T

L (u

.a.)

Temperatura (°C)

Gamma rays Ions

Ion irradiationIon irradiation• Non-uniform irradiation

(spatially localized)

• Non-uniform dose profile within each track

• High ionization density (LET=E/x) along the ion trajectory

X or

ions

Radial distance (Å)101 102 103

Dose

(Gy)

10-2

10-1

100

101

102

103

104

105

106

107

ABCD

Avila et al., J. Phys. D: Appl. Phys. 32 (1999) 1175-1181

ion

A B C D

0

When a charged particle traverses a medium,

it deposits energia via secondary electrons. The energy tranfer is quantified by

the lineal energy transfer, LET,LET= E/x

LET z2/v2

Ion high energy Ion low energy

Low LET High LET

Intermediate energy ions Intermediate energy ions (TAMU cyclotron)(TAMU cyclotron)

• In-air rradiation1H, 3He, 12C, 16O, 20Ne beams, up to 40 MeV/A incident energy

• Average fluence determined by an independent detector

• TLD-100 chips in “stacks” (i.e. various energies and LET simultaneously)

E2 E3 E4 E5 E6

E1

E1 E2 E3 E4 E5

Ion glow curveIon glow curve

For similar doses, glow curves

depend on LET

200 mGy

LiF:Mg,Ti glow curve shape LiF:Mg,Ti glow curve shape All ionsAll ions

G Massillon, I Gamboa, MEB, J. Phys. D: Appl. Phys. 40 (2007)

Glow curve shape depends on the

incident particle ionization density

Efficiency (relative Efficiency (relative to gamma rays) to gamma rays) for individual for individual peakspeaks

= TLsignal dose-1 mass-1)ion

ion, TLsignal dose-1 mass-1)

G Massillon, I Gamboa, MEB, J. Appl. Phys. 100 (2006)

Models describe the general features

of the data, not the detail

The relative efficiency for high-The relative efficiency for high-temperature peaks displays a maximum temperature peaks displays a maximum

as a function of LETas a function of LET

This reminds us (the 70s…) the induction of damage in cells exposed to ions

LET at the maximum, 100 keV/m, implies ionizations each ~ 2 nm ~ the DNA helix diameter: If you ionize at the right distance, both strands are broken, and the repair gets very difficult.

LET(keV/m)

Some resultsSome results• Low- and high- temperature peaks in TLD-100 exposed to ions display distinct behavior in terms of

o relative efficiency, o dependence of efficiency as a function of LET and

particle type. o Use of TLD-100 as a set of independent detectors?

• Model descriptions suggest a much stronger “two-hit” structure in the high-temperature peaks. Agreement between data and model do not permit a quantitative evaluation of target sizes, or the like • Details in the data suggest the possibility of correlation in the excitation of peaks 6a, 6b and 7• Presently, we study the “high LET” features of the TL response to low-energy X-rays

JBN, corresponding member, Mexican Academy of Sciences, Nov 2009