M.E. BrandanInstituto de Física UNAM
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