Radiation Characteristics of a Gated Fibre-Optic-Coupled Detector

• Chester Reft• University Of Chicago• Department of Radiation and Cellular Oncology

Purpose: to study some dosimetriccharacteristics of a gated fibre-optic-coupled

detector

Energy dependence for 6 and 18 MV photons, orthovoltage

Energy dependence for 6 20 MeV electrons

Dose linearity

Dose rate response

Reproducibility

Small field dosimetry

Radioluminescence is the phenomenon by which luminescence is produced in a material by ionizing radiation

Prompt luminescence exhibited by all glasses

Incident native impuritiesluminescene

radiation defects

Mechanisms for Irradiation-induced Luminescence in Materials

recombination of EHPs created by irradiation

transient or permanent defects produced by radiation and subsequently serve as recombination centers for mobile electrons or holes

Features of Optical Fibre Radiation Dosimeters

in-situ, real time dose or accumulated dose measurementswide dynamic range – 10-5 4x103 Gysmall sizerelatively isotropic responsesmall energy dependence for megavoltage γ and e-

all optical no electromagnetic interferenceminimal environmental effects

Potential Radiation Therapy Applications

In-vivo, real time patient dose monitoring3DCRTIMRT (?)

Small field dosimetry

Fluoroscopic procedures (skin dose)

CT or PET procedures

The detector is a transparent fused-quartz glass

doped with Cu+1 ions

~ 1 mm long and 400 μm diameter

Comparison of Detectors

0.330.451.0sens vol th(mm)

3.6x1010

e-/Gy3.3x1010

e-/Gy5x105

cts/Gysens 60Co

2.52.40.4diameter(mm)

4.35.00.13sen area (mm2)

2.622.332.2ρ(g/cm3)

61410.8Z

DiamondDiodeFiberCu1+ silica

Optical fiber Detector Assembly

Irradiation

Detector Optical Fiber

native fluorescence+

Cerenkov

+phosphorescence

native fluorescence

+

Cerenkov

Gating

Phosphorescence from detector

B.L. Justus et al.Applied Optics 43 (2004)

X-rays irradiate Cu1+ detector and segment of optical fibreOptical fibre coupled to photon counting moduleAccelerator trigger pulses synchronized with the current serves as a gate

measures the duration of each gate pulse measures the interval between pulses

Data from the counters are buffered and then analyzed

Schematic diagram of the electronics

B.L. Justus et al.Applied Optics 43 (2004)

Luminescence decay of the Cu1+ emission following pulsed excitation (solid curve)Also shown is the 8 μs wide accelerator trigger pulse (dashed curve)

SW Phantom

100 cm

Fixed Collimator

Adjustable Collimator

Target

FlatteningFilter

Figure Schematic diagram of experimental set-up showing NRL detector and monitor detector

NRL detectorTo electronics

NRL monitorTo electronics

1.02020 MeV1.00616 MeV0.99812 MeV0.9769 MeV0.9506 MeV0.99318 MV1.0006 MV

Q(Energy)/Q(6 MV)Energy

Detector energy response normalized to 6 MV photons

Mass Energy Absorption Coefficient of water to Si Dioxide

0.96

0.98

1

1.02

1.04

1.06

1.08

1.1

1.12

0 2 4 6 8 10 12

Energy (MeV)

[(μen/ρ)watSiO]18MV

6MV = 0.97

(R/D)6MV18MV ≈ [(μen/ρ)wat

SiO]18MV6MV

1.01 ≈ 0.97

Mass Stoping Power Ratio of water to pyrex

1.04

1.06

1.08

1.1

1.12

1.14

1.16

1.18

1.2

1.22

0 5 10 15 20 25

E (MeV)

[(S/ρ)watSiO]6 MeV

20MeV = 1.07

[(R/D)]20MeV6MeV ≈ [(S/ρ)wat

SiO]6MeV20MeV

1.07 ≈ 1.07

NRL Detector Linearity

-

2.00

4.00

6.00

8.00

10.00

12.00

14.00

- 2.00 4.00 6.00 8.00 10.00 12.00 14.00Dose (Gy)

NR

L si

gnal

nor

mal

ized

to 1

G

6 MV18 MV12 MeVr = 0.99999Linear (6 MV)

Deliver 50 MU at a depth of 10 cm for a 10x10 cm2 fieldnormalized to 100 MU/min

1.00 ± .020.96 ± .02600

1.001.00 100

18 MV6 MVMU / min

Detector variation with dose rate relative to the Farmer chamber

0.96

0.97

0.98

0.99

1

1.01

1.02

1.03

1.04

1.05

0 2 4 6 8 10 12Dose Rate

Q(N

RL

) / Q

(Far

mer

18 MV

6 MV

Linear <6 & 18 MV>

Linear (Linear <6 & 18 MV>)

0.33 0.67 1.0

Gy/minmGy/pulse

P.W. Hoban et al.PMB 39 (1994)

Detector Reproducibility

± 0.3 %± 0.5 %± 1.0 %± 1.5 %

Ion chamberdiodediamondOptical Fiber

0.6460.6540.647

0.5150.5240.505

0.6300.6450.632

0.5060.4990.493

0.6280.6250.621

0.4910.4730.479

25DiodeNRLDiam

0.7480.7530.748

0.6420.6470.632

0.7340.7230.733

0.6340.6240.622

0.7480.7010.723

0.6230.6240.611

20DiodeNRLDiam

.08650.8690.865

0.8010.8110.796

0.8580.8540.858

0.7980.7650.792

0.8790.8630.851

0.7900.8020.780

15DiodeNRLDiam

1.0001.0001.0001.0001.0001.00010

DiodeNRLDiam

1.1431.1301.138

1.2481.2771.250

1.1721.1491.159

1.2741.2811.261

1.1771.2011.168

1.2211.2921.278

5DiodeNRLDiam

18X6X18X6X18X6X

2.0x2.01.0x1.00.6x0.6d(cm)

Small field TPR measurements with detectors

1.20.2

0.70

0.50

-1.1-0.4

18 MV

2.40.3

-1.5-0.1

-0.50

-1.9-1.1

18 MV

-0.5-1.1

-6.3-3.3

-1.8-3.2

2.0-0.8

18 MV

1.7-1.9

-1.4-2.6

-3.7-2.4

25 – NRL- Diam

0.7-1.5

-1.6-1.9

0.2-1.9

20 – NRL- Diam

1.2-0.6

-4.1-0.8

1.5-1.3

15 - NRL- Diam

1.10.2

0.5-1.0

5.84.7

5 - NRL- Diam

6 MV6 MV6 MV

2.0x2.01.0x1.00.6x0.6d(cm) - det

% difference of small field TPR measurements with NRL and diamond detectors relative to diode detector

0.992 ± .0120.991 ± .018Diamond

0.994 ± .0221.000 ± .025NRL

18 MV6 MV

Averaged ratio of TPR measurements with NRL and diamond detectors relative to the diode detector

Future work

Modify the software to allow measurements for orthovoltage radiation

Therapy

Diagnostic

1.003.60110250 kVp1mm Cu

1.113.9992200 kVp1.0mm Cu

1.435.1464200 kVp.2mm Cu

1.686.0457150 kVp0.2mm Cu

1.977.0936125 kVpNo filter

<cts>/cGyNorm 110KeV

<cts>/cGy<E0>KeV

Quality

Mass Energy Absorption Coefficient of water to silicon dioxide

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250 300 350 400 450Energy (KeV)

0.8010.02590.0323110

0.2560.1040.40736

(μen/ρ)watSiO(μen/ρ)wat(μen/ρ)SiOE(KeV)

[(μen / ρ)]110kVp36kVp = 3.13

[(μen/ρ)watSiO]110KeV

36KeV = 3.13

(R/D)36KeV110KeV ≈ [(μen/ρ)wat

pyr]110KeV36KeV

1.97 ≈ 3.13

Conclusions

• Favorable Dosimetric properties compared to other detectors

– Energy response• Megavoltage – small variation• Orthovoltage – large variation

– Linearity– Dose Rate– Reproducibility

•Small size ideal for small field dosimetry such as stereotactic radiosurgery

•Small size and optical interface make them potentially useful for in-vivo dosimetry

Conclusions Continued