at the National Superconducting Cyclotron Laboratory (NSCL), Michigan State University, USA. The average linear energy transfer of the accelerated 86Kr ions ...
Radiation Protection Dosimetry Vol. 100, Nos 1–4, pp. 537–539 (2002) Nuclear Technology Publishing
RELATIVE THERMOLUMINESCENCE EFFICIENCY OF TLD-600 AND TLD-700 DOSEMETERS IRRADIATED WITH 59.8 MeV PER NUCLEON KRYPTON-86 IONS B Mukherjee†, R. M. Ronningen‡ and P. Cross§ † ANSTO, Safety Division, Menai, NSW 2234, Australia ‡ NSCL, Michigan State University, East Lansing, MI 4824–1321, USA § Radiological Physics, St Vincent’s Hospital, Sydney, NSW 2010, Australia Abstract — A batch of LiF thermoluminescence dosemeters (TLDs), each containing five TLD-600 and TLD-700 thermoluminescence dosemeter chips, was irradiated with 59.85 MeV per nucleon 86Kr20+ ions from the K1200 superconducting cyclotron at the National Superconducting Cyclotron Laboratory (NSCL), Michigan State University, USA. The average linear energy transfer of the accelerated 86Kr ions and the resulting dose imparted to the TLD chips were calculated to be 3343 keV.m⫺1 per ion and 1.68 Gy respectively. A similar batch of TLD chips was irradiated with 1.3 MeV gamma rays from a 60Co source to 1.0 Gy. The TLD chips were evaluated at a ramp heating rate of 10°C.s⫺1 to 400°C using a hot-finger type TLD reader. The thermoluminescence efficiency of the TLD-600 and TLD-700 dosemeters, relative to 60Co gamma rays was calculated to be 0.0025 and 0.0027 respectively
INTRODUCTION The thermoluminescence efficiency of LiF thermoluminescence dosemeters (TLDs) to various types of ionising particle, such as protons (1–3), alpha particles (1,4– 8) and energetic heavy charged particles (8,9) has been studied by various researchers. The main objectives of these investigations were threefold: understanding the physical phenomenon of the interaction of the charged particle with the TL phosphor (10), examination of the suitability of the TLD ion space radiation dosemetry (9,11) and feasibility studies of the application of TLD for dose mapping of heavy ion radiotherapeutic beams (8,12). Previous results indicated that the TL (light conversion) efficiency (2) drops sharply with increasing atomic number and linear energy transfer (LET) of the projectiles bombarding the dosemeter material. This paper highlights the experimental method for the estimation of the relative thermoluminescence efficiency of TLD-600 and TLD-700 dosemeters irradiated with 59.85 MeV per nucleon 86Kr20+ ions generated by the K1200 superconducting cyclotron at Michigan State University, USA.
plate avalanche counter (PPAC) measured the fluence and uniformity of the beam. Its effective thickness was 1.0 mg.cm⫺2 of carbon. The beam then emerged into air via a vacuum window constructed from a 14.01 mg.cm⫺2 zirconium foil. The energy loss of the ion beam as it traversed the PPAC, zirconium foil, air gap and aluminised Mylar packet material containing the dosemeters was computed using the SRIM code (14). The total energy of the ion beam entering in the LiF dosemeters was calculated to be 4.37 GeV. The experimental set-up at the heavy ion irradiation station of the NSCL is shown in Figure 1. The TLD-700 (7LiF: Mg,Ti) and TLD-600 (6LiF: Mg,Ti) chips (dimension 3.2 ⫻ 3.2 ⫻ 0.9 mm3) used in this experiment were commercially supplied by Har-
T
MATERIALS AND METHODS An ion beam of 86Kr20+ was accelerated by the K1200 cyclotron to 5.16 GeV total energy. The beam passed through a thin Al foil located after the exit of the cyclotron, which further stripped the beam. The fully stripped ions were magnetically selected using the NSCL’s A1200 fragment separator (13), operating in a beam transport mode. The subsequent beam had a total energy of 5.15 GeV. This beam was transported to the irradiation endstation. In that position a two-dimensional parallel
Contact author E-mail: mukherjee얀ieee.org. 537
1
2
Figure 1. Photograph showing the pouch (1) containing TLD600 and TLD-700 chips positioned at the heavy ion irradiation port of the K1200 superconducting cyclotron. The pouch (2) with the control (TLD-600 and TLD-700) chips is attached to the external wall of the beam tube (T).
B. MUKHERJEE, R. M. RONNINGEN and P. CROSS
recorded with the PPAC (13) (8.49 ⫻ 105 ions.cm⫺2), a is the cross sectional area of the TLD chip (0.32 ⫻ 0.32 cm2 ⫽0.102 cm2), L is the average LET of 86Kr20+ ions in LiF (3343 keV.m⫺1 per ion), d is the thickness of the TLD chip (0.09 cm), v is the volume of the TLD chip (0.32 ⫻ 0.32 ⫻ 0.09 cm3 ⫽9.44 ⫻ 10⫺4 cm3) and is the density of LiF (2.7 g.cm⫺3). By substituting the above numerical values in Equation 1 the imparted dose (D86Kr) of the 59.8 MeV per nucleon 86Kr20+ ions in the LiF TLD chips was calculated to be 1.68 Gy. The relative thermoluminescence efficiency 86Kr of 86 Kr20+ ions in the TLD chips relative to 60Co gamma reference radiation is defined as follows (2): HZE ⫽ (A86Kr D86Kr)/(A60Co/D60Co)
where A86Kr is the area under the glow curves of the Kr20+ ion irradiated TLD-600 and TLD-700 chips (Figure 3(a)), A60Co is the area under the glow curves of the gamma ray irradiated TLD-600 and TLD-700 chips (Figure 3(b)), D86Kr is the dose delivered to TLD chips by 86Kr20+ ions (1.68 Gy) and D60Co is the dose delivered to TLD chips by 60Co gamma reference radiation (1.0 Gy). By substituting the numerical values in Equation 2 the relative TL efficiencies for TLD-600 and
350
370
390
350
370
330
310
290
270
250
230
210
190
170
10,000
310
290
270
250
230
110
210
5000 0
2000
150
15,000
190
3000
20,000
170
4000
330
25,000
5000
Temperature ( C) 0.90
0.80
Depth in LiF (mm)
0.70
0.60
0.50
0.40
0.30
0.20
0.10
1000 0
110
ions
150
Kr
30,000
130
20+
(b)
(1)
6000
0
Ionisation: (keV.mm-1 per ion)
where N is the integrated fluence of
86
Temperature ( C)
TL output (counts)
D86Kr ⫽ 1.602 ⫻ 10⫺13 NaLd/v
200 180 160 140 120 100 80 60 40 20 0
130
(a)
RESULTS AND DISCUSSION The average LET of the 86Kr ions in the LiF was evaluated from the energy loss curve shown in Figure 2 and calculated to be 3343 keV.m⫺1 per ion. The heavy ion induced dose in TLD chip D86Kris given as:
(2)
86
TL output (counts)
shaw-Bicron Co., USA. Two pouches each containing five TLD-600 and five TLD-700 chips were made from 10 m thick aluminised Mylar foil with a thin cardboard backing. Pouch 1 was placed at the irradiation port directly intercepting the ion beam path and pouch 2 was attached to the external side the beam tube (T) and served as the control group (Figure 1). The 86Kr20+ beam was switched on and irradiation was continued until an integrated particle fluence of 8.49 ⫻ 105 ion.cm⫺2 was evaluated by integrating the PPAC spectrum. The impinging 86Kr20+ ions were completely stopped in the LiF dosemeter chip. The energy loss (ionisation) of the 86 Kr20+ ion beam in LiF was calculated using the SRIM (14) code and is shown in Figure 2. Upon return to Australia, a third batch of dosemeters consisting of five TLD-600 and five TLD-700 chips were irradiated with 1.3 MeV gamma rays from a 60Co source to 10 mGy. All TLD chips were evaluated at a ramp heating rate of 10°C.s⫺1 to 400°C using a hotfinger type TLD reader. The TL signal produced by the control dosemeters (pouch 2) was considered as background. This background was primarily caused by the secondary gamma radiation produced during the course of heavy ion bombardment at the cyclotron (Figure 1) and cosmic radiation during the passage of the dosemeters from the USA during the intervening commercial passenger flight (15). This was subtracted from the TL output of the dosemeters irradiated with heavy ions (pouch 1). The TL glow curves of the dosemeter chips irradiated with 86Kr20+ ions and 60Co gamma rays are shown in Figures 3(a) and 3(b) respectively.
Figure 2. Energy loss (ionisation) distribution of 59.85 MeV per nucleon 86Kr20+ ions in the LiF dosemeter chips evaluated with the SRIM ((14)) program. 538
Figure 3. Glow curves of the TLD-600 (쑗) and TLD-700 (쎲) chips irradiated with (a) 59.8 MeV per nucleon 86Kr20+ ions produced by the K1200 superconducting cyclotron and (b) with 1.3 MeV gamma rays from a 60Co standard source. The areas (A) under the glow curve are (a) TLD-600 6802 counts, TLD700 10,195 counts; (b) TLD-600 1,647,481 counts, TLD-700 2,272,826 counts.
RELATIVE TL EFFICIENCY OF TLD-600 AND TLD-700 DOSEMETERS
TLD-700 chips were calculated to be 0.0025 and 0.0027 respectively. These are shown in Figure 4 as a function of LET. The TL efficiency of 59.85 MeV per nucleon 86Kr20+ 1
Relative TL efficiency
10-1
10-2
10-3
10-4
1
10
102 103 104 LETLiF (keV.mm-1 per ion)
105
ions for the TLD-700 dosemeter evaluated by us fits well within the data cluster for various heavy charged particles evaluated by other investigators. These results confirm the phenomenon of the saturation effect (8–10) of the secondary particle tracks produced in the dosemeter. The track saturation effect results in the rapid decrement of TL efficiency with the ionisation density or LET of the interacting heavy charged particles. Furthermore, the result also reveals that at very high LET, the linear relationship between the area under the high-temperature (HTR) glow peak and LET (11) is found to be invalid. The TL efficiency of 86Kr20+ ions evaluated in this work could be of importance for the dose planning of heavy charged particle radiotherapy. Further investigations are planned in this connection. ACKNOWLEDGEMENT
Figure 4. The relative TL efficiency of TLD-700 dosemeters irradiated with various types of energetic heavy charged particle ranging from 1H to 131Xe as a function of LET. Key: 䉬, 1 (2) H ; 앳, 1H(1); 왖, 4He(8); 왕, 7Li(8); ⫻, 12C(8); 쐌, 86Kr (this work); ∗59Ne(8); 쏔, 122Sb(8); 쮿, 131Xe(8).
The authors thank the K1200 cyclotron operations staff for providing the 86Kr beam and technical support. The principal author (BM) thanks the NSCL for supporting his visit and this research. This research was supported by the US National Science Foundation under Grant no PHY-9528844.
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