|Year : 2013 | Volume
| Issue : 3 | Page : 133-137
Development of prototype fiber optics dosimeter for remote radiation level measurements
Asha Jayaprakash, DN Krishnakumar, A Dhanasekaran, MT Jose, B Venkatraman, S.A.V. Satya Murty
Radiological Safety Division, Electronics, Instrumentation and Radiological Safety Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamilnadu, India
|Date of Web Publication||28-Jul-2014|
D N Krishnakumar
RSS/RSD, IGCAR, Kalpakkam, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Measurement of radiation levels in difficult-to-access and hazardous areas, such as hot cells, high active source storage areas, require refined and sensitive remote radiation level measurement techniques. Optical fiber dosimetry has been studied as an emerging method of monitoring radiation remotely and is suitable for use in confined environments that may be inaccessible using existing conventional electronic dosimeters or radiation survey meters. Being light weight and nonintrusive, optical fibers based dosimeters provide several advantages in the field of remote radiation dosimetry and in-vivo medical applications. A prototype fiber optic dosimetry system with extrinsic architecture is designed and developed using optically stimulated luminescence (OSL) technique at Radiological safety division, Indira Gandhi Centre for Atomic Research. The fiber optic dosimetry system uses OSL material like BaFBr: Eu to detect radiation and a bifurcated optical cable to illuminate the sensor with the suitable light source and also to guide the light from the sensor to the detector. Indigenously developed hardware is used for pulse processing and application software of the system is developed in Microsoft Visual Basic.Net. This paper depicts the characterization of the dosimetric material, development of hardware and software for the system and calibration of the system using standard source. The system uses Advantech APAX 5570 base controller with suitable modular add-on cards for data acquisition and controlling. Indigenously developed electronics is used for processing the pulses from the sensor attached to the tip of the bifurcated optical cable. The acquisition of the counts from the electronic circuit and illumination and bleaching time for the sensor pellet is controlled by application software developed in VB.Net. The system is calibrated by irradiating the pellet with different absorbed doses. The system explores the possibility of remote radiation monitoring using OSL technique in real time. The system is portable, simple to use and requires less user intervention to operate.
Keywords: Radiation dosimetry, optically stimulated luminescence measurements, remote radiation level measurements
|How to cite this article:|
Jayaprakash A, Krishnakumar D N, Dhanasekaran A, Jose M T, Venkatraman B, Murty SS. Development of prototype fiber optics dosimeter for remote radiation level measurements. Radiat Prot Environ 2013;36:133-7
|How to cite this URL:|
Jayaprakash A, Krishnakumar D N, Dhanasekaran A, Jose M T, Venkatraman B, Murty SS. Development of prototype fiber optics dosimeter for remote radiation level measurements. Radiat Prot Environ [serial online] 2013 [cited 2020 Apr 4];36:133-7. Available from: http://www.rpe.org.in/text.asp?2013/36/3/133/137479
| Introduction|| |
Real-time remote dose measurements have wide applications in radiation protection, dismantling operations in nuclear facilities, industrial process controls (X- or e-beam polymerization), nuclear medicine, and radiotherapy. Commonly used conventional electronic dosimeter systems have several shortcomings like use of external power supply with a high voltage (HV), degradation and sensitivity changes under irradiation.  Moreover, measurement of radiation levels in difficult-to-access and hazardous areas, such as hot cells, high active source storage areas; require more refined and sensitive remote radiation measurement techniques. Optical fiber dosimetry has been studied as an emerging method of monitoring radiation remotely and is suitable for use in confined environments that may be inaccessible using existing dosimeters.  They are also capable of measuring radiation for medical in-vivo applications and distributed measurements in environments of high dose rates.  Being light weight and nonintrusive, optical fibers provide several advantages in the field of dosimetry. Since the response of scintillator sensors are optical signals, these can be transmitted through optical fibers without loss and are not susceptible to electromagnetic interference.
Optical fibers can be utilized for radiation sensing using two methods. The first type of fiber radiation sensor has an intrinsic architecture, where the optical fiber is used as the radiation-sensitive material, in addition to guiding the optical signal to the detector. This architecture has been demonstrated based on the mechanisms of scintillation and photo darkening. , The example for this type of architecture is monitoring of radiation dose around the container filled with radioactive waste.  The second type is an extrinsic architecture, where the radiation-sensing component is spliced or coupled to an optical fiber. , In such cases, the fiber acts only as a wave guiding component to carry an optical signal from the sensing component to a detector. Scintillation and optically stimulated luminescence (OSL) are commonly utilized for this architecture. Two examples of materials used as the radiation-sensing component are Cu + -doped silica  and Al 2 O 3 :C.  Recent work with these materials demonstrates the ability of this sensor architecture to perform nonintrusive, in-vivo monitoring during radiotherapy. , If thermo luminescence material attached to the end of a multimode fiber optic cable, it emits light when it is heated by means of laser.  In applications where heating of the tip of the fiber is unacceptable, such as monitoring of dose to tumor during radiotherapeutic treatment of cancer patients, OSL dosimeter has importance where the dose can be read by stimulating with light.  Single channel BeO ceramic sensor based fiber optic dosimeter of small sensitive volume has a potential for use a reliable dosimeter in radiotherapy applications. 
This paper depicts the development of prototype single channel fiber optic dosimetry (FOD) system for remote radiation measurement at Radiological Safety Division (RSD), Indira Gandhi Centre for Atomic Research (IGCAR). The primary sensing mechanism used here is OSL in an extrinsic fiber sensing architecture. OSL material like BaFBr: Eu, which has a wide linear dose response is used as the sensor material. The characterization of the dosimetric material, development of hardware and software and calibration of the system is given in the paper.
| Materials and methods|| |
Characterization of dosimetric material
High sensitive BaFBr: Eu 2+ storage phosphor has been successfully synthesized using high temperature solid state diffusion route in a reducing atmosphere. , BaFBr: Eu pellets of 10 mm diameter and thickness 4 mm were prepared by sintering the pellets prepared from a mixture of high pure (99% purity) analytical reagent grade BaF 2 , BaBr 2 2H 2 O and EuF 3 ( 0.2 mol%) pressed at pressure of 1 kg/m 2 using a pelletizer. These pellets were fired in reducing atmosphere at 950°C for 4 h. The maximum OSL is obtained with the nonstoichiometric BaF 1.015 Br 0.985 :Eu 2+ phosphor composition. Optical characteristics of the BaFBr: Eu pellet are studied by taking photoluminescence (PL) and photo-stimulated luminescence measurements. The PL measurements were carried out using a spectrofluorometer. The PL emission and excitation spectra of BaFBr: Eu are shown in [Figure 1]. The PL emission wavelength is found to be 390 nm with the excitation of 330 nm light in this material. The blue light emitted by the material at 390 nm is matching with high sensitive region of most commonly used photomultiplier tubes (PMTs). The BaFBr: Eu is irradiated in a gamma chamber using Co-60 standard source (Board of Radiation and Isotope Technology make) available at RSD, IGCAR. The present dose rate of the source is 213 Gy/h. OSL emission and stimulation spectra of the phosphor pellet were measured using the spectrofluorometer. The OSL emission and stimulation spectra of BaFBr: Eu are shown in [Figure 2]. The stimulation spectrum obtained for the gamma irradiated pellet is quite broad, extending from 450 to around 600 nm, with two dominant peaks at 500 and 560 nm. The sharp small multiple peaks overlapped on the stimulation spectrum between 450 and 500 nm are related to lamp spectrum. In order to have a better signal to noise ratio, the stimulation and emission wavelengths should be well separated from each other. The spectrum clearly indicates that the emission light is in the blue region. Hence, in order to have a well separated emission and stimulation wavelengths, light source in the green region (~560 nm), a green light emitting diode (LED), is selected for illumination of the pellet.
|Figure 1: Photoluminescence's emission and excitation spectra of BaFBr:Eu|
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|Figure 2: Optically stimulated luminescence emission and stimulation spectra of BaFBr:Eu|
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For determining the variation of the OSL signal with dose, BaFBr: Eu pellets were irradiated to different doses using the gamma chamber. After irradiation, the BaFBr: Eu pellet was stimulated with green LED through an optic fiber and the OSL intensity was measured using flourimeter. The OSL spectra of the pellet for different doses are shown in [Figure 3]. From the figure it is evident that OSL intensity increases with absorbed dose up to 10 Gy, beyond which the response of the material tends to saturation level. It should be noted that the maximum OSL intensity in all absorbed doses occurs at the same wavelength, 390 nm.
|Figure 3: Optically stimulated luminescence spectra of irradiated BaFBr:Eu for various absorbed doses|
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| Results and discussions|| |
Design and development of fiber optic dosimetry system
The system design is classified into two viz., hardware design and software design. Hardware of the system consists of the main controller with its add-on modules, bifurcated optical fiber cable, PMT, signal conditioning electronics and sensor pellet. The application software is so developed that it controls and automates the operation of the whole system with very less user intervention.
Block diagram of the FOD system is shown in [Figure 4]. Ultra volt M series miniature module is used to generate HV for PMT from 12 V external battery supply. The voltage +12 V required for the amplifier is generated using Cosel DC-DC converter. A high gain operational amplifier buffer amplifier is used for conditioning the PMT signal. It is observed that the noise from the circuit is well below 50 mV. The LM311 series is a monolithic, low input current voltage comparator is used for discriminating the pulses from the amplifier. The lower level of the discriminator (LLD) is set by using a 10 K pot connected at the noninverting input of the comparator LM311. LLD for the circuit is set to 50 mV as the noise level from the amplifier circuit is <50 mV. The discriminator generates a 1 μs transistor transistor logic (TTL) pulse if the input pulse exceeds the 50 mV. The pulses from the discriminator cannot directly input to the embedded add-on card as it accepts pulses of 12 V. Hence, the TTL pulses were shifted to +12 V pulses without compromising the frequency information. A level shifter circuit using a TTL buffer integrated circuit, DM7407, is designed for the purpose. It contains six independent gates each of which performs a buffer function. The level shifting is achieved by providing external pull-up resistor to the open-collector outputs. The output of the circuit is pulses of the same input frequency with level being shifted to +12 V. The LED driving circuit consists of a voltage source and two components connected in the series, a current-limiting resistor (called the ballast resistor), and an LED. The LED used will have a voltage drop, specified at the intended operating current.
Embedded data acquisition controller
Front end of the data acquisition systems is based on Advantech APAX series add-on modules connected to the base APAX 5570 system. The APAX module is embedded windows XP based system with a Celeron processor of speed 1 GHz. It has 512 MB of RAM and 6 GB of hard disk memory. The system has a RJ-48 port to connect to the network and RS-232 serial port for data communication. The system has a universal serial bus port for fast data transfer. Suitable add-on cards such as digital input module, analog modules, and resistance temperature detector input module can be inserted to base APAX 5570 system depending on the requirement. It has also an additional external secure digital memory slot to plug in external memory card for data storage. Apax 5080 counter/frequency input module is used for counting the pulses from the conditioning electronics. It is a plug-in card with four inbuilt counter/frequency channels with maximum rating of 1 MHz. Apax 5046 is a 24 channel sink type digital output (DO) plug in the module and the LED is driven through the DO channel of this add-on card. Photograph of the system is shown in [Figure 5].
All the features of the system like, driving of LED for illumination and bleaching of the sensor, the data acquisition from the circuit and counting, applying calibration factor and instantaneous display of data on the screen are controlled by application software developed in VB.Net. In order to have a better understanding and easy extendibility, full software is organized in different modules and functions. Commercially off the shelf based method is followed for software development. The "dynamic-link library" files given by the vendor are modified suitably for acquiring the signals through add-on cards. The software controls the acquisition of the counts from the level shifter through the counter module, APAX 5080 plugged in to the controller and then applies the calibration factor and display the instantaneous dose in the screen. The LED is driven through the DO module, APAX 5046. Software gives privilege to the user for variable illumination time and thereby facilitating the user to adjust the illumination time according to the LED source intensity. The system is optimized for an illumination time of 5 s to record representative counts. Bleaching time for the given LED source is estimated to be <30 s.
Calibration of the system
The system is calibrated by irradiating the sensor pellet with known absorbed doses using Co-60 standard source from the gamma chamber. [Figure 6] shows the linear fit of the counts obtained for various absorbed doses and it is observed that the response of the system is linear up to 10 Gy. BaFBr: Eu showed a linear dose response from 250 mGy to 10 Gy. The calibration factor of the system is estimated as 4126.2 cps/Gy. The error in calibration factor is + 81 cps/Gy. The high value of the calibration factor is due to low intensity of stimulated light used, i.e. from a single green LED. The results would be better for high intensity Laser LED cluster source. The calibration factor is hardcoded in the application software. Minimum detectable activity of the system is calculated to be 35 mGy. 
| Acknowledgments|| |
The authors would like to thank for Dr. R. Bhaskaran, Head, RIAS for providing infrastructure and useful discussions for carrying out the work. The authors would also like to thank Ms. Anjukumari for the help rendered during the work.
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