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ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 2  |  Page : 70-76  

Development of a combination detector system for simultaneous measurement of α and β/γ radioactivity


1 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
2 Department of Physics, Nowrosjee Wadia College, Pune, Maharashtra, India

Date of Submission15-Feb-2018
Date of Decision25-Mar-2018
Date of Acceptance09-Apr-2018
Date of Web Publication24-Aug-2018

Correspondence Address:
Mrs. Vaishali Manojkumar Thakur
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, Maharashtra
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_26_18

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  Abstract 

Accurate estimation of radioactivity content of various samples and effluents is essential in all stages of operation at a nuclear facility. Generally, two separate systems are used for estimation of α and gross β activity. A single counting setup with a detector probe consisting of a combination of ZnS (Ag) and plastic scintillator detectors can be applied for this purpose. This paper describes design, development, and characterization of a combination detector system for simultaneous assessment of α and β/γ radioactivity content of an electrodeposited or planchetted sample or that of a filter paper. Pulse height discrimination technique has been utilized to distinguish between α and β radiations. The β activity counts had been observed to contribute <0.7% in α activity. Counts from α activity are added up in β channel counts, and it was observed that for 239Pu source, it is about 2.5 times. Efficiency of the system for β sources was found to have varied from about 14% (137Cs) to 23% (90Sr/90Y). The efficiency for 238Pu α source was found to be 13.4%.

Keywords: Combination detector, pulse height discrimination, thin plastic scintillator, ZnS (Ag)


How to cite this article:
Thakur VM, Ashokkumar P, Rekha A K, Jain A, Rath D P, Chaudhury P, Chaudhari L M. Development of a combination detector system for simultaneous measurement of α and β/γ radioactivity. Radiat Prot Environ 2018;41:70-6

How to cite this URL:
Thakur VM, Ashokkumar P, Rekha A K, Jain A, Rath D P, Chaudhury P, Chaudhari L M. Development of a combination detector system for simultaneous measurement of α and β/γ radioactivity. Radiat Prot Environ [serial online] 2018 [cited 2018 Dec 11];41:70-6. Available from: http://www.rpe.org.in/text.asp?2018/41/2/70/239682


  Introduction Top


Measurement of radioactivity content of various system and effluent samples is necessary at an operating nuclear facility or radiological laboratory. Accurate estimation of the radioactivity is very much essential for augmenting the required quality in process standard and radiation protection in all stages of operation at a nuclear facility. Generally, ZnS (Ag)-based detection systems are used for estimation of α activity, and end-window Geiger–Muller counting systems are made use for β/γ activity measurement. Here, two separate instruments are employed for gross radioactivity estimation of various samples. Among the various available radiation detectors, scintillation-based detectors [1],[2],[3] are preferred due to their better performance in terms of efficiency and ruggedness. Radiation detectors using silicon PIN diodes are extensively used for α spectrometry and allied applications such as radon measurement.[4],[5] However, such systems are generally not popular for applications involving β detection. α and β activity estimation using a single detection system reduces the workload and can be achieved using a combination of ZnS (Ag) and plastic scintillator detectors.[6],[7] Measurement systems with a combination of two different detectors in a single instrument are reported before.[7],[8] Pulse shape discrimination (PSD) technique based on the pulse rise timing information from different detectors as a sandwich has been employed successfully for simultaneous counting of different types of radiations in past.[9] A simple and easy to handle equipment is required for the analysis of radioactivity samples on a day-today basis. In order to address the requirement of simultaneous measurement of α and β/γ activity content of radioactive samples, a combination detector system (CDS) based on pulse height discrimination has been designed and developed. Activity deposited on a 30 mm diameter aluminum planchette or filter paper can be counted under the CDS. The system has a sandwich of two scintillation detectors, namely (i) a conventional ZnS (Ag) detector for α and (ii) a thin plastic scintillator for detection of β/γ radiations that are emitted from the sample. The design and fabrication of the combination detector, development of associated electronics, its characterization, and application are presented here.


  Materials and Methods Top


A CDS was developed for the simultaneous assessment of α and β/γ radioactivity content of an electroplated or activity-deposited disc samples or airborne activity-loaded filter sample. The detector system consists of a detector (combination of ZnS (Ag) and plastic scintillator) module, sample holder assembly, processing electronics, controller electronics, and LCD display. The system can be powered using a12V rechargeable DC battery or directly from AC supply mains by means of a 12V DC eliminator.

The detector module consists of combination detectors coupled to photomultiplier tube (PMT) assembly having an in-built preamplifier and high voltage (HV) power supply. The module is then mechanically coupled to a base support unit and sample holder assembly. The combination detector was developed by sandwiching a disc of 2” diameter ZnS (Ag) (4.09 mg/cm 2) and a thin plastic scintillator (13.4 mg/cm 2) together. A 10 mm thick 2” diameter light guide was fabricated from Perspex material and fine polished for proper light transit. The detector was then optically coupled to the 2” Ф PMT by means of the light guide. This assembly is enclosed inside a cylindrical metallic enclosure. The detector end of the enclosure is covered using a removable cap having a 2” diameter 1 mg/cm 2 thick mylar window on it. [Figure 1] shows the detector module along with (a) unfinished light guide, (b) polished and finished light guide, and (c) light guide coupled with the combination detector at one side. A cylindrical PVC enclosure was utilized for protecting the detector module. One end of the enclosure was coupled to the base support unit. Two connectors are provided for detector module; a limo connector used for 12 V low voltage supply input to power HV and preamplifier circuits, and a BNC connector to carry the CDS output pulses for further processing.
Figure 1: Detector module with (a) unfinished (b) polished light guide and (c) light guide + detectors

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The assembly of the combination detector has been mechanically fitted to a base support unit with an arrangement for introducing sample in an aluminum planchette using a sample holder. A 110 mm diameter 16 mm thick solid Aluminum base serves as support stand for the system. A 95 mm diameter 12 mm thick stainless steel (SS) annular base plate of inner diameter 50 mm was fitted to the support stand. The base plate was fabricated with a milled space to accommodate a rotating sample holder. The sample holder is a circular (70 mm diameter) SS plate with a central arrangement to position an aluminum planchette along with sample disc below the detector. A SS square piece with a hole was welded along the diameter of the sample holder. The sample holder was fitted in the space between the base plate and support stand with the help of a SS pin. The sample holder which was fitted to the detector-PMT base can be rotated about the axis of the pin using a lever [Figure 2].
Figure 2: Sample holder fitted with the detector - photo multiplier tube base

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Pulses from the detector module are analyzed using the subsequent electronics. Block diagram of the CDS is given in [Figure 3]. The system consists of a pulse-shaping amplifier followed by two discriminators, associated transistor transistor logic (TTL) converters, microcontroller, input keypad, and LCD display. A miniature pulse-shaping amplifier-printed circuit board was developed in-house for use in the system. The primary purpose of shaping amplifier is to magnify the amplitude of the preamplifier output pulse from the millivolt (mV) range into the 0.1–10V range. To assure complete charge collection from a detector, preamplifier circuits are normally adjusted to provide a long decay time. Since the pulses occur at random times, it might lead to pulse pile up, especially if the count rate is large. Shaping the pulses to produce a pulse train can alleviate the pile-up problem. The pulses have been shaped in such a way that their pulse duration is reduced without affecting the pulse amplitude by Resistor-Capacitor (RC) differentiation-integration method. In this technique, signal is processed through a cascaded Capacitor-Resistor (CR) differentiator and two cascaded RC integrator as shaping networks. The pulse is also filtered at low (differentiator) and high (integration) frequencies, resulting in an improvement in signal to noise ratio. A baseline restorer was used to ensure that the baseline between pulses is held rigidly at ground potential in spite of changes in counting rate or temperature so as to have accurate amplitude measurements of the pulses.
Figure 3: Block diagram of the measurement system

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The shaped pulse is then fed simultaneously to two discriminators (hereafter called as discriminator 1 and discriminator 2). A comparator IC (LM311) has been used for this purpose. The analog input pulses that cross the discriminator threshold (VTH) are converted to standard logic pulses at the output of the discriminator. The VTH can be set manually so that all the input pulses having amplitude greater than the set value will be counted. By this process, discriminator eliminates the low amplitude noise and the signal trigger; the discriminator is then transferred into logic pulses for further processing.

The output logic pulses from discriminators are fed to two TTL converters. TTL converter provides well-defined square pulses to the input counting port of microcontroller. We used dual retriggerable, resettable one-shot IC9602 for this purpose. This IC allows the designer to employ either leading-edge or trailing-edge triggering, which is independent of input transition times. When input conditions for triggering are met, a new cycle starts and the external capacitor is allowed to rapidly discharge and then charge again. The retriggerable feature permits output pulse widths to be fixed for all pulses having variable pulse duration. In fact, a continuous true output can be maintained by having an input cycle time which is shorter than the output cycle time. The output pulse may then be terminated at any time by applying a low logic level to the reset pin. A pulse width of 2 μSec was set and fed to microcontroller for further processing.

The controller unit is 80C51 family-based Philips-make P89V51RD2 microcontroller. It is Single-Chip 8-Bit microcontroller manufactured in advanced CMOS process. The microcontroller is interfaced with 20 × 4 character LCD display unit and keypad. Block diagram of controller system is shown in [Figure 4]. The system's operational assembly language program was developed in Keil environment and embedded into the microcontroller's in-built flash memory using in-system programming. It allows the user to modify the code as well as system parameters. It is interfaced with 24C512 memory of 64 Kbyte Electrically Erasable Programmable Read Only Memory using I2C bus interface. The data with date/time are stored in memory along with parameters. The data can be downloaded to PC as and when requested by serial communication at the end of counting time interval. At power on reset, system initializes the two counters, LCD display and baud rate for communication. Three key keypad or RS-232 serial communication inputs the parameters such as counting time, baud rate, and memory Read or Erase options. It acquires background data for 4 times the acquisition time duration, displays it on the 4 line alphanumeric LCD, and stores in the memory. The system thereafter starts data acquisition for a preset minute and updates the counts per minute (CPM) on display, stores it, and sends it to PC for further analysis.
Figure 4: Block diagram of controller system

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  Results and Discussions Top


Presently developed CDS for α and β/γ activity measurement is shown in [Figure 5]. Sample disc is placed in the sample holder [Figure 2] and positioned it below the combination detector by inward rotating the holder. The ZnS (Ag) scintillation detector and plastic scintillator detector respond for α and β particle events, respectively. α particle, coming out of the sample, would penetrate the aluminized-mylar layer and deposit its energy within the ZnS (Ag) layer, resulting in emission of scintillation photons (emission peak at 450 nm 2). β particle, emitted by the sample, will lose only few tens of keV in its short path within the ZnS (Ag) and will interact within the plastic scintillator layer, resulting in emission of scintillation photons (emission peak at 423 nm). The fraction of β energy deposited within the ZnS (Ag) does not generate any measurable scintillation in the detector. Pulses from ZnS (Ag) and plastic scintillator combo detector generated by α and β ray were discriminated using pulse height discrimination technique. The threshold voltage of discriminator-1 is adjusted to 50 mV to eliminate noise pulses. All the scintillation pulses above this level were counted and be referred as α and β/γ radiation response pulses.
Figure 5: The α, β/γ counting system being used in laboratory

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Among the conventionally used radionuclides, practically encountered maximum β energy is that corresponding to the end point energy of 90 Y (2280 keV). Therefore, the threshold level of discriminator 2 was adjusted to 4.3 V by using 90 Sr/90 Y standard source. By setting this level, the response due to β particles in this channel is brought to a minimum. The oscilloscope screen shot of the shaped pulses for 90 Sr/90 Y and 239 Pu sources is shown in [Figure 6]. Practically, all the TTL pulses that are available at this α channel are due to α particles. Background in CPM for both α and β channels under set discriminator levels is 0 and 37.9 ± 8.3, respectively. A detailed characterization study of the system was undertaken using different available α and β reference sources. [Table 1] gives the properties of various sources [10],[11] used during the study.
Figure 6: Screenshot of oscilloscope for shaped pulses of (a) 90Sr/90Y and (b) 239Pu sources, respectively

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Table 1: Properties of various sources used during the study

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Efficiency of the system

The β detection efficiency of the system (Eβ) was determined using various standard sources. The CPM was recorded, and the efficiency in percentage was calculated using the known activity of the source used. The observed efficiency values for various available β sources are tabulated in [Table 2]. The efficiency for β sources was found to vary from about 14% (137 Cs) to 23% (90 Sr/90 Y). This observation is in conformity with the fact that the end point energy of 137 Cs β is lowest [Table 1] and that of 90 Sr/90 Y is the largest among the sources utilized for the experiment. The thickness of ZnS (Ag) detector which precedes plastic scintillator is 4.09 mg/cm 2 and an additional 1 mg/cm 2 mylar entrance window together has β energy cutoff of about 50 keV. The percentage of β particles falling in this energy range depends on the β spectrum of the nuclide. It may be noted that the plastic scintillator used here has a thickness of 13.4 mg/cm 2 and it is just enough to stop energy up to about 100 keV [12] after passage through ZnS (Ag) detector. Hence, the β particles of 50 keV and above are detected, and partial energy of the β's above 100 KeV is lost for the purpose of detection. Hence, the variation in efficiency with the end point energy of the β radiation source is expected due to the loss of counts during the initial part of the energy spectrum. The usage of thin plastic scintillator helped in reducing background counts due to γ radiation. β channel counts interfering in α channel is as shown in last column of [Table 2]. It varied from 0 to ~0.3%. Spillover counts from β channel to α channel were found to be maximum for 90 Sr/90 Y source.
Table 2: Observed efficiency values for various β sources

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An experiment was conducted to determine the α-detection efficiency of the system. The percentage efficiency as calculated using the available working laboratory standard 239 Pu sources is tabulated in [Table 3]. Average efficiency of the system for α (Eα) was found to be 10.7% with 239 Pu sources. [Figure 7] gives the relationship between count rate (counts per se cond [CPS]) and activity of 239 Pu laboratory standard sources observed during this experiment. Even though a slight difference in calculated efficiency was observed, the ratio of counts in β to α channel for 239 Pu source was found to be nearly constant as 2.58.
Table 3: Efficiency of the detector system for 239Pu sources used in laboratories

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Figure 7: Relation between counts per second and activity of 239Pu laboratory standard sources

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Standard 238 Pu and 241 Am sources yielded efficiency of 13.4% and 13.9%, respectively. For 238 Pu (5499 keV) and 241 Am (5486 keV) sources, β to α ratio was found to be 1.816 and 1.179, respectively.238 Pu and 241 Am emits γ photons with energy 43.5 keV (0.04%) and 59.54keV (35.9%), 33.2keV (0.13%), and 26.3keV (2.41%), respectively. Furthermore, note that 239 Pu has 13.6 keV and 9.9 keV X-ray with an abundance of 4.9% and 3.9%, respectively. These low energy γ photons are simultaneously detected by plastic scintillator along with corresponding instantaneously emitted α particles by ZnS (Ag) detector. The generated scintillation from plastic scintillator will be added up to that from ZnS (Ag) detector. Hence, effectively, even if the α count was falling in β channel, that will be raised to α channel due to simultaneous detection of γ lines. These arguments can explain the observed reduction in the β channel to α channel counts ratio for different sources. Therefore, an instrument using CDS which applies pulse height discrimination method will be useful only when the α-emitting radionuclide handled is known. Knowing the nature of activity measurements, CDS is most useful and can serve the purpose of simultaneous measurement of α and β/γ activity. Occurrence of overlap of α counts in β channel makes accurate activity measurement of mixed unknown sources difficult while pulse height discrimination technique is applied.

Estimation of activity content

For estimation of α activity content of a sample (αs), the α efficiency (Eα) value is used with the α channel count rate and can be calculated using the formula



β sample activity (βs)can be estimated using the formula



Where, A and B are the net CPM in α channel and β channel, respectively.

(Eβ) corresponding β emitting radionuclide efficiency.

For 239 Pu Radionuclide n in equation (2) is 2.54, and depends on the type of alpha source in the sample, requires to be determined individually.

Linearity in response

A study was carried out to check the linearity in response of the system with increasing activity of 137 Cs. The efficiency of the detector was determined in an activity range of about 100–2500 Bq. The sources are prepared by depositing known activity on aluminum planchette using standard 137 Cs solution. The observed CPM in both β and α channels for different standard activity levels is given in [Table 4]. It was observed that maximum 0.03% of the β channel counts are spilled into α channel which is negligible for all practical applications. The CPS was calculated and plotted against the corresponding sample activity. [Figure 8] shows the relation between CPS and activity of 137 Cs sources (Bq). A very good linearity for β activity response was observed and the efficiency was found to be 14.3%. Spill over counts from β to α is less than 0.07% for 137 Cs source.
Table 4: Efficiency of the detector system for 137Cs sources prepared using standard solution

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Figure 8: Relation between counts per second and activity of 137Cs sources prepared from standard solution

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  Conclusions Top


A CDS has been developed using scintillation detectors for simultaneous assessment of α and β radioactivity content of electrodeposited, planchetted, or filter paper radioactive samples. The study indicated that the contribution due to β activity in α channel is negligible and that due to α activity, which is added up in β channel, is about 2.5 times for 239 Pu. This factor has been accounted during β activity estimation. Currently, the system is being successfully used for routine activity measurement in a radiological laboratory. Introduction of PSD techniques is being tried to minimize α to β counts spillover in such a system in our laboratory.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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