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ORIGINAL ARTICLE
Year : 2013  |  Volume : 36  |  Issue : 4  |  Page : 160-163  

Assessment of neutron dose in Indus accelerator complex using CR-39 SSNTD


1 Health Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai, India
2 Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, India
3 Indus Operation and Accelerator Physics Design Division, Raja Ramanna Centre for Advanced Technology, Indore, Madhya Pradesh, India

Date of Web Publication8-Oct-2014

Correspondence Address:
Dimple Verma
Room No. 12, Indus-1 Building, RRCAT, Indore - 452 013, Madhya Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.142392

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  Abstract 

Indus accelerator complex (IAC) consists of two synchrotron radiation sources namely Indus-1 and Indus-2. The radiation environment here is mainly due to bremsstrahlung and photo-neutrons. Major problems faced in neutron detection in IAC are the severely pulsed nature and gamma (bremsstrahlung) interference. Thus, to assess the neutron dose rates in the accessible and inaccessible areas of IAC, passive integrating type neutron detectors CR-39 and bubble detectors are used. The dose rates observed at microtron body; booster injection and extraction septum are significant (few mSv/h) when compared to other locations in IAC. From bubble detector data, it can be seen that the dose rate during injection is high (maximum 112 μSv/h) compared with storage mode (maximum 2.6 μSv/h) indicating high beam loss during injection. During the injection, the personnel are not allowed in these areas due to high radiation doses on account of the beam loss. The neutron dose rates observed for accessible areas are three orders of magnitude less than the inaccessible areas in the complex.

Keywords: Bremsstrahlung, CR-39, Indus accelerator complex


How to cite this article:
Verma D, Haridas G, Pal R, Dev V, Bandopadhyay T, Chougaonkar M P, Tripathi R M, Babu D. Assessment of neutron dose in Indus accelerator complex using CR-39 SSNTD. Radiat Prot Environ 2013;36:160-3

How to cite this URL:
Verma D, Haridas G, Pal R, Dev V, Bandopadhyay T, Chougaonkar M P, Tripathi R M, Babu D. Assessment of neutron dose in Indus accelerator complex using CR-39 SSNTD. Radiat Prot Environ [serial online] 2013 [cited 2019 Nov 11];36:160-3. Available from: http://www.rpe.org.in/text.asp?2013/36/4/160/142392


  Introduction Top


Indus accelerator complex (IAC) consists of two synchrotron radiation sources, Indus-1 and Indus-2. Indus-1 is a 450 MeV electron storage ring and Indus-2 is a 2.5 GeV electron booster cum storage ring. The radiation environment in IAC is mainly due to bremsstrahlung and photo-neutrons. They are produced due to beam losses occurring at different sections of the accelerator and at different operating modes. The pulsed structure of the beam is different at various sections of the facility and hence the emitted bremsstrahlung and photoneutrons also follow nearly the same pulsed structure. The pulsed structure encountered in IAC is as low as 10 -9 s. Neutrons are produced in IAC mainly by two processes namely giant resonance and quasi-deuteron effects. In the case of the giant resonance, neutrons are produced from neutron production threshold to 30 MeV of photon energy. In this process, a photon transfers its energy to the entire nucleus in which the protons as a group move opposite to the group of neutrons. The cross-section integrated over photon energies from threshold to 30 MeV has a trend expressed as,

.

Where A n is the photo-neutron cross-section as a function of photon energy and N = A - Z. And Z, A and N being the atomic number, mass number and the neutron number, respectively.

For photon energies above 30 MeV, neutrons are mainly generated through quasi-deuteron effect. In this process, the photon interacts with a neutron-proton pair rather than the entire nucleus unlike in giant resonance mechanism, where the photon interacts with the nucleus as a whole. The cross-section [1] for the process is qualitatively expressed as,



Where L is a dimensionless coefficient having value in the range 3-13 and A d is the deuteron photo-disintegration cross-section which varies as a function of photon energy. The cross-section for this process is an order less than that at the giant resonance peak. The contribution of the quasi-deuteron effect to the neutron spectrum is to add a high-energy tail to the giant resonance spectrum. [1]

Neutron spectra found in the radiation environment of electron accelerators peak somewhere near 1 MeV, it is usual to assume that the energy is effectively 1-2 MeV. [1] The majority of the neutrons ~85% in IAC are giant resonance neutrons having isotropic emission which follows fission spectrum with most probable energy as 0.7 MeV, the average energy as 2 MeV and a maximum energy of 6-7 MeV. [2] Quasi-deuteron neutrons are forward peaked with most of the neutrons above 5 MeV with a maximum neutron energy being less than half of the primary electron energy. [1]

Major problems faced in neutron detection in IAC are the severely pulsed nature and gamma interference. The response of flux meters and Rem meters are not adequate for severely pulsed neutron field in IAC. [3] Hence, online neutron measurement is a challenging problem in the radiation environment of IAC. Thus, the integrating type of neutron dosimeters is preferred for measurements in pulsed neutron fields. Preliminary measurements were carried out previously using CR-39, bubble detectors and the rem meter. [4] After this, the measurements were repeated using CR-39 foils and the neutron dose rates in IAC could be established with the measurements. To assess the neutron dose rates in the accelerator complex during various operating conditions, passive integrating type neutron detectors CR-39 and bubble detectors were used. CR-39 detectors are insensitive to beta and gamma radiation over a wide dose range, and there is negligible post irradiation fading due to environmental conditions. The detector covers the range of neutron energies expected at IAC. The paper describes the details of the measurements carried out and the results.


  Materials and methods Top


Neutron dose measurements at Indus accelerator complex

The dosimeter used in neutron dose measurements at IAC comprises of CR-39 detector of dimensions 3 cm by 3 cm with 1mm polyethylene radiator in front of it, sealed in an airtight triple laminated aluminized pouch. The CR-39 sheets, 625 micron thick (dosimetric grade) and curing time of 32 h, were procured from Pershore Moulding, UK. The irradiations were performed for (1) accessible areas and (2) inaccessible areas around the entire IAC in Indus-1 and 2. The measurement locations in case of microtron, booster and Indus-1 are shown in [Figure 1]. The several positions 1, 2, 3 and 4 marked in [Figure 1] indicate the same location for different beam lines. Position 1 refers to the central hole location (inner side), position 2 refers to the central hole location (outer side), position 3 refers to monochromator location and position 4 refers to the experimental station location. As there are 5 beam lines, position 1 indicated at 2 different locations indicates the locations at the respective beam lines and so on. [Figure 2] gives the schematic of Indus-2 showing the measurement locations. These detectors were exposed to all machine operating conditions namely injection, ramping, storage and beam killing. Thirty-one locations were identified for accessible areas and the detectors were placed at 1.25 m height from the floor level (beam plane is at this height). The detectors at accessible areas were exposed for a maximum period of 6 months where the exposure time obtained from the machine data was up to 1160 h. For inaccessible areas, a total of 39 locations were selected and the detectors were exposed for a period of 2-9 days where the actual exposure time was ~40 h, whereas the exposure time for beam line hutches was 258 h. For accessible areas of Indus-1, some preliminary measurements have been carried out during Indus-1 injection mode and Indus-1 storage mode separately using bubble detectors and the results are included. The bubble detectors procured from BTI, Canada (BD-PND, energy >200 KeV) with sensitivity of 2 bubbles/μSv (22 bubbles/mrem) were also used for measurement at selected locations. In the case of inaccessible areas of Indus-1 and Indus-2, bubble detectors could not be used due to two reasons (1) anticipated high neutron dose rates leading to saturation of the detectors (2) one cannot remove the same after desired operating time due to continuous operation of the machine.
Figure 1: A schematic of Indus-1 with microtron and booster showing the measurement locations

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Figure 2: A schematic of Indus-2 showing some of the measurement locations

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Processing and calibration

The irradiated detectors were etched electro-chemically. The electrochemical etching technique is two-step technique carried out in specially designed cell with 7 N KOH as etchant, maintained at a temperature of 60°C. For the first step-etching at low frequency, the power supply operates at 1250 volts constant potential and low frequency of 100 Hz for 4 h, followed by the second step-etching at high frequency of 3.5 kHz for 40 min duration, using a field strength of 20 kVcm−1 . The processed dosimeters were evaluated by counting tracks in an area of 1 cm 2 (track cm−2 ) using a PC based image analysis system. [5] The CR-39 dosimeters were calibrated with 241 Am-Be neutron source (dose equivalent average energy is 4.4 MeV) having a yield of 2.5 Χ 10 6 n/s at 50 cm distance from the source in free air. The calibration factor obtained is 0.0055 mSv-cm 2 /track in air. After counting of tracks, the calibration factor is multiplied to the net track density to arrive at the measured integrated neutron doses. The dose rates were calculated using the machine ON time during the irradiations.


  Results and discussion Top


The dose rates as observed by CR-39 at inaccessible areas such as the areas around the microtron and the booster ring were of the order of 0.29-1.74 mSv/h. At the injection septums, in Indus-1 it was up to 0.16 mSv/h and in Indus-2 it was up to 0.95 mSv/h. The accessible areas of IAC showed neutron dose rates up to 2.9 μSv/h. [Table 1] shows the maximum neutron dose rates observed in some of the important areas.
Table 1: Maximum neutron dose rate observed at some important locations in IAC

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As the neutron dose rates expected at experimental hall is low, bubble detectors were used to find out the dose rates. The observed neutron dose rates in Indus-1 experimental hall during Indus-1 injection and storage are shown in [Table 1] using the Bubble detectors. Here, the CR-39 and the bubble detectors cannot be compared because the minimum detectable limit for CR-39 is 200 μSv, whereas for the bubble detector (taking a sensitivity of 2.1 bubbles/μSv), the minimum dose that can be read is 0.5 μSv. If we try to place both the CR-39 and bubble detectors together during Indus-1 injection which lasts for nearly 8-10 min, the dose seen by the detectors would be ~20 μSv (calculated from bubble detector data of 112 μSv/h during injection). This small dose can be easily recorded by bubble detector but will not be recorded by CR-39 where the minimum dose that can be recorded is 200 μSv. Hence irradiating the two detectors together at the same position for the same interval too will not suffice for a data comparison between the two detectors. From bubble detector data, it can be seen that the dose rate during injection is high compared with storage mode. This indicates high beam loss during injection. It is to be noted that during the injection the personnel are not allowed in these areas due to high radiation levels in comparison with storage (user) mode.


  Conclusions Top


Inaccessible areas

  • The dose rates observed at microtron body, booster injection and extraction septum are significant (few mSv/h) when compared to other locations in IAC
  • The neutron dose rates observed inside ring area are mainly near the injection septum of Indus-1 and Indus-2.


Accessible areas

  • Dose rates observed for accessible areas of IAC are insignificant (few μSv/h)
  • The experimental station of all beam lines in Indus-1 showed <0.6 μSv/h, which is mainly during Indus-1 injection
  • Indus-2 experimental hall showed dose rates ranging up to 2.9 μSv/h which needs to be rechecked
  • It is observed that the neutron dose rates observed for accessible areas are three orders of magnitude less than the inaccessible areas.


 
  References Top

1.Radiological Safety Aspects of the Operation of Electron Linear Accelerators, IAEA Technical Reports Series No. 188; 1979.  Back to cited text no. 1
    
2.Cember H. Introduction to Health Physics. 3 rd ed. McGraw Hill, 1996.  Back to cited text no. 2
    
3.Haridas G, Sathian V, Ponraju D, Nayak MK, Dhariyawan MP, Thakkar KK, et al. Inter comparison of neutron detectors in pulsed photo neutron field. IARP Conference, November 23-25, Mumbai; 2005.  Back to cited text no. 3
    
4.Verma D, Nayak MK, Dev V, Sahani PK, Kumar V, Haridas G, et al. Response of neutron dosimeters to pulsed neutron fields in high energy electron accelerators at Indus complex RRCAT. Radiat Protec Environ IARP 2009;32:50-3.  Back to cited text no. 4
    
5.Pal R, Jayalakshmi V, Sathian D, Chaurasiya G. Dosimetric systems and characteristics of CR-39 for use in individual neutron monitoring. IEEE Trans Nucl Sci Part 2 2009;56:3774-8.  Back to cited text no. 5
    


    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1]



 

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