Primary objective of establishing Health Physics Unit at Indus synchrotron facility, was to identify hazards in the facility, devise procedures and methodology to assess the hazards, procure necessary instrumentation for quantifying the hazard, recommend necessary remedial measures to facility in-charge, to keep the hazard within permissible regulatory limits and train working personnel in safe practices to be followed during work. Health physics activities of Indus-1 started in mid-eighties with radiation source term evaluation and the subsequent radiation shielding calculations. Several revisions of the calculations were done before being approved by the Atomic Energy Regulatory Board (AERB) and the civil construction actually started.

Evolution of the present shield design

The approved shielding thickness for the accelerators in Indus-1 was arrived at, based on   design beam loss scenario. The recommended sidewall thickness for microtron and booster was 1 – 2 m of ordinary concrete (O.C), depending on the shielding criteria prevailing at that time. Respective roof thickness was 0.90 m and 0.30 m for both the machines. For Storage ring, a lateral shielding of 0.12m lead was recommended but, due to practical difficulties, an equivalent thickness of concrete was put uniformly around the ring. During the commissioning trials of various accelerators in Indus-1, several radiation hot spots were detected and were to be locally shielded with lead. More number of hot spot was detected around the concrete shielding of storage ring. For installation and commissioning of beam lines, the radiation shielding around the ring was to be re-designed and a modular shielding comprising of 8 cm lead + 8 cm MS was designed and installed around the ring with ordinary concrete maze entries to the ring area. After installation of the modular shield structure, extensive radiation surveys were carried out, at injection and storage mode of operation of the ring, which suggested some modifications in the shield. After incorporating those, majority of the experimental hall (except IUC and BARC High Resolution beam line area) have radiation fields similar to background radiation levels and hence entry prohibition to those areas during injection mode was relaxed to entry restriction presently. There is no special roof shielding for the ring except the roof of the building.

Radiation hazards in Indus-1 facility

a) Ionizing radiation

Radiation hazard in Indus-1 comprises of ionizing radiation as well as non-ionizing radiation. Ionizing radiations are produced due to the interaction of electron beam with vacuum chambers, other solid components in the vacuum envelope and residual gas molecules in it. The main prompt radiations (which are present only when accelerator is “ON”) of ionizing nature is bremsstrahlung x-rays and photo-neutrons. Bremsstrahlung x-rays have a broad spectrum with energies extending upto the primary electron energy and highly angle dependent. Intensity of these x-rays peaks in the forward direction of the beam. Photo-neutron is the other form of ionizing radiation, produced due to the interaction of bremsstrahlung x-rays with machine components and structural materials. However the photo-neutron dose contribution is significantly less, in comparison with x-rays. Inside the machine area, in addition to x-rays and photo-neutrons, high energy electrons also may be present at specific locations where beam loss takes place.

b) Non-ionizing radiation

Apart from ionizing radiation there are non-ionizing radiation present mainly due to various radio-frequency sources like klystron, RF cavity, RF amplifier, wave guides , co-axial cables etc. The operating frequencies are 2856 MHz (for microtron) and 31 MHz (for booster and storage ring). Initial RF leakage measurements indicated power density levels (~ 3 mW/cm2) at 31 MHz , exceeding the limits, according to ANSI standards. Shielding and proper grounding reduced the power density levels to less than 1 mW/cm2, in all the RF devices operational at the facility. Routine power density measurements ensure that the limit is not exceeded.

Leakage magnetic field measurements around the storage ring dipole magnets were carried out when the magnet is fully energized (1.5 T) to ascertain the hazards due to magnetic field to personnel working near them during testing. Leakage field up to 30 Gauss was measured at 30 cm from the coils and recommendations and necessary instructions were given.

c) Induced activity

Activity induced in the machine components, where the beam strikes is measured to be the minimum, externally. The maximum dose equivalent rate due to induced activity obtained from the injection septum of booster, externally, immediately after shutdown from normal operation was 1.5 mSv/h. The radio- frequency cavity of microtron showed an average dose equivalent rate of 1.0 mSv/h, around it immediately after microtron is opened for cathode replacement. The cavity lid showed a maximum contact dose equivalent rate of 10 mSv/h. The internal probe of microtron after several months of beam optimization showed an induced activity of 0.06 mSv/h.  Study of the decay showed that Cu-64 is the prime radionuclide responsible for the induced activity, which is a b+ emitter with a half-life of 12.7 h. Other places in the facility showed negligible induced activity. Immediately after beam dump experiments on copper and stainless steel in TL-1 at 20 MeV, the maximum dose equivalent rate due to induced activity were found to be ~0.5 mSv/h. However, the induced activity was comparatively less during beam dump at 450 MeV at TL-2 end in booster and storage ring area. Induced activity in air and cooling water was found to be insignificant. Ozone levels were found to be less than 0.1 ppm in microtron, booster and storage ring area. Chemi-luminiscence method was used for the measurement of ozone.

Operational aspects during various stages of commissioning.

During the various stages of commissioning of accelerators in Indus-1 building, radiation survey was of prime importance as far as safety is concerned, to ensure safe working environment for the workers. It was a difficult time to quantify the radiation field due to lack of proper instruments for the radiation field, characterized by,

a)      High energy
b)      Pulsed 
c)      Mixed field

To find out the suitability of the radiation monitors for monitoring in such fields, response of certain commercially available monitors were studied. Radiation field mapping for bremsstrahlung x-ray and photo-neutron was carried out using several active and passive dosimeters and their comparison gave confidence in choosing the right instrument for a radiation survey at a certain location. Pulsed x-ray monitors were developed by Electronics Division (ED), BARC for area monitoring in Indus-1. The proto-type monitors developed, were tested in the 10 MV x-ray field from a copper target using extracted electrons from the microtron at 10 MeV (when type-1 RF cavity was in use) The results were compared with Thermo-Luminescent Dosimeter (TLD) and theoretical estimate from the measurement of beam current using Faraday Cup. The agreement we got was in good agreement within +15% and as a result mass production of the monitors was undertaken by ED, BARC. The x-ray monitors were extensively used during the commissioning phase of all the accelerators in Indus facility, not only for the measurement of radiation fields but also to help detect beam losses and tuning the machine properly thereby minimizing the beam losses and subsequent reduction in the radiation levels. Operational experience during the commissioning trials indicated the presence of heavily pulsed radiation, where all the monitors, which uses proportional counters and GM tubes, ceased to work.

While storage ring commissioning, initially the dose equivalent rates were high. Various factors like poor injection, non-survival of beam in the ring, vacuum conditions etc. contributed to the high field. Occasions were faced when radiation streamed to accessible areas. This was detected at the early stage of commissioning trials, the sources were identified  and was immediately contained by augmenting shielding near the source itself. Later on when good storage was achieved, after optimization of the storage ring parameters, the radiation fields around the ring reduced drastically. However during injection of beam into the ring, the radiation fields were found to be high (0.30- 6.0 mSv/h), especially in the direction of injection. During both injection and storage mode bending dipole-3 regions showed the maximum radiation field among other regions in the ring. Another major observation we made is the reduction in the radiation field, as the operation continues for several months. But when the ring is vented to atmosphere for any reason, and when operation is resumed, the radiation levels around the ring were found to increase due to increased rate of out gassing from the ring. Before installation of the beam line the shielding had to be modified and a modular shielding was installed around the ring.  Several problems were encountered during the characterization of synchrotron beam at the front ends and in beam line commissioning phase. Some of the major problems were:

1. Streaming of high-energy bremsstrahlung x-rays through the front end.
2. Streaming of high-energy bremsstrahlung x-rays through the left out annular space in between the modular shielding and the beam lines.
3. Radiation leakage through the other joints of the modular shield structure.

To limit streaming out through front ends, during photon characterization at front ends, beam current was always kept at an optimum lower value. On the other hand, streaming through the left out space was stopped by filling it with lead shot filled cloth bags. But this posed a problem while removing for any reason like baking, alignment etc and later refilling, where the shield uniformity gets disturbed and hence no leakage is to be confirmed every time. Moreover, wherever leakage was observed, it was a difficult task to confirm, whether the photons are of low, medium or high energy. Radiation levels at Spectroscopy beam line (BM-3) was found to be above permissible levels even during storage < 100 mA and hence work was to be stopped at the beam line on several occasions. Later on, shield augmentation has been done to reduce the radiation levels to within permissible limits. Another difficulty faced was the inconsistency of radiation field at certain locations on different days for the same operating conditions. This might be due to optimization of operating parameters during the commissioning phase. The search and scram system and door interlocks installed in the facility, within the accelerator halls and at entry points to the halls with the approval of AERB was found to be very effective in preventing any inadvertent entry to high radiation areas and unwanted exposure to any working personnel. Radiation physics experiments. Several experiments were carried out using 20 MeV and 450 MeV electrons to generate sufficient data for use in radiation safety of workers in Indus-1 facility. The important among them are listed below.

Figure 1 Measured dose buildup at the photon field of BM-3 of storage ring

1. Study of response of radiation survey instruments to high energy photon radiation.
2. Study of response of several active and passive neutron detectors in the pulsed photo-neutron field at TL-2 in booster hall.
3. Determination of dose equivalent index for different target thickness at 20 MeV.
4. Study of electro-magnetic shower propagation in Cu, Pb, water and air using 450 MeV electron / 450 MV photons.
5. Angular distribution of 450 MV photons at TL-2 termination and dipole magnet-3 of storage ring
6. Bremsstrahlung x-ray spectrum measurements around the modular shielding of Indus-1 storage ring, in collaboration with RSSD, BARC
7. Photon dose buildup studies in copper, water, sodium iodide and stainless steel at 450 MeV
8. Determination of the adequacy of TLD badge filter for use in high energy radiation environment of electron accelerators.
9. Radiation dose mapping (photon & neutron) using TLD, ion chamber, bubble detector and CR-39.
10. Integrated dose measurement at front ends of storage ring using TLD to generate data for radiation damage purpose.

All these experiments were carried out to understand the nature of radiation field in the facility and arrive at the necessary modifications in the commercially available radiation monitors or for deducing correction factors for the instrument response in high energy and pulsed fields. Study of the response of survey instruments to high energy photon radiation indicated underestimation of dose rates by a factor of 2 to 4.5. Hence a correction factor of 5 is applied to all instrument response while measurement in such fields. A typical dose build up curve obtained in water at bending dipole magnet  (BM-3) of storage ring is shown in Figure1. The angular distribution of bremsstrahlung photons measured at the same location for optimization of modular shielding around the ring is shown in Figure 2

Figure.2 Bremsstrahlung x-ray angular distribution measured at BM-3 of storage ring

Optimization of radiation shielding around the storage ring and the design of modular shielding was executed based on actual experimental data generated from Indus-1.

Beam line safety

There are five beam lines planned in Indus-1, of which 4 are installed, commissioned and operational now. Initial phase of commissioning of beam lines revealed hot spots at the external surface of the nearby modular shielding, which showed increased radiation levels at the experimental stations, above the background. By augmenting necessary shielding where ever necessary, the radiation levels were brought down to background levels at all the experimental stations during storage mode of operation ( at 100 mA stored current). The experimental stations of spectroscopy beam line (BM-3), CAT reflectivity beam line (BM-2) and the Angle Integrated Photo-electron Spectroscopy (AIPES) beam line (BM-2) are declared safe during the injection mode of operation too, recently (in 2003). The experimental stations of each beam line have a radiation level of 0.1mSv/h presently.

However regular radiation survey is carried out at the experimental stations to ensure safety of working personnel in those areas. Also instructions have been issued to the beam line coordinators to take extreme care while removing the lead bags from the annular space of beam line and modular shielding (for baking, alignment etc.), through which the beam line penetrates out, as high energy photon radiation can stream out and reach experimental station.

  Radiation safety systems

Various radaiation safety systems were designed, procured, installed and commissioned  in the facilty to ensure a safe working environment. They include,

• Bulk shielding
• Search & Scram system
• Area Monitors (hooked to main control room)
• Audio-visual warning system
• Safety inter-lock system & kori-locks.
• Radiation hazards caution boards
• Ventilation