RADIATION PHYSICS DIVISION

Introduction

Radiation Physics Division (RPD) was created in April 1992 as an upgradation of the world-renowned Solid State Nuclear Track Detectors (SSNTD) laboratory established in the Nuclear Engineering Division of PINSTECH, in 1974. With the status of a Division, the laboratory was expected to expand and diversify its research programmes in areas other than track-detectors applications. Consequently, the research activities in nuclear data processing, radiation dosimetry, simulations and radioactive waste-disposal management were initiated apart from the usual research work based on the use of SSNTDs.

Manpower and Experience

At present the Division has an experienced team of seventeen scientists having diverse backgrounds of work in different fields such as Nuclear Physics, Nuclear Geology, Nuclear Engineering, Electronics and Computer Science.

The Division has well-equipped laboratories for the Research and Development work related to Nuclear Reaction Studies, Nuclear Data Management, Computer Simulations, Environmental Dosimetry, Radiation Shielding, Mineral/Gem Identification, Rock Dating, Petrographic Studies of Rocks, and Autoradiography. In view of the involvement with largely computer-based work, there exists a general expertise in the division relevant to word-processing, desktop publishing, computer graphics, database programmes and software development.

Collaborations

The division has active collaborations with a number of Pakistani and foreign universities. About 10 students from various universities visit RPD every year and complete their M.Sc theses. Two scholars have completed their Ph.D theses while five more are working for their Ph.D degrees under the supervision of scientists of this Division. The Division is also involved in joint research work with Geoscience Laboratory (Islamabad), Gomal University (D.I. Khan), Punjab University (Lahore), Govt. College (Lahore), AJK University (Muzaffarabad, Azad Kashmir) and National Institute of Oceanography (Karachi). The Division has also very active collaborations with certain research groups based at ICTP (Italy), JINR (Russia), Colorado University (USA), Marburg University (Germany) etc. The Division has contacts and collaborative research projects with groups working on accelerators at CERN (Switzerland), LAMPF (USA), GSI (Germany), and JINR (Russia). With the help of these groups, this Division remains abreast of the latest developments in the field of nuclear interaction studies.

IAEA Research Projects

Due to vast experience and well-equipped laboratories available in RPD, fourteen research projects were awarded by International Atomic Energy Agency (IAEA) to the scientists of this Division, which have been completed successfully.

Publications

The Division has the distinction of publishing about 8-10 research papers every year in foreign and local journals. The total number of publications (including those of former SSNTD - Laboratory) would soon surpass the figure of five hundred.

Technical Activities

The present activities of the Division are distributed among the four groups,

  1. Nuclear Interaction Studies Group

  2. Nuclear Geology Group
  3. Environmental Radiation Group.
  4. Track Detector Applications Group

  1. Nuclear Interaction Studies Group

  1. Programmes

The low energy heavy ion reactions involve a variety of mechanisms like compound nucleus formation, quasi-fission, elastic and deep inelastic scattering etc. These reaction modes are characterized by a set of well-defined features. In order to study the reaction process, it is necessary to detect all the reaction products originating at the point of interaction. This can be achieved either by using electronic detectors or solid state nuclear track detectors. The later method is relatively simple and inexpensive but still remarkably successful in studying and interpreting the heavy ion reactions. For the analysis of track data collected with optical microscopes, the researchers at RPD have developed a set of highly sophisticated computer programmes. With the help of these programmes, it is possible to identify in-flight projectile fission, elastic and in-elastic channels as well as multiprong events originating from sequential fission. Based on this technique, a number of reactions have been studied and many others are under investigation.

A Complete kinematical analysis of several heavy ions reactions has been done. It has been concluded on the basis of computed relative velocity of adjacent tracks of reaction products that the formation of more than two particles in the final exit channel are due to sequential fission process as shown in figure. Cross sections have been measured for a number of heavy ions reactions at different energies, using the statistics of inelastic exit channel and the elastic channels (i.e. based on quarter point angle q1/4 ). These results have also been compared with theoretical values. Scattering cross sections of low energy ions in different targets have also been done.

The study of nuclear reactions between heavy nuclei at relativistic energies is aimed at understanding the properties of nuclear matter at high temperatures and pressures. In order to study nuclear reactions, a series of experiments using radio-chemistry and track-detection techniques are underway in collaboration with the Philipps University, Marburg (Germany). The exposures of the Cu targets with relativistic 16O and 19F ions have been made at the Joint Institute of Nuclear Research, Dubna, (Russia). Further research work in this regard is in progress. CR-39 detectors have been used to identify different charges at relativistic energies and a calibration curve has been obtained. Charge changing cross sections of relativistic heavy ions in different light and heavy target materials have been calculated and compared with that of theoretically calculated values.  

Track Measuring System

Automatic track measuring system for Heacy Ion Reactions

Measurements of pion induced fission cross sections have been made at Radiation Physics Division for the last six years. For this purpose the exposures are made at accelerator facilities in USA while the fission fragments registered in dielectric detectors are counted off-line at PINSTECH. Total and differential cross sections as well as mass yields have been obtained for the fission induced in U, Pb, Au, and Ho using 80 MeV and 100 MeV p + and p - beams. Recently eight stacks of target-detector assemblies have been exposed at LAMPF and BNL (USA) using negative pions of four different energies. The stacks are being analysed for fission cross section studies.

Different target-detector assemblies were exposed to pions of different energies (500, 672, 1068 & 1665 MeV)  at Low Energy Pion (LEP) channel of Clinton P. Anderson Meson Physics Facility (LAMPF), Brookhaven National Laboratory, USA.

A marked forward - backward asymmetry in the emission of fission fragments has been observed which can be explained on the basis of momentum transfer to target atom due to momentum of incident pion. Fission cross sections of target nuclei used in the experiments have been determined.

The microscopic neutron-nucleus reaction data constitutes the core of fission reactor technology. Such a complete data-base consists of millions of data points for which specialized computer programmes are required to store, retrieve and manipulate data sets. The latest libraries of data sets are available in the R.P.D. for dissemination to users. 

  1. Facilities

  1. Expertise Available for Industry

  1. Nuclear Geology Group

  1. Programmes

Conventional uranium-exploration methods are expensive, time consuming and require huge financial input. On the other hand, the radon measurement technique is inexpensive, simple, and the time required is much less. Results are reliable and helpful in deciding drilling targets. This technique is also helpful in areas, where ore bodies were mobilized and have not yet attained equilibrium to respond to gamma sensitive counters. A  uranium mine is shown in the picture.

More than 25000 points have been investigated in; Isa Khel, Kallar Kahar, Sohawa, Bhimber, Dina, Dera Ghazi Khan, Rajanpur and Dadu District, some of which are shown in the map. 

Technique proved useful in Pakistan as it gave an accurate picture of the sub-surface are bodies present in sand stones.

Radon (Rn) generated within the earth’s crust, as a result of the decay of 238U, is exhaled usually in small amounts at uniform rate. Abnormally high and erratic
Rn-exhalation has been noted before the occurrence of an earthquake due to the earthquake making processes. Based on this phenomenon, we are trying to develop an earthquake prediction system. Our studies have indicated a correlation between excessive Rn-exhalation and the occurrence of an earthquake.

The geological faults are the sites of earth’s crust movement, geothermal energy, mineralization and water reservoirs. Location of geological faults is important for the designing and site selection of dams, high-ways, bridges, reactors and similar structures. A fault is an extensive fracture within the earth’s crust, hence an excessive Rn-exhalation over the fault zone is expected. This is the basis for studying and locating the fault zone. We have located Main Boundary Fault around Islamabad at three places. The technique can be used to locate any other suspected fault zone.

The FTD of rocks and minerals is based upon the counting of tracks formed by fission fragments of 238U. The FTD-Technique has many advantages over the conventional rock dating methods. Based on FTD studies the phenomenon of sea floor spreading (picture) has been established. We have studied samples from various areas of Pakistan and useful results have been obtained. The FTD studies will help to understand the overall geology of Pakistan and can help in the identification of areas as potential sources for various minerals. The NGG has developed expertise in FTD and is doing extensive work in Pakistan.

Carbonatite rocks are mainly composed of carbonates of Ca & Mg and some silicate minerals with unusual higher percentage of Nb, Ta, Y, Fe, PO4, U and some rare earths. They are found in Pakistan at Loe Shilman, Malakand, Swat, Koga, and Tarbela. On the basis of our studies, high contents of U, Nb, Ta, Y, rare earth elements and PO4 have been found in the rock samples from Loe Shilman and Malakand areas. A more detailed work has been planned in collaboration with Geoscience Lab. & Nuclear Materials Division (NMD) of PINSTECH to evaluate the economic worth of these rocks. A view of Loe Shilman is shown in the picture.

A strict elemental control in steel and alloys is observed due to its extensive use in almost every industry. Boron plays an important role in quality control of these materials. The estimation of boron is very important in the fabrication of nuclear fuel, cladding and manufacture of structural components of reactors. The NGG has gained enough experience of boron determination in steel and other materials up to a few ppm.

This method is based on the measurement of radon emanation from the ground. A possible locally decreased emanation can be identified over oil/gas reserves as compared to surroundings as ahown in the picture. Two areas, Fim Kassar (Chakwal) and Golra (Islamabad) have been selected to study radon and its  correlation with sites containing oil / gas reserves.

Pakistan has a promising geological terrain for oil and gas reserves. But we produce only 56000 barrels of crude oil per day, which is less than 20% of requirement. Our dependence on imported petroleum is so critical that we spend $ 2.76 billions on the oil import annually The Government of Pakistan attaches top priority to the speedy indigenous development of oil and gas to arrest the heavy oil bill. The oil generation and maturity is time-temperature dependent as shown in the figure. This time-temperature window is same that anneals fission tracks in apatite, that can be determined using the fission track technique. The "fission track" technique can emerge out to be a mile stone in locating the areas of high priority for oil and gas exploration. Based on this strategy, a program has been chalked out to use the technique of fission track for oil and gas exploration in Pakistan.

  1. Facilities:

  1. Expertise Available for Industry

Exact age of certain minerals and rocks can be determined using FTD technique. The technique is fairly accurate having no upper or lower limits.

Services of minerals and gem identification using various techniques are available.

Petrographic analysis of rock samples can be carried out using advanced research microscopes. 

Boron impurity in steel is a serious problem. It increases the hardness of the steel to undesired level. As a result the malleability and ductility of the steel decreases to a very low level.

  1. Environmental Radiation Group

  1. Programmes

In order to protect the workers from deleterious effects of radon and its progeny, development and improvement of the existing environmental and personnel radon dosimeters is needed. This group is engaged in doing research and development work in these areas. Moreover, dose calculations in different geometries of existing and proposed dosimeters are evaluated both experimentally and theoretically.

The SSNTDs are widely used in the environmental radiation monitoring as well as in other projects of ionizing radiation measurements. The SSNTDs are used to monitor ionizing radiation often in severe environmental conditions. Therefore, the studies related to the sensitization of different types of SSNTDs by UV light, laser beam and microwaves are required to be carried out experimentally. Development of SSNTDs with high detection efficiency is also a requirement for certain studies. The experimental and theoretical work is in progress to achieve these aims.

Sometimes, professionals either in medical science or in industry have to deal with ionizing radiations such as X-rays, g -rays, and neutrons etc. In order to avoid any health hazards to those radiation workers biological-shielding design is required. Biological shielding design calculations are carried out using Monte Carlo Simulation techniques, which is tantamount to doing the real experiments.

This project is being carried out to develop a low cost method to detect the trace levels of radon/thoron and other radionuclides in drinking waters.

  1. Facilities

  1. Expertise Available for Industry

  1. Track Detector Applications Group

  1. Programmes

The main objective of the research in this area is to study the etching process with respect to the physical and chemical changes. Some of the main results are:

Many physical parameters like concentration, electrical conductivity, density and total dissolved solids have been measured after etching CR-39 in NaOH for various times up to 15 hours. From the relationship of the concentration with electrical conductivity and density, we have devised a quick method for keeping the normality of the etchant at constant value during the prolonged etching.

CR-39 interacts with NaOH according to the following chemical equation:

  (C12H18O7)n + NaOH + H2O à           Na2CO3.H2O + (C3H6O)n + C4H10O3

After studying the products using different techniques, the synthesis of the crystal Thermonatrite was identified and separated from the reaction products. This crystal has been Synthesized for the first time in the Laboratory, which has the potential of usage in many industrial products.

Bulk-etch rate VB of various SSNTDs can be determined using either diameters of the normally incident fission fragments or employing the changes in physical parameters (thickness, mass,  area etc) after performing etching for a given time interval.  We have evaluated various methods for the determination of bulk etch rates, reported so far by various groups active in the field. It has been found that all the methods yield the similar and correct values provided a number of different detectors are used independently and then the mean of the Gaussian fit is taken.

It has been observed that thickness  and weight of detectors as measured just after etching are not true. The etchants such as NaOH not only removes the general surface but also causes swelling in the detectors. A series of experiments were conducted to find the magnitude of swelling and hence correction factor for determining the bulk etch rate just after etching. The effect of normality as well as temperature of the etchants on the swelling has also been investigated. A correction factor has been obtained for the correct value of the thickness and mass.

Annealing of tracks in insulating solids is a reverse process of formation of damage trails in the materials. Hence to understand the energy transfer mechanism of a charged particle along the path traversed in CR-39. Various annealing models have been suggested by a number of authors over the years. Track lengths of fission fragments were measured in 25 different CR-39 detectors, after annealing the tracks for various time intervals at different temperatures. All the models have been fitted to the experimental data and hence their validity has been tested.

  1. Applications of SSNTDs

SSNTD offer a unique opportunity to study the heavy ion interactions. For the long times it has been the only technique with the help of which the many fragments in the exit channel could be registered in coincidence. Since the head of the group has vast experience in the field and has been involved in studying various heavy ion interactions in collaboration with the group at Kernchemie, Marburg, Germany, we have continued the studies at intermediate energies. Based on the different values of the partial cross sections observed in two different threshold detectors, in spite of the same value of the total reaction cross sections, it has been inferred that intermediate mass fragments have been emitted in the heavy ion interaction of Pb + Au at 14.0 MeV/u.

Annihilation of antiprotons in nuclei leads to a high degree of nuclear excitation (~ 5000 MeV), resulting in the emission of energetic light nuclear fragments (p, d, t, 3He & a). Further decay of the excited nucleus takes place through evaporation followed by binary or ternary fission. To study the annihilation of antiprotons with matter, a number of CR-39 detectors were exposed with the beam of 5.9 MeV antiproton at CERN, Geneva.

The ranges of light fragments resulting from the annihilation of 5.9 MeV antiprotons with the constituent nuclei of CR-39 detector (C12, H18, O7) have been calculated to be within 350-500 um . Hence 200-300 um of the detector surface was removed by polishing to reach the new surface where the tracks of the light particles were revealed after proper etching. The dimensions and orientation of these tracks confirm that they have been produced by the light particles emitted as a result of the annihilaztion of antiprotons with the constituents of CR-39 near the original surface.

Nuclear Track Micro Filters (NTMF) are produced by the bombardment of various ionizing ions of specific energy on various Solid State Nuclear Track Detectors (SSNTDs) with subsequent etching in the appropriate etchants. The number, size and shape of the pores in these filters are controlled by the beam flux and etching conditions. These filters have been extensively employed in aerosol research, environmental studies as well as in bio-medical and chemical sciences. We have also employed the NTMF in the following applications. 

In industrial and radiochemical processes, there exists a problem for complete separation of emulsions into their constituent phases. In this regard a gadget, comprising two parts namely specimen holder and pure solvent collector, separated by the filter housing and attached to a vacuum suction filtration system was fabricated. For different emulsion systems, equal amount of distilled water was mixed with the organic liquids such as Kerosene oil, Petrol, Mustered oil, Xylene, Toluene, Diesel & Carbon-tetra-chloride. Red ink and Diphenyl-thio-carbazone dyes were used to distinguish inorganic and organic phases respectively. 

Strongly mixed immiscible liquid phases were successfully separated by using Mica Nuclear Track Micro Filters (NTMF). It was observed that Mica filter with 8 mm pore size was suitable for separation of the emulsions up to 92%. 

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