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Table of Contents
- Solid State Nuclear Track Detectors
- Brief History
- Material of Solid State Nuclear Track Detectors
- Models of Track Formation Mechanisms
- 3.1. Total Rate of Energy Loss, dE/dx
- 3.2. Primary Ionization Model...............................................................................................................
- 3.3. Thermal Spike Model
- 3.4. The Ion Explosion Spike
- Track Formation in Plastics
- Advantages and Disadvantages of SSNTDs
- CR-39 Polymeric Track Detector
- Chemical etching
- Methods of Bulk Etch Rate Determination
- 8.1. Thickness Change Method
- 8.2. Fission Fragment Tracks Diameter Method
- 8.3. Mass Change Method...................................................................................................................
- Mathematical Details of Track Formation
- 9.1. Constant Track Etching Velocity, VT...........................................................................................
- 9.2. Determination of Track Parameters R and VT
- Apparatus
- Experimental Procedure
- Observations and Calculations
- Calculating Etching Parameters
- 13.1. Bulk Etch Rate
- 13.2. Track Etch Rate...........................................................................................................................
- 13.3. Efficiency
- 13.4. Critical Angle
Solid State Nuclear Track Detectors
1. Brief History
The story of detection of charged particle tracks began in 1958, when D.A. Young who was working at the Atomic Energy Research Establishment (AERE) at Harwell in England, discovered that LiF crystals, held in contact with a uranium foil and irradiated with thermal neutrons, revealed a number of etch pits after treatment with a chemical reagent. In 1959, Silk and Barnes, also working at AERE Harwell, directly observed hair like tracks of fission fragments which were less than 300 oA in diameter and greater than 4 μm in length in thin sheet of mica using transmission electron microscope. During early 1960s the team of Fleischer, Price and Walker who were working at General Electric Research Laboratories at Schenectady, New York pioneered extensive developments of this method. They extended the etching technique of Young whose work was unknown to them at that time to mica, glasses, plastics, various crystals, etc. Very soon the use of SSNTDS started in almost every field of science and technology which includes radiation dosimetry, nuclear physics, space physics, geology, medicine, etc. Having observed tracks in SSNTDs, the next step was to understand the track formation and its development mechanism which is briefly described below. Heavily ionizing particles passing through insulating media leave narrow trails of damage that ranges from ~ 30 – 100 Å. In crystals this consists of atomic displacements, manifesting themselves as interstitials and vacancies which are surrounded by a region of considerable lattice strain. In plastics, the radiation damage produces broken molecular chains, free radicals, etc. Certain reagents dissolve or degrade these damage regions at much higher rate than the undamaged material thereby resulting in etched tracks which can be seen under an optical microscope. These reagents are called etchants
2. Material of Solid State Nuclear Track Detectors
Etch able tracks are formed in a variety of materials. All track recording materials, reported so far, are electrical insulators. Some wide-band gap semiconductors are also known to record tracks. These materials fall into two main categories
loss (dE/dx), primary ionization, restricted energy loss, the density of atomic displacements or the creation of a "thermal spike" of molten material along the particle track. Another hypothesis, known as the ion explosion spike model, is based on the momentary electric field created along the particle track by charge imbalance, caused by the ejection of many electrons from the immediate region of the track. Some of these models are briefly discussed below. None of these approaches can fully account for some of the observed differences in etching behavior. The threshold of track etching is best correlated with the restricted energy loss, which is tied to the energy deposition rate only along the primary particle path, excluding long-range delta rays. Because the energy imparted to the delta rays is one component of the more traditional specific energy loss dE/dx, the restricted value is somewhat smaller, particularly at high particle energies. As mentioned above, attempts have been made in past to correlate the etching behavior of different track materials to the physical phenomena occurring in the detector materials due to the passage of heavily ionizing radiation through different detector materials. Based on these efforts following models have been proposed by different groups.
3.1. Total Rate of Energy Loss, dE/dx The earliest, and perhaps the most natural, explanation of track formation was that it depended on the total amount of energy deposited per unit path length by the incident ion. This criterion was proposed by Fleisher et al.. The proposition was that tracks are formed when dE/dx exceeds some critical value (dE/dx)c. This approach became untenable when results of more extensive irradiations, particularly in the high-energy region, became available and was rejected by Fleischer et al. in favour of the primary-ionization criterion described below.
3.2. Primary Ionization Model In this model, the formation of etch able tracks is related to the number of primary ionizations produced close to the ion path. A relation for the primary ionization, J , based on the work of Bethe is usually employed:
/ 2 2 2 eff 2 [ln( 1 2 ) ]
J C Z^ ^ K
Where Zeff and β are, respectively, the effective charge and the velocity of the ion relative to the velocity of light, c (i.e. β = v/c). The terms K and C/^ are constants for a given stopping medium, and δ is a relativistic correction term which is related to the polarization of the stopping medium.
It has not yet proved possible to calculate K from first principles or to make an absolute determination of J for most stopping media. K is frequently used as a fitting parameter and J is calculated in arbitrary units. This procedure does not affect the correlations, since the fitting of the data depends on the shape of the J versus β curves, rather than on the absolute values of J. It was found that primary ionization did indeed give a good fit to the results of heavy-ion irradiation experiments and that a consistent value of primary-ionization threshold, J c, could be obtained for a given stopping medium. The reason of the success of the primary-ionization criterion in comparison to dE/dx is that the higher-energy δ-rays carry off an appreciable energy outside the central track region (∼50- oA), and this energy is unlikely to play a part in track formation. In the formula for the total rate
of energy loss, the energy of these δ-rays makes a significant contribution to dE/dx; whereas in the primary-ionization formula such high-energy events are given the same weighting factor as low-energy δ-rays and thus assume comparatively less importance. The primary-ionization criterion is open to the criticism that it takes no account of the ionization produced by even the low-energy δ-rays and that the ionization potentials implied by the values of K used in obtaining the best fit to experimental data are physically unrealistic. With regard to the second of these criticisms it must be said that Fleischer et al. regard Eq. (1.1) as phenomenologically useful rather than of deep theoretical significance. Moreover, since in crystals the radial extent of un- etched tracks is only a few tens of angstroms, it seems reasonable that primary processes should predominate over secondary ones as the basis for track formation although this may not be true for polymers, or generally in the cases of low-energy ions (< 1 MeV/nucleon). The primary-ionization criterion has proved highly successful in practice, and has survived to the present day except that the concept of a threshold J is regarded now a day as being less decisive. Thus, for both plastics and minerals it has been found that the track etch velocity VT is a continuous function of J. Typically, VT ∝ J x, where as a value of x ≅ 2 has been found to hold for some plastics. Nevertheless, VT versus J curves generally rises very steeply, and particularly for minerals, the concept of a threshold remains a reasonable approximation to reality 3.3. Thermal Spike Model Bonfoglioli have concluded that tracks in solids are formed by a thermal spike process. According to this model the passage of energetic particles is assumed to produce intense heating of the localized region of the lattice. This region is therefore raised to very high temperature and
Figure 1 The ion explosion spike mechanism for track formation in inorganic solids.
Figure 2 Track formation in polymers
Ionizing radiations directly produce ionized and excited molecules, and electrons. Some excited molecules may de-excite through the emission of radiation or through non-radioactive transitions. Excitation energy can also be transferred from one molecule to another. Electrons are
6. CR-39 Polymeric Track Detector
CR-39 (Columbia Resin-39) is the commercial name for the thermoset plastic which is a polymeric form of diethylene glycol bis (allyl carbonate). Its simple formula is (C12H18O7)n. It has widespread commercial use as it is very transparent and mechanically rigid. The casting techniques employed in commercial undertakings are mainly meant for the optical industry. The materials, depending upon the chemical concentrations of the constituents and processing conditions, available in the market may have slightly different physical as well as chemical properties. Although CR-39 was already in use for multiple purposes (e.g. in sun glasses, wind screen of aero planes, etc.), its excellent nuclear track recording properties were reported in 1978 by Cartwright et al. It is the most sensitive detector material of the SSNTDs family and can register particles as small as protons over a wide band of energy. CR-39 is now in use in many fields of Science and Technology that include the development of a personnel fast neutron dosimeter, radon dosimeter, etc. The track recording efficiency of CR-39 also depends on the processing conditions and can be modified, to some extent, by the processing conditions adopted. This includes etchant type, strength, temperature, and etching time. Extensive work has been done to optimize the etching conditions using NaOH and KOH. Therefore, the main purpose of this project was to discover some new etchants for CR-39 such that the new etching solutions should be more efficient than conventionally used 6M aqueous NaOH at 70 OC. Moreover, besides having higher etching efficiencies than 6M NaOH at 70 OC, the etchant should also have the advantage of drastic decrease in the processing time
7. Chemical etching
Chemical etching is the most widely used method of fixing and enlarging the image of the latent damage trail in a solid state nuclear track detector. Essentially, etching takes place via rapid dissolution of the disordered region of the track core which exists in a state of higher free energy than the undamaged bulk material. The linear rate of chemical attack along the track is termed the track etching velocity, VT. The surrounding undamaged material is attacked at a rate VB , the bulk etching velocity. The bulk etching rate is generally constant for a given material and for a given etchant applied under a specific set of etching conditions, although in crystals it will often depend on the crystallographic orientation, and in some polymers it may vary with depth below the original surface. The chemistry of track etching has been little studied. The highly sensitive track recording plastic CR-39, produced from diethylene-glycol bis (allyl carbonate), has been
studied by Gruhn et al. They found that attack by the hydroxyl ion results in the hydrolysis of the carbonate ester bonds and the release of poly-allyl alcohol (PAA) from the polymer network. In addition to the polymeric etch product PAA, 2, 2'-oxydiethanol is also formed. The most intensive studies of track etching have concentrated, however, on track shape geometry and on the effects of environmental conditions on track etching e.g., effects of temperature, concentration of etchant and of etch products, etc. When an irradiated detector is put in a chemical solution (called etchant which can be an acid, base, etc.), it rapidly and preferentially attacks the damaged material. This chemical attack dissolves off the material at a linear rate VT (track etch rate) along the track and bulk of the material at a lower rate VB (bulk etch rate). The etching characteristics depend upon the incident particle parameters, detector material and on the nature, concentration and temperature of the etchant. A latent track will be enlarged by etching only if the rate of etching along the track, VT, exceeds the rate at which the surface is etched (VT/V (^) B > 1). If VT/V (^) B = 1 (i.e. VT = VB ), then the etched track length L = 0. Therefore, etched tracks are not produced on surface of the detector. The ratio VT/V (^) B depends on the amount of molecular damage along the core of the track.
8. Methods of Bulk Etch Rate Determination
As mentioned earlier, bulk etch rate VB is the thickness removed per unit time from any surface of the detector during its chemical etching. VB is an important parameter for CR-39 detector because it serves to: (i) Facilitate the reduction of the measured parameters of the tracks into quantities, which have more absolute significance. (ii) Act as a calibration parameter for the detector and the etching process.
Three methods are mainly employed for the measurement of VB depending upon the type of detector's material and facilities available. 8.1. Thickness Change Method This method is based on the direct measurement of change in thickness of the detector due to etching process.
2 1 B 2 V t^ t t
^ (2)
Where t 1 = thickness of the detector before etching
parameters of an etched track are the etched-cone length Le or the length S projected onto the detector surface and the major and minor axes D, d of the etch pit opening (see Figs.3 and 4). The main purpose of this topic is to relate these latter parameters to the track and bulk etch rates VT and VB and to the track length R (I shall use R in the treatment that follows to denote the length of the damage trail in the detector, whereas Le will refer to the length of the conical etched-out track), so that quantities of interest such as Z, M, or β may ultimately be deduced. The track etching geometry also determines quantities such as the efficiency of a given detector which is defined as the ratio of the number of observed etched tracks to the number of latent damage trails crossing a unit area of the “original” surface of the detector (i.e. where the etching first starts). In the remainder of this section, some of the general features of track etching geometry are indicated. Some of the simpler derivations are also presented.
Figure 3 some parameters used to describe the geometry of the etched tracks.
Some parameters that are used to describe the geometry of etched tracks are shown in Fig.3. As may be seen in this figure, R = full length of the latent track; L = length of track attacked by the etchant up to a given moment; Le = observed length of the etched track; h = thickness of surface removed by etching; d = diameter of the etch-pit opening.
9.1. Constant Track Etching Velocity, VT The calculation of etched-track parameters is comparatively simple when the track etch rate VT can be taken to be constant along that portion of the latent damage trail which is etched out. This condition will apply in many instances where the ionization rate of the particle is not varying rapidly such as the case of an energetic cosmic-ray nucleus. Furthermore, the constant VT model allows a number of features of the track etching process to be established fairly simply. Initially, some further simplification shall be made by concentrating on a track which is normally incident upon the detector surface. Now, the linear rate of attack down the track is VT (the radial extent of the enhanced etch ability is assumed to be very small compared with the final dimensions of the etched track), so that in an etching time, t, the etch pit will extend to a distance L from the point of origin, where L = VT t. However, the surface is also being removed at a rate VB. so that the length of the etch pit is Le = VT t - VB t (5)
At each point along the track, the etchant moves outwards at a rate VB. Any point at a distance,
y, from the beginning of the track is reached by the etchant at a time ( ) T
t y Y (^) V and there is a
residual time t - t(y) available for the etchant to attack radially outwards from the point at y to a distance VB × (t - t(y)) in the medium. The three-dimensional pit wall is then formed by the locus of all the spheres of radius VB × (t - t(y)), where t(y) is the variable. It will be seen from Fig. 4 that this leads to the formation of a cone with semi-cone angle δ given by
sin( ) B^ B^ B T T
V t V t V
L V t V (6)
This angle sin 1 ( B ) T
V
V
is also known as the critical angle of etching (θ c ). From the triangle
O'PT in fig.4 it is apparent that
(1) The semi-cone angle δ = sin-1^ (VB/VT). In certain materials such as glasses where VT is not very much greater than VB, etched tracks with large cone angles are produced. In plastics and minerals, long needle-like tracks of much smaller cone angle are usually produced. This is a consequence of the fact that VT >> VB in these cases. (2) The diameters of the surface openings of etched tracks increase with increasing VT, reaching a maximum of 2VB t when VT >> VB 9.2. Determination of Track Parameters R and VT Among the most important parameters of the latent damage trail which are often required to be determined are the length R and the mean value of the track etch velocity VT. It is relatively easy to measure the track parameters S, D, and d by using a micrometer attachment to the microscope eyepiece or from a photomicrograph/video screen projection. VB can also be measured from the changes in the thickness of the plastic or the glass sheet. It may be noted here that thickness method cannot be applied to CR-39 detector because irreversible swelling take place in the case of CR-39 detector at elevated temperatures. Alternatively, sheet of plastic is irradiated with 252 Cf fission fragments at normal incidence. Then, for θ = 90°,
2 (^ 1) B ( 1) D d V t V V
^ (11)
Where V = VT/VB. Since for most plastics and etching conditions used the track etch rate for fission fragments is very much higher than the bulk etch rate (so that V >> 1), we have D = d ≅ 2VB t
B 2
V D
t^ (12)
Thus a measurement of the diameter of the normally incident fission-fragment tracks at a known etching time yields a value for the bulk etch rate. If the ratio of the diameters of some tracks (having an etch rate ratio V, say for alpha particle) to those of fission fragments is denoted by x, then we can write
( 1) ( 1) x V V
and upon re-arranging, we get the useful formula 2 2
T B
V x^ V x V
2 2
(^1 )
T B 1
V V x x
Most treatments of track geometry are formulated directly in terms of the thickness of the removed surface layer h = VB t.
10. Apparatus
Following apparatus is required for the study of Solid State Nuclear Track Detectors.
CR-39 detectors 241 Am and 252 Cf radioactive Sources Etchant (6 M NaOH) Etching Bath Vacuum Chamber Optical instrument (Microscope)
11. Experimental Procedure
Calibration of microscope: First we have to calibrate microscope.
Figure (5); The microscope used in experiment
Figure (7); Microscopic view during the calibration
Irradiation of CR-39 ; Given 1’ x 1’ CR-39 pieces irradiated with Am-241 and Cf- for one minute. For the study of activity, the time should be carefully noted Etching; First, we have to prepare 6M NaOH solution in beeker and then dip these pieces in the solution. Now, we put this in the etching bath at70 OC. The etching bath is shown below.
Figure (7); Outside view of etching bath
Figure (8); Inside view of etching bath
Measurement of Diameters; Now, we measure the diameters of the tracks. Tracks views are shown as below
Figure (9); View of tracks on CR-39 irradiated by Am-
