Characterization and enhancement of the response of polyallyl-diglycol carbonate etched-track detectors for neutrons

(1)

U

NIVERSITÀ DI

P

ISA

Dottorato di Ricerca in Ingegneria Industriale

Curriculum in Ingegneria Nucleare e Sicurezza Industriale Ciclo XXXI

Characterization and enhancement of the response of

polyallyl-diglycol carbonate etched-track detectors for

neutrons

Author

Daniel Andrade Azevedo de Vasconcelos

Supervisor(s)

Prof. Francesco d’Errico Prof. Riccardo Ciolini

Coordinator of the PhD Program

Prof. Giovanni Mengali

(2)

(3)

List of figures

Figure 1: The breaking of the polymeric bonds by a charged particle. The chain ends form new

species (color) that are chemically highly reactive (Source: Fleischer et al., 1969). ... 16

Figure 2: Track geometry at different dip angle conditions: (a) no track is detected if the incident angle θ < θc (Vtt sinθ < Vbt), (b) limiting case when the incident angle θ = θc (Vtt sinθ = Vbt), and (c) and (d) formation of the post etched cone for a particle with an incident angle θ > θc (Vtt sinθ > Vbt) (Source: L’Annunziata, 2012) ... 20

Figure 3: Geometry of the track development. The incident angle is normal with respect to the detector surface, and Vt is constant (Source: Nikezic and Yu, 2004). ... 21

Figure 4: Three phases in the track development. I is the initial detector surface, O and E are the entrance and end points of the particle path, R is the particle range in the detector material, and Vb is the bulk etch rate. (1) Conical track; (2 and 3) the track wall is partially conical and partially spherical; (4) the track is fully spherical (Source: Adapted from Nikezic and Yu, 2004). ... 23

Figure 5: Phases of track development for oblique incidence. ... 24

Figure 6: Structure of the polycarbonate monomer. ... 27

Figure 7: Structure of the cellulose nitrate with 2 –O–NO2 groups. ... 28

Figure 8: Structure of the CR-39 monomer, the polyallyldiglycol carbonate. ... 29

Figure 9: a. diffusion coefficient of carbon dioxide in the PADC detector as an inverse function of the temperature, and b. flux of carbon dioxide on the surface of the PADC detector with a thickness of 0.9 mm during the chemical etching at 70 °C. ... 33

Figure 10: Increase of sensitivity on detectors irradiated with alpha-particles of 0.54, 1.68, 2.78, 3.83 and 4.94 MeV due to 0.6 MPa CO2 treatment against treatment time. Plots for 6 MeV are according to Csige (1997). ... 34

Figure 11: Changes in sensitivity with treatment gas pressure. The treatment time was a. 3 days, and b. 1 day (Source: a. Yamauchi et al, 2009, b. Csige 1997). ... 34

Figure 12: Exploded view of the CR-39 Radosys® dosimeter badge. (PP: Polypropylene; HDPE: High-density polyethylene; PA Polyamide) ... 40

Figure 13: Irradiation geometry showing the 241_{AmBe neutron source, the detectors and the PMMA} phantom. ... 41

Figure 14: Stainless steel chamber used for the CO2 sensitization treatment. ... 41

(6)

Figure 16: Differential track size distributions of untreated CR-39 irradiated detectors, and irradiated detectors treated with CO2 under a partial pressure of 0.5 MPa for 1, 3 and 5 days,

respectively. ... 43 Figure 17: Differential track size distributions in CR-39 detectors exposed to monoenergetic neutrons. ... 44 Figure 18: a. support made from the badges itself, b. irradiation geometry showing the 241AmBe neutron source, the detectors and the PMMA phantom. ... 46 Figure 19: a. 3D printed support for detector slides, b. Stainless steel vessel built for pressures up to 3.7 MPa. ... 46 Figure 20: Radosys® Nano etching unit. ... 47 Figure 21: Gradual increase of the background noise with the increase of treatment time, for samples treated under a CO2 partial pressure of 2.4 MPa for a. 48 h, b. 96 h, and c. 144 h. ... 48

Figure 22: Differential track size distribution in CR-39 irradiated detectors treated under various partial pressures of carbon dioxide and durations. The reported error is referred to 1 s.d. calculated on the 5 detectors irradiated for each condition. ... 51 Figure 23: Increase of the carbon dioxide sensitization effect for various partial pressures against treatment time... 51 Figure 24: The variation of the differential number of tracks as a function of the partial pressure of carbon dioxide for all the treatment times. ... 52 Figure 25: Radosys’ gold standard image used to calibrate the scale of the analysis software. The grid has 40 lines per mm... 56 Figure 26: Comparison between Radosys and Image-Pro Premier analysis of the standard detector’s track size distribution. ... 56 Figure 27: a. One of the 9 original images exported by the Radosys’ track reader; b. image digitally processed by ImageJ system, rid of most noise and background; c. tracks effectively counted and analyzed by ImageJ... 57 Figure 28: a. Track size distribution comparison between the analysis system of the Radometer 2000 track reader and ImageJ analysis method; b. the same comparison after an application of a calibration factor (CF) on the ImageJ’s track size distribution. ... 58 Figure 29: a. Image of the sealed UV irradiation facility with the UV lamp on the top, b. Image of the facility opened, showing the slide support. ... 59 Figure 30: Images of recoil proton tracks on the surface of CR-39 detectors treated with, (a) CO2

(0.6 MPa for 6 days), UV light (254 nm for 20 h at 5 cm S/D distance), (c) CO2 and UV

(7)

Figure 31: Differential track size distributions in irradiated CR-39 detectors treated with CO2

(0.6 MPa for 6 days), UV light (254 nm for 20 h at 5 cm S/D distance), CO2 and UV light

combined (Test 1 and Test 2) and untreated detectors. ... 62 Figure 32: a. Zoomed image of the surface of one detector treated with Method B, showing “mirrored” tracks; b. scheme explaining the cause of “mirrored” tracks. ... 66 Figure 33: a. Image of the spinning base moved by an electric continuous current motor; b. set up of the whole UV irradiation system. ... 66 Figure 34: Image of the surface of detectors treated by UV light with the spinning base. ... 67 Figure 35: Differential track size distributions in irradiated CR-39 detectors treated initially by CO2

(2.4 MPa for 16 h), followed by the first etching of 1, 2 or 3 h, then treated with UV light (254 nm for 48 h), and ending up with an etching of 3, 2 or 1 h, respectively. ... 69 Figure 36: Images of recoil proton tracks on the surface of CR-39 detectors treated using the two-step etching method combining CO2 treatment and a 48 h UV treatment, intercalated by etchings

of a. 1+3 hours, b. 2+2 hours, and c. 3+1 hours. ... 70 Figure 37: Differential track size distributions in irradiated CR-39 detectors treated initially by CO2

(2.4 MPa for 16 h), followed by the first etching of 2 or 3 h, then treated with UV light (254 nm for 72 h), and ending up with an etching of 2 or 1 h, respectively. ... 71 Figure 38: Images of recoil proton tracks on the surface of CR-39 detectors treated using the two-step etching method combining CO2 treatment and a 72 h UV treatment, intercalated by etchings

of a. 1+3 hours, b. 2+2 hours, and c. 3+1 hours. ... 72 Figure 39: Differential track size distributions in irradiated CR-39 detectors treated under the standard commercial conditions (CO2 under 0.6 MPa for 144 h + 4 h etching), by the carbon

dioxide treatment, alone, used in this work (CO2 under 2.4 MPa for 16 h + 4 h etching), and

under the best condition found in this study (CO2 treatment + 3 h etching + UV treatment

for 72 h + 1 h etching). ... 73 Figure 40: Differential track size distributions in irradiated CR-39 detectors treated using the optimum two-step etching method (CO2 treatment under 2.4 MPa for 16 h + 3 h etching +

254 nm UV treatment for 72 h + 1 h etching), and changing the UV exposure for a UVB at 302 nm and UVA at 365 nm. ... 76 Figure 41: Images of recoil proton tracks on the surface of CR-39 detectors treated with the best condition found in this study (CO2 treatment + 3 h etching + UV treatment for 72 h + 1 h

etching), buy varying the wavelengths of the UV treatment. Were used the a. 254 nm UVC, b. 302 nm UVB and c. 365 nm UVA lamps. ... 76

(8)

List of tables

Table 1: The major effects of UV exposure on irradiated CR-39 detectors ... 38 Table 2: Track densities for carbon dioxide pre-treated CR-39 detectors, compared to untreated detectors (Errors are 1 standard deviation). ... 42 Table 3: Carbon dioxide treatment conditions. The treatments carried are marked with an “X” and the ones with “*” are the limit of readability before the saturation caused by the high background noise. ... 48 Table 4: Total number of tracks for CR-39 detectors treated under various pressures of carbon dioxide from 6 to 144 h, compared to untreated detectors (Errors are 1 standard deviation). ... 51 Table 5: Summarized description of the sequence of tests elaborated to investigate the effects of a simple combination of CO2 and UV sensitization treatments on CR-39 detectors. ... 60

Table 6: Total number of tracks for detectors treated with CO2 alone, UV alone and both combined in

as described in Test 1 and Test 2, and untreated detectors. (Errors are 1 standard deviation). ... 63 Table 7: Summarized description of the sequence of tests elaborated to investigate the effects of the two-phase etching technique combining CO2 and UV sensitization treatments on CR-39

detectors. ... 65 Table 8: Total number of tracks for irradiated CR-39 detectors treated by the two-step etching method with a UV treatment time of 72 h, and intercalated etchings of “1+3”, “2+2”, and “3+1” hours, respectively. ... 68 Table 9: Total number of tracks for irradiated CR-39 detectors treated by the two-step etching method with a UV treatment time of 72 h, and intercalated etchings of “2+2” and “3+1” hours, respectively. ... 70 Table 10: Total number of tracks for irradiated CR-39 detectors treated under the standard commercial conditions (CO2 under 0.6 MPa for 144 h + 4 h etching), by the carbon dioxide

treatment, alone, used in this work (CO2 under 2.4 MPa for 16 h + 4 h etching), and under

the best condition found in this study (CO2 treatment + 3 h etching + UV treatment for 72 h

+ 1 h etching), respectively. ... 73 Table 11: Comparison between the major sensitization effects on detectors treated with the commercial conditions, CO2 under high pressure (alone), and the optimal two-step etching

condition found in this study. ... 74 Table 12: Total number of tracks for irradiated CR-39 detectors treated with the best condition found in this study (CO2 treatment + 3 h etching + UV treatment for 72 h + 1 h etching), but

varying the wavelength of the UV treatment. Were used the 254 nm UVC, 302 nm UVB and 365 nm UVA lamps. ... 75

(9)

Abstract

The growing need for accurate neutron dosimetry of the past decades, led to an increasing number of studies in this area, especially on CR-39, currently one of the main passive neutron personal dosimeters. Among the shortcomings to be addressed, counting automatically the smallest recoil proton tracks is a particularly difficult one. The present thesis was developed in this context, by studying two of the main sensitization techniques available in literature, the post-irradiation treatment with carbon dioxide under pressure, and the ultraviolet exposure treatment.

The work is divided in two main branches, the first is the study of the sensitization effects of the carbon dioxide treatment on CR-39 detectors exposed to fast neutrons and the second is the development of an original sensitization method combining carbon dioxide (CO2) and ultraviolet

(UV) treatments.

For the carbon dioxide treatment, we did the first extensive investigation of its effects on CR-39 detectors irradiated to fast neutrons (most of the previous literature was based on alpha-particles). With these information we were able to optimize the treatment conditions, which improved the detector’s sensitivity, increased the average diameter of the tracks and reduced the treatment time, when compared to the routine protocol used commercially.

We also developed a novel sensitization method combining CO2 and UV treatments which,

compared to the already optimized CO2 treatment, further increased the sensitivity of the detector

and the average track diameter. The treatment time for this technique ended up being longer than the one of the optimized CO2 treatment, but shorter than the commercial protocol.

(10)

1 Introduction

In the past decades, the need for accurate neutron and charged particle dosimetry has been growing (Bos and d’Errico, 2006). This is due to increasing exposure levels in the nuclear industry, deriving from more frequent in-service entries at commercial nuclear power plants, and from increased plant life-extension or decommissioning activities. Medical activities also cause increasing neutron exposures, mainly traceable to a larger availability of radiotherapy facilities using proton beams with energies between 50 and 250 MeV.

Solid state nuclear track detectors (SSNTDs), particularly polyallyl-diglycol carbonate (PADC), i.e. CR-39, have almost replaced albedo thermoluminescent detectors as the most widely used passive neutron personal dosimeters. This is due to great improvements occurred since the 1980s in etching procedures, read-out equipment, and quality of the detector materials (d’Errico and Bos, 2004).

A standard size CR-39 neutron dosimeter can record hundreds of individually detectable charged particle tracks. The damage from ionization and molecular excitation by the neutron-induced recoil ions in CR-39 provides useful information about the energy deposition and fluence of particles interacting with the material. Based on this information, the neutron dose can be determined from the total number of tracks formed in the material by means of a calibration factor obtained using standard neutron sources (Sahoo et al., 2014). The tracks’ characteristic geometric parameters can also be used to generate the neutron spectrum (d’Errico et al., 1997; Luszik-Bhadra et al., 1997; Paul et al., 2013) or LET spectrum (Sahoo et al., 2013; Spurný et al., 1996) in order to obtain improved dosimetric assessments.

However, imaging these tracks and determining their statistics presents a formidable challenge (d’Errico et al., 1997). Particularly difficult is counting automatically the smallest recoil proton tracks, which can be erased by chemical etching procedures due to their limited dimensions (Hulber and Selmeczi, 2005).

In this context, detector processing techniques yielding tracks of larger sizes and thus enhancing readability are extremely valuable and warrant further investigation. Two of the main sensitization techniques are the post-irradiation treatment with carbon dioxide under pressure, first reported by Fujii et al. (1995), and the ultraviolet (UV) exposure treatment, introduced for CR-39 detectors by Hussain (1982).

(11)

It is known that the sensitivity can be increased by treating the detectors under high pressures of carbon dioxide for several days (Fujii et al., 1995; Csige, 1997; Yamauchi et al., 2009). It was also reported (Saad et al., 2001) that the amount of carbon dioxide in the detector is closely related to the latent track formation and track detection sensitivity, assuming that the CO2 produced in the polymer

through the polymerization process, if not stored properly, can escape by diffusion, reducing its sensitivity. The treatment at high pressure can thus recover this loss of sensitivity. Although the fundamental mechanism of the sensitivity enhancement of the CO2 treatment is, yet, not fully

understood, there is enough evidence to support the idea that, when the etchant is penetrating into the detector during the chemical attack, and the carbon dioxide is passing through the opposing direction, from the detector to the solution, the carbon dioxide interacts with the etching solution (Tse et al., 2007) and the detector’s material itself, making it easier to remove smaller molecules from the surface of the detector, which enhances the track etching rate (Enge, 1995; Hassan et al., 2013).

For what it concerns the second sensitization technique, it has been observed that the exposure of a polymer to ultraviolet (UVA, UVB, and UVC) radiation causes chemical and physical changes on its surface, which are dependent on the nature, intensity, duration and the wavelength of the incident radiation (Tse et al., 2006). The energy transferred to the polymer leads to excitation and ionization of the molecular chains, hence to radical formation, main chain bond scission, and also crosslinking of polymeric chains (Tse et al., 2008). These chemical changes cause deterioration of the mechanical and optical properties of the polymer, which in turn affects its bulk and track etch responses.

Both previous treatments were first studied for heavy particle applications, such as radon detection. However, today, mainly due to improvements in the quality of the detectors’ material, these sensitization techniques are no longer required to obtain relatively large tracks from high LET particles, such as alpha-particles and fission fragments. For neutron dosimetry applications, instead, they are much more promising, as the energy of neutron recoils is very wide and so is the range of resulting track sizes, therefore, shifting the track size distribution upwards greatly facilitates the recognition and counting process (Hulber, 2009).

Nevertheless, very few have studied the enhancement effects of these sensitization techniques on CR-39 detectors irradiated by fast neutrons. For the CO2 treatment, the only reports are (Hulber and

(12)

from Matiullah and Kudo (1990), who only used sunlight (broad band) as the UV source to treat their samples, but there is no study that analyses the effects for specific wavelengths.

Therefore, the aim of this work was to further investigate and characterize the sensitization effect of carbon dioxide and ultraviolet light on the proton recoil tracks produced by fast neutrons, and ultimately identify an optimal treatment condition for fast neutron dosimetry.

The thesis was structured in six chapters, the first three presents a literature review, made in order to support the understanding of the results, which were presented in Chapters 5 and 6, followed by the conclusions in Chapter 7. A more detail overview is shown below:

• The basic principles of the detection of heavy charged particles using solid state nuclear track detectors is introduced in Chapter 2, with focus to the track formation mechanism and criteria, and the principles of track etching;

• In Chapter 3, the pros and cons of the main types of SSNTD materials used for neutron detection are described;

• The mechanism of sensitization (to the extent of what’s known so far) and the sensitization effects of CO2 and UV treatments, on CR-39 detectors, are detailed in Chapter 4;

• The results were divided in two parts, the first being the investigation of carbon dioxide treatment alone, detailed in Chapter 5, and the second, a novel technique combining both carbon dioxide and ultraviolet sensitization treatments described in Chapter 6. The conclusions for each of these studies are presented at the end of each chapter;

• The Chapter 7 is the last one and it contains the general conclusions of this thesis, as well as future perspectives.

(13)

2 Solid state nuclear track detectors

The story of SSNTDs began in 1958 when D. A. Young, working at the Atomic Energy Research Establishment at Harwell in England, discovered that LiF crystals, held in contact with uranium foil and irradiated with thermal neutrons, revealed a number of etch pits after treatment with a chemical reagent. The number of these pits showed complete correspondence with the estimated number of fission fragments which would have recoiled into the crystal from the uranium foil (Durrani and Bull, 1987).

At the same laboratory, Silk and Barnes (1959) used the magnification of a transmission electron microscope (TEM) to see such tracks directly in mica, but only momentarily, since the tracks faded and were lost in the electron beam of the TEM. Neither Young nor Silk and Barnes pursued the subject, and only in the early 1960s the method was further developed by the team of R. L. Fleischer, P. B. Prince, and R. M. Walker, who worked at the General Electric Research Laboratories at Schenectady, New York (Fleischer et al., 1967). They extended the etching technique of Young (whose work was unknown to them at that time) to mica and eventually to a variety of other materials such as glasses, plastics and various mineral crystals. Since then, the durability, simplicity and the markedly specific nature of the response of these detectors have led to its rapid application in a wide variety of fields (Durrani and Bull, 1987), until there is, today, hardly a country in the world where the solid state nuclear track detection is not used, nor a scientific and technological field where it does not have an actual or potential application (Durrani, 2008).

2.1 Track formation mechanisms and criteria

The operation of the SSNTD is based on the extensive ionization and excitation that a heavy charged particle (protons, alpha particle, or heavy ion) causes when it passes through a medium. This energy loss creates narrow trails of damage whose dimensions are usually between 30 and 100 Å, the so-called latent track, which is a cylindrical region with altered structure, composition and physicochemical properties along the ion trajectory (Tse, 2007).

The interaction of the charged particle in the medium occurs through Coulomb force; if the distal interactions are neglected, the focus is on the particle interactions with atoms and molecules that are close to its path. Most of these interactions takes place with electrons, and only a small

(14)

number are with nuclei. Normally only heavy charged particle, which are much heavier than electrons, can produce tracks. These tracks are almost completely straight lines as the direction of the particle effectively does not change during its interaction with the medium. This may not be true if the particle interacts with a nucleus, where a significant deviation from the initial direction may occur. However, as stated before, such interactions are relatively rare and here they can be neglected. Some deviations from the straight line can happen close to the end of the particle range, when the energy of a particle becomes very low (Nikezic and Yu, 2004).

However, not all heavy charged particles will form latent tracks. To a first approximation, track formation can be regarded as occurring when the number of ions produced by the incident particle exceeds a certain threshold value. This threshold varies from one type of material to another. Some plastics are sensitive enough to record slow protons, whereas in most minerals even an argon ion at its maximum ionization rate will be unable to form an etchable track. Electrons and photons, even at high doses, have relatively little effect on SSNTDs. This is often a very useful property when trying to pick out charged particles in a high field of mixed radiations (Durrani and Bull, 1987).

The goal of studying track formation mechanisms is the determination of how charged particles are formed in solids and what is the criterion or standard to expect which solids can record what particles. There are some facts that a track formation mechanism must fit (L’Annunziata, 2012):

• Insulating solids and poor semiconductors with electrical resistivity higher than about 2000 Ω cm are basically track-recording materials;

• Metals and good semiconductors are not track-recording solids;

• Electrons (β-rays), X-rays, and γ-rays cannot create etchable tracks in solids;

• The sensitivities of different insulating solids are different, such as muscovite mica, which can record the nucleus of Ne (atomic number Z = 10) and heavier nuclei, but not a lighter nucleus with Z < 10. Polycarbonate can record alpha particles but not protons; CR-39 can record protons and heavier particles. In other words, the solid-state nuclear track detector is a type of threshold detector;

• Plastic or polymer detectors are more sensitive than inorganic solids;

• Before etching, the recorded tracks in solids may be annealed out (i.e. track fading) at higher temperature.

(15)

What mechanisms can cause the above phenomena and what criterion can be used to judge the differences are essential questions for the application of nuclear track detectors.

2.1.1 Chain breaking mechanism in polymers

The latent track formation occurs differently in organic and inorganic materials, but for the purpose of this study, we will focus only on organic solids, or polymers.

Although the track effect is relatively well known, and the technique is rather simple and straightforward, there is not a unique theory that explains the latent track formation, but the basic process can be divided in two phases: the physical and the physiochemical.

The first “physical” phase can be described as the stopping power, in which the incident particle delivers its energy to the atoms and molecules surrounding its path, thus creating free electrons which will slow down through a series of ionizations and excitations, producing more free electrons. Some of these may go further away from the initial particle path creating the so-called delta-rays.

In the second physiochemical phase, the damaged molecules interact between them forming new chemical species, which have a stronger interaction with the etching solution than the undamaged detector material (Nikezic and Yu, 2004).

The net effect of these two phases is the production of many broken molecular chains on the polymer (Durrani and Bull, 1987), as can be seen in Figure 1.

(16)

Figure 1: The breaking of the polymeric bonds by a charged particle. The chain ends form new species (color) that are chemically highly reactive (Source: Fleischer et al., 1969).

2.1.2 Criteria of track formation

Various authors have suggested that track formation should be related to several different parameters, such as total energy loss rate, primary ionization, restricted energy loss, etc. Thus different track formation criteria can be adopted, each one taking the form of a statement that tracks are formed in a medium when, and only when, the chosen parameter exceeds some critical value. This quantity is a property of the solid and has no relationship with any kind of charged particles (Durrani and Bull, 1987). But none of these parameters that have been proposed for the criteria have satisfactorily fit to all data. Every parameter is successful for certain properties but it fails with respect to other properties of the solids. The main ones are:

a) Primary ionization criteria

This criterion was proposed by Fleischer et al. (1967). According to it, the formation of etchable tracks is related to the number of primary ionizations produced along the trajectory of the particles. If this number exceeds a certain threshold, which varies from one type of material to another, then the track formation can be regarded as occurring. The primary ionization rate is given by the Bethe-Bloch formula:

(17)

𝐽 =2𝜋𝑛𝑒𝑍𝑒𝑓𝑓2 𝑒4 𝑚𝑣2 𝑓𝑒 𝐼0[ln 2𝑚𝑣2 𝐼0 − ln(1 − 𝛽 2_{) − 𝛽}2_{− 𝛿 − 𝐾]}

where ne is the number of electrons in the unit volume of the detector, v is the velocity of the incident

particle, m is the rest mass of the electron, fe is the effective fraction of the electrons in the least

energy bound state of the detector material, I0 is the ionization potential of the most loosely bound

electrons in the detector, β = v/c is the particle velocity divided by the speed of light in a vacuum c, δ is a correction term for the effect of polarization of a medium at relativistic velocities, K is a constant depending on the composition of the stopping medium (detector), and 𝑍𝑒𝑓𝑓 = 𝑍[1 − exp(−130𝛽/

𝑍2/3_{)] is the effective charge of the incident particle, where Z is the atomic number of the incident}

particle.

The criterion of primary ionization rate can give a good fit to the experimental results both for inorganic solids and for organic polymers. Nevertheless, it is still subjected to a number of criticisms, such as: 1) it does not consider the ionization induced by delta-rays, 2) the calculated primary ionization rate is only a relative value, and the formulation has not been scientifically approved, 3) in organic polymers, the parameter I0 must be taken as small as about 2 eV, but this energy is not

enough to ionize any atoms even though a good fit can be obtained for an organic polymer. Indeed, 𝐼₀ ≈ 2 eV indicates that the tracks are formed by molecular chain breaking, not by primary ionization (L’Annunziata, 2012).

b) Restricted energy loss

Benton and Nix (1969) suggested that not all the energy loss in solids contributes to track formation. A significant fraction of the energy is transferred to electrons of the detector with sufficiently high energy and with a range much larger than the scale of latent tracks, named delta-rays. Only the delta-rays emitted with an energy less than a value ωo will contribute to the formation

of etchable tracks.,. Thus, the restricted energy loss (REL) is calculated as:

𝑅𝐸𝐿_𝜔₀ = 2𝜋𝑛𝑒𝑍𝑒𝑓𝑓 2 _𝑒4 𝑚𝑣2 [ln 2𝑚𝑣2 𝐼 𝜔₀ 𝐼 − ln(1 − 𝛽 2_{) − 𝛽}2_{− 𝛿]}

(18)

where I is the mean ionization potential of the detector material, and 𝜔₀ is the upper limit of the delta-ray energy below which the deposited energy contributes to the formation of tracks.

The REL model considers the insignificance of delta-rays with energy higher than 𝜔₀, for this reason it fits better the experimental data for the most part, only in the region of low energy incident particles some deviation occurs (L’Annunziata, 2012). Ahlen (1980) showed that the issue with this criterion is that it cannot fit the track etch rate values in a straight line for ion particles with different atomic number.

c) Energy deposition model

The energy deposition criterion was suggested by Kobetich and Katz (1968), and it suggests that a charged particle will create an etchable track in a dielectric solid if the particle deposits an energy Ev which reaches a critical dose (volume density) by means of delta-rays at a critical distance r from the path of the particle. The calculations for obtaining Ev at a distance r by delta-rays are quite

complicated, so they will not be described in this thesis (see the original article by Kobetich and Katz, 1968).

This criterion takes into account the energy density deposited by delta-rays and the critical radius from the trajectory of the particle, and the results can approximately predict the thresholds both for inorganic and organic solids. However, it still has some problems, first, it ignores the energy deposition of the primary incident particles; second, it cannot give an horizontal line to denote the existing threshold; and third, the sharp cutoff at low energy of incident particles does not permit to detect very low energy incident particles (L’Annunziata, 2012).

2.2 Principles of track etching and counting systems

2.2.1 Chemical etching

Out of a number of techniques available for track revelation (i.e., electrochemical etching, microwave-induced chemical etching, ultrasonic chemical etching, and plasma etching), the chemical etching technique has been most widely used for all types of SSNTDs, due to its simplicity and convenience (Durrani and Bull, 1987). The process consists in immersing the detector in a

(19)

chemically aggressive solution, usually aqueous solutions of NaOH or KOH, which will cause the etching of the surface of the detector material, but with a faster rate in the damaged region. Indeed, the radiation damage trails are more vulnerable to chemical reactions as compared to other bulk material because of the large free energy associated with their disordered structure (Bhagwat, 1993; Nikezic and Yu, 2004).

In the etching process, the etching solution is so chosen to have the maximum efficiency, but at the same time to maintain the surface of the detector optically transparent after etching. For maintaining constant temperature, an electrical water bath is used, which can control the temperature of the etchant within the accuracy of ± 1°C. Stirring also has an importance in etching process to keep the temperature as well as the etchant concentration uniform, but it is particularly important to avoid the buildup of etching products on the surface of the detector, which affects the etching rates (Rana and Qureshi, 2002).

Each SSNTD has its own etching parameters. The shape and size of the tracks formed by chemical etching depend on many factors such as the angle of incidence of the charged particle on the detector surface, the type and velocity of the charged particles, the nature and crystal orientation of the detector material, etc. But even the same SSNTD might require different etching parameters as the dimension of the tracks might change depending on the type of radiation field under study and the analysis method to be utilized. Thus, normally the parameters such as the concentration, type, temperature of the etchant, and duration of the process have to be adjusted.

2.2.2 Definition of bulk etch rate Vb and track etch rate Vt

The simplest description of track geometry is based on two parameters, the bulk etch rate Vb

and the track etch rate Vt, which were first introduced by Fleischer and Price (1963). The linear rate

of chemical attack along the track is termed the track etch rate Vt, while the surrounding undamaged

material is attacked at a rate Vb, named the bulk etch rate. These two quantities are the basis of the

simplest description of the track geometry, which is determined by their ratio V = Vt/Vb. The bulk

etch rate depends on the used detector, the type and concentration of etchant, the etching temperature, and the pre-treatment of the detector. The track etch rates will depend, in addition to the above mentioned factors, on the amount of damage located in the track core region: the higher is the energy loss of the ionizing particle, the higher is Vt (Durrani and Bull, 1987; Tse, 2007).

(20)

2.2.3 Critical angle θc and etching efficiency 𝜼

For etched-track to appear, there is a certain critical value 𝜃_𝑐 of the angle , the angle between the plane of the material and the direction of incident particles, below which tracks will not be registered by chemical etching although the condition Vt > Vb is satisfied. To understand it, consider

a charged particle is incident at an angle θ with respect to the detector surface, as shown in Figure 2. After the etching time t, the etched-track length will be Vtt and the thickness of removed bulk

material will be Vbt. If Vtt sinθ < Vbt, the track will be removed away by the etchant, as illustrated in

Figure 2a. If Vtt sinθ = Vbt, as shown in Figure 2b, we have the critical situation for formation of an

etched-track. The recorded track will be observable only if Vtt sinθ > Vbt, as observed in Figure 2c

and Figure 2d.

Figure 2: Track geometry at different dip angle conditions: (a) no track is detected if the incident angle θ < θc (Vtt sinθ <

Vbt), (b) limiting case when the incident angle θ = θc (Vtt sinθ = Vbt), and (c) and (d) formation of the post etched cone for

a particle with an incident angle θ > θc (Vtt sinθ > Vbt) (Source: L’Annunziata, 2012)

The critical angle is an important parameter for nuclear track detectors, since it relates to the detector efficiency. The track registration efficiency (𝜂) strongly depends on the critical angle and is defined as the fraction of latent tracks in a detector that can be revealed by etching under specific conditions, as also expressed below:

𝜂 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑡𝑟𝑎𝑐𝑘𝑠 𝑟𝑒𝑣𝑒𝑎𝑙𝑒𝑑 𝑏𝑦 𝑒𝑡𝑐ℎ𝑖𝑛𝑔 𝑜𝑛 𝑡ℎ𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 𝑖𝑛𝑡𝑒𝑟𝑠𝑒𝑐𝑡𝑖𝑛𝑔 𝑡ℎ𝑒 𝑠𝑢𝑓𝑎𝑐𝑒

(21)

As tracks due to incident particles which approach the surface at an angle less than 𝜃_𝑐 cannot be revealed by chemical etching, the etching efficiency is given by:

𝜂 = 1 − sin 𝜃_𝑐

From this expression, it is clear that, for small critical angles, one has large track registration efficiencies. High registration efficiencies (85–99 %) are found for most plastic nuclear track detectors, having critical angles of 2–5° (Manzoor, 2007).

2.2.4 Geometry of track development

One of the challenges that have attracted significant amounts of attention was a formal description of the track development, i.e., the growth of tracks, which is a problem rather geometrical in nature. In addition, there are theories that describe the physical aspect of track formation. However, until now, there is not a single complete theory that satisfactorily explain track formation and calculates the parameters related to tracks (Enge, 1995; Nikezic and Yu, 2004).

The simplest case of track development refers to an incident particle which enters perpendicularly to the detector surface, as shown in Figure 3. In this figure, I is the initial detector surface, I’ is the surface after the etching, O is the entrance point and E is the end point of a particle in the detector material, while OE = R is the particle range in the detector material. The distance between I and I0 is equal to h, i.e., the thickness of the layer removed by etching, L’ is the total

distance traveled by the etching solution along the particle track, and L is the track depth (Nikezic and Yu, 2004).

Figure 3: Geometry of the track development. The incident angle is normal with respect to the detector surface, and Vt is

(22)

In one aspect, track development is analogous to wave propagation. According to Huygen’s principle, each point in the wave front is the source of a new spherical wave. In the case of the track development, a hemisphere with radius h = Vbt (t is the etching time) is formed around each point on

the detector surface, except in the direction of the particle path where the etching progresses with the rate Vt (Nikezic and Yu, 2004).

Track development is governed by the ratio V = Vt/Vb and the track formation is not possible if V is smaller than or equal to 1. In three dimensions, the track is a cone with a developing angle δ,

which is obtained by rotation of the track wall around the particle path. From the similarity of triangles in Figure 3, is can be observed that:

sin 𝛿 = 1 𝑉

The circle A in the Figure 3 represents the revolution of a point on the track wall around the particle path. During the etching, the track wall moves parallel to itself.

2.2.4.1 Track etching geometry for constant Vt and normal incidence

The calculations of the parameters of the etched particle tracks are comparatively simple when the track etch rate is assumed to be constant. Although the track is a three-dimensional structure, for the sake of simplicity, the track depth L can be calculated considering a two-dimensional appearance, as shown in Figure 3.

The track depth is given by:

L = (Vt – Vb)t = (V – 1)h

where t is the etching time. From Figure 3, we can see that:

tan 𝛿 = 𝐷 2⁄ 𝐿 =

ℎ √𝐿′2_{− ℎ}2

(23)

and combining the previous equations, one can find the diameter of the track opening as:

𝐷 = 2ℎ√𝑉 − 1 𝑉 + 1

The evolution of an etch pit profile with prolonged etching can be divided in three phases: conical, transition, and spherical, as shown in Figure 4. During the etching, the etchant progresses towards the end point E of the trajectory (full length of latent track R) at a rate Vt, reaching that point

at time t0 = R/Vt. Up to this point of time, the track end is sharp and fully conical. In this conical

phase, the relevant etched surface is denoted as surface 1 in Figure 4. After the time t0, the etching

progresses in all directions at the same rate Vb, which is referred to as the transition phase. During the

time that the etchant descends from surfaces 1 to 3, the pit profile becomes progressively rounded at the bottom, forming a sphere around the point E. Finally, when the etchant has reached surface 4, the whole track profile becomes completely spherical, and the track has gone into the spherical phase (Nikezic and Yu, 2004; Tse, 2007).

Figure 4: Three phases in the track development. I is the initial detector surface, O and E are the entrance and end points of the particle path, R is the particle range in the detector material, and Vb is the bulk etch rate. (1) Conical track; (2 and 3) the track wall is partially conical and partially spherical; (4) the track is fully spherical (Source: Adapted from Nikezic

(24)

2.2.4.2 Track etching geometry for constant Vt and oblique incidence

In most realistic applications, the incident particles strike the detector with oblique instead of normal incidence, i.e., irradiation of a detector by radon or cosmic rays. Somogyi and Szalay (1973) considered that problem in detail, coming up with the following conclusions.

The cross-section between a track in the conical phase (surface 1 in Figure 5) and the post-etching surface (surface 2 in Figure 5) is an ellipse and the corresponding track opening is elliptical. The ellipse is characterized by its major and minor axis, D and d, respectively. If the track is over-etched, the post-etching surface might cut both the elliptical and spherical parts of the track wall. In this case, the track-opening contour is a complex curve that consists of an ellipse and a circle jointed at some points. With prolonged etching, the spherical part of the track wall, and thus the circular part of the track opening, are enlarged leading the track to a completely circular form, as shown in Figure 5, surface 3. (Nikezic and Yu, 2004; Tse, 2007). For each phase of the track opening (major axis D1, D2 and D3, and minor axis d1 and d2), analytical formulas were derived and expressed in terms of

removed layer h, by Somogyi and Szalay (1973). In Figure 5, the incident angle θ is measured with respect to the detector surface, and h0 is the removed layer when the etchant reaches the end point E

of the particle path.

Figure 5: Phases of track development for oblique incidence (Source: Nikezic and Yu, 2004).

In the last sections, track development with a constant Vt for normal and oblique incident

(25)

growth for a variable Vt is still being established. It is generally accepted that the track etch rate

varies along the particle trajectory (Dörschel et al., 2002b), and it is correlated with the particle energy loss (Dörschel et al., 2002a).

2.2.5 Counting and analysis systems

In the past decades, new imaging techniques have been increasingly applied in etched-track detector research. Among these, coherent light scattering has been introduced and used to extend the linear range of track densities that can be scanned. Confocal microscopy has been used for the three-dimensional reconstruction of the tracks and the analysis of charge and energy of the initiating particle. Finally, atomic force microscopy, recording depth and size of the tracks at nanometric levels, has been used to investigate the early stages of track development, to analyze surface roughness and possibly discriminate between tracks and spurious defects in the detector (d’Errico and Bos, 2004).

However, the most common method of viewing and making quantitative measurements of the observable track parameters like number, length, diameter, etc. is through an optical microscope with calibrated eye pieces and mechanical stages under transmitted light, and with magnifications ranging from 400 to 1000 X. Images of the tracks focused under the microscope are scanned through a camera to an image digitizer in the PC, where they will be analyzed by a proper software. This method is used by all the commercially available track reader, which are only four, the Autoscan 60 (Thermo Electron Corporation, Santa Fe, MN, USA), the Radometer 2000 Series (Radosys Ltd., Budapest, Hungary), the Taslimage System (Track Analysis Systems Ltd., Bristol, U.K.), and the HSP-1000 (Seiko Precision, Chiba, Japan).

The main reason why this technique of optical image digitalization and image processing became so popular, was due to the advances and general availability of computing and imaging technologies over the past decades. Processing speed and data storage of personal computers have increased by several orders of magnitude, while high-resolution digital cameras with high grey-scale depth have become common. In combination with motorized-stage microscopes and image-analysis software, these equipment permit investigations of track parameters with extraordinary ease and power (d’Errico and Bos, 2004).

(26)

3 Neutron detection using SSNTDs

The neutron is a neutral particle and thus it cannot directly produce ionization and excitation of atoms. Therefore, a neutron cannot be recorded directly by any kind of radiation detector, including etch track detectors, but it can be measured indirectly through its recoil nuclei and reaction products with the nucleus inside the detector material itself or in an external converter layer (also called radiator) in front of the detector materials. Recoil nuclei H, C, O in CR-39 (C12H18O7) can be

recorded directly in the detectors (L’Annunziata, 2012). In the case of slow neutrons, the converter is normally a solid containing 6Li or 10B. For fast neutrons, these target nuclei have very low reaction cross sections, and elastic scattering from hydrogen is the preferred way to generate charged particles (recoil protons). The converter can consist of a layer of polymer containing hydrogen such as polyethylene or polypropylene, and the recoil protons escaping from its surface will be incident on the solid state nuclear track detector (Knoll, 2010).

Together with TLDs, etched-track detectors have become the most commonly used passive neutron personal dosimeters. This is the result of the great efforts spent since the 1980s to improve etching procedures, read-out equipment, and especially the quality of the detector plastics, aimed at reducing extent and variability of their background track density (d’Errico and Bos, 2004).

Among the many types of etched-track detectors developed during the years, the one of polyallyl-diglycol carbonate (PADC), i.e. CR-39, have become the most popular. The reasons for that along with a review of the other main types of detectors will be presented in the next topics.

3.1 Polycarbonates

Before the discovery of the CR-39 as a track detector, the most commonly used type of detectors was made by polycarbonate (PC). This material is still frequently used when proton tracks may be a strong background, however, due to its lower sensitivity, it was almost completely replaced by CR-39 (L’Annunziata, 2012).

The composition of polycarbonate is C16H14O3, with a molecular weight of 254.2855 g/mol,

and a density of 1.29 g/cm3_{. The benzene ring in its structure (Figure 6) has a tendency to absorb}

(27)

minimum recordable charge (atomic number) Z of particles is two alpha-particles (L’Annunziata, 2012).

Figure 6: Structure of the polycarbonate monomer.

General Electric Company produces the Lexan, which is a commercial name of the product of Bisphenol-A polycarbonate foil, and one of the most widely used detectors in the polycarbonate series. Makrofol is a polycarbonate material originally manufactured as an electric insulator by Bayer AG of Leverkusen, Germany. Both Lexan and Makrofol can be used as a nuclear recoil detector without radiator for fast neutron detection. Bare polycarbonate sheets will record recoil nuclei of oxygen and nitrogen produced in air by neutrons. Usually, an aluminum plate is placed in contact and in front of the detector to shield off the recoil nuclei from the air, or alternatively, to treat air as a radiator. Polycarbonate sheets sandwiched with fissile foils can record fission fragments (fission rate measurement) and consequently derive the neutron fluences and neutron spectrum (L’Annunziata, 2012).

There are many types of Makrofol (Makrofol-E, Makrofol-KG, Makrofol-D, etc.), the most recent one being Makrofol-DE. It can be found in many thicknesses, from 125 to 750 µm, and is considered a promising material for dosimetry due to its high sensitivity to alpha-particles, and compared to CR-39 detectors, it costs less and has a more stable background. Nevertheless, it doesn’t treat the CR-39 detectors when it comes to fast neutron dosimetry, mainly due to its high energetic threshold of detection, of 1.5 MeV (Barreto, 2011).

3.2 Cellulose Nitrate

Cellulose nitrate (CN) is one of the most sensitive nuclear track detector materials. Its sensitivity is just less than CR-39, with the least ionizing ion being 0.55 MeV protons. It is made

(28)

from absorbent cotton (cellulose) reacted with concentrated nitric acid. The three –OH groups in the molecular structure of cellulose are partly or all replaced by –O–NO2 groups in the reaction. The

formed structure of cellulose nitrate with 2 –O–NO2 groups is shown in Figure 7, with formula

[C6H8O9N2]n and density of 1.4 g/cm3.

Figure 7: Structure of the cellulose nitrate with 2 –O–NO2 groups.

The most know cellulose nitrate detector commercially available today is the LR-115. Developed by Kodak-Pathé, France, this detector is mainly intended for the dosimetry of small quantities of ionizing particles (mainly alpha-particles), but they can also be used for neutrons. It is formed by a polyester base of 100 µm thick, with a thin reddish layer of cellulose nitrate deposited on one of the faces. That being the sensitive area.

There are many types of LR-115 to detect neutrons. If the intention is detecting thermal and epithermal neutrons there are the Type 1 and Type 1B, both 6 µm thick and designed to detect alpha-particles arising from (n,α) reaction of 10_{B (1.6 MeV) and}6_{Li (2.4 MeV), respectively. If a cadmium}

screen covers one part of the film, it is possible to differentiate thermal and epithermal neutrons. To detect fast neutrons, the Type 2 is recommended. On this detector the tracks arise from the transfer of energy from the nuclei of the constituents of the cellulose nitrate itself, that have been bombarded by neutrons of high energy, from 1 to 14 MeV. To increase sensitivity the LR-115 Type 2 has a cellulose nitrate layer of 12 µm thick, twice as the Type 1 and Type 1B (DOSIRAD, 2018).

Although the LR-115 is able to detect a wide range of neutron energy, it’s not the most sensitive SSNTD available. Besides that, cellulose nitrate is a flammable material which represents a risk on experimental procedures in the laboratory.

It is worth mentioning that the company Kodak-Pathé also developed another type of cellulose nitrate detector that had received significant attention, the CN-85 (previously known as CA 80-15).

(29)

But according to the company currently producing and selling Kodak-Pathé nuclear films, DOSIRAD, the CN-85 had its production discontinued due to the low demand (DOSIRAD, 2018).

3.3 CR-39

The chemical composition of the monomer of CR-39 (Columbia Resin 39), is C12H18O7, with

its molecular weight being 274.2707 g/mol. It has a chain-like structure, as shown in Figure 8. In polymerization of CR-39 monomers, the double bonds of allyl groups (CH2CH=CH2) open and form

a cross-linked net structure in three-dimensional directions with other molecules, forming polyallyldiglycol carbonate (PADC).

Figure 8: Structure of the CR-39 monomer, the polyallyldiglycol carbonate.

It had been discovered earlier in 1945, originally for lens manufacture (Bartlett, 2008). But only in 1978 Cartwright et al. reported its ability to detect recoil protons produced by fast neutrons.

Since then, the CR-39 has become one of the most commonly used nuclear track detectors due to characteristics such as (L’Annunziata, 2012; Padilha, 1992; Vilela, 1990):

• Low detection threshold: Its minimum recordable ionizing particle is a 20 keV proton;

• Physical characteristics: The possibility of being used in small dimensions, light weight (density 1.31 g cm-3_{), optically transparent, and resistant to weather factors when stored for a}

long time;

• Can determine particle’s properties: It can not only record track number, positions (X, Y, Z), directions (zenith and azimuth angles), but also derive particle charge, mass, and energy from the track parameters;

(30)

• The detection of wide range of neutron energy: It is possible due to the use of converters such as high-density polyethylene (HDPE) for fast neutrons, and 6Li, 10B or 14N for slow neutrons; • Insensible to photons and electrons: It is practically insensible to these radiations even in

relatively high doses (MGy), which allows its use in mixed field environments;

• Fading stability: Its tridimensional lattice structure makes it difficult to for any recombination to happen after the break of its molecular chains, which qualifies it as a “thermoset” type of polymer.

The main drawbacks of the CR-39 are the high energetic and angular dependence and the instable background which makes it to have a high minimum detectable dose.

Due to its popularity, the CR-39 is produced by many companies, such as: Radosys, TASL, American Acrylics and Plastics, Intercast Europe SpA, Chiyoda Technol Ltd., Landauer Inc., and Page Mouldings (Pershore Mouldings). It is currently the most widely used solid-state nuclear track detector (L’Annunziata, 2012).

(31)

4 Sensitization methods

In the practice of the automatic track counting, an artificially increased sensitivity means that the average size of the smallest, or even invisible, tracks is increased to the size range that can be more efficiently treated by the object recognition algorithm (Hulber and Selmeczi, 2005). There are various forms of sensitizing PADC type track detectors, the most known ones, which will be discussed in this chapter, are the treatment with carbon dioxide under pressure and UV-irradiation.

4.1 Carbon dioxide treatment

The sensitization effects of carbon dioxide gas under several atmospheres on PADC track detectors, was first reported by Fujii et al. (1995, 1997), and soon followed by a more detailed examination by Csige (1997). Since then, various works have been published aiming to understand the mechanism of the sensitization enhancement (Yamauchi et al., 2009; Hassan et al., 2013), from which a way to control the response of the PADC detectors could be learned. Nevertheless, this process has not been completely resolved yet, and further research still needed.

Most of these studies have investigated the effects of carbon dioxide treatment on the alpha-particle and fission fragments registration sensitivity, however, for most commercial uses, i.e. radon measurements, this technique is currently unnecessary. The advances on counting systems, sensibility of CR-39 detectors and chemical etching conditions, allowed tracks from high LET particles to be counted with ease without the need of a further sensitization technique.

On the other hand, this method is very important in the field of individual neutron dosimetry, where it is particularly difficult to automatically count the tracks of the recoils protons from fast neutrons, which can be completely erased by chemical etching procedures due to their limited dimensions (Hulber and Selmeczi, 2005). Nevertheless, until this day, only a few studies have focused on the enhancement of the CR-39 response to fast neutrons and its recoil protons (Hulber and Selmeczi, 2005; Shimada et al., 2011).

(32)

4.1.1 Mechanism of sensitization

The sensitization mechanism has not been fully understood yet, but there are some results from which a model is being constructed around. One of the most basic is that the sensitization enhancement is only seen if there is a certain amount of carbon dioxide inside the detector during the etching process. In 1986, the Bristol group reported a rapid drop of the sensitivity of newly manufactured CR-39 over the first several days (Portwood et al., 1986), and thus they supposed that the effect might be due to mechanical stress in the plastic or to an excess of CO2 inside the plastic

after manufacture. In 1995, Fujii et al. proved that the diffusion of this excess CO2 was causing the

loss of sensitivity, by replacing the gas into the detectors and seeing a sensitivity increase. Later on, this condition was further studied and confirmed by Yamauchi et al. (2009) and Hassan et al. (2013). Based on this fact it was hypothesized that when the etchant is penetrating into the detector and carbon dioxide is passing through the opposing direction, from the detector to the solution, the carbon dioxide somehow interacts with the etching solution and/or the detector’s material itself. Tse et al. (2007) showed that the carbon dioxide can interact with K+ ions in the etching solution forming, for example, K2CO3. Hassan et al. (2013) showed how a continuous flow of carbon dioxide

can make it easier to remove smaller molecules such as ethylene glycol from the surface of the detector, which enhances the track etching rate. The idea of carbon dioxide washing the detector’s surface and bringing segmented parts of PADC network into the etching solution was also corroborated by Enge (1995). These results are supported by the detection of ethylene glycol in the etching solution, along with other organic compounds (Kodaira et al., 2012). An important point is that the ethylene glycol was detected only for the irradiated detectors. The products of the bulk etching were limited to diethylene glycol and polyallylalcohols because the attack of the hydroxide ion results in the hydrolysis of the carbonate ester bonds (Gruhn et al., 1980), which along with ether bonds are known to be the dominant breaking points in PADC (Yamauchi et al., 2005, 2008; Tse et al., 2007; Sahoo et al., 2014).

Although incomplete, this idea is able to explain why the increase in treatment time or in the pressure of carbon dioxide enhances the sensitivity of the detectors (Csige, 1997; Yamauchi et al., 2009). It makes the flux of CO2 last longer, enhancing its effect. When the treated detector is inside

the chemical solution, which is usually at temperatures ranging from 60 to 90 °C, the diffusion coefficient of the carbon dioxide increases exponentially when compared to a treated detector kept in

(33)

air (Figure 9a). The gas is then released away very fast, especially in the early stage, at a certain flux, as shown in Figure 9b. Therefore, it is logical to assume that the more CO2 present in the detector

before the etching, the longer and more intensely it will act on removing smaller molecules from the detector’s surface, enhancing the track etch rate.

Figure 9: a. diffusion coefficient of carbon dioxide in the PADC detector as an inverse function of the temperature, and b. flux of carbon dioxide on the surface of the PADC detector with a thickness of 0.9 mm during the chemical etching at

70 °C (Source: Hassan et al., 2013).

4.1.2 The sensitization effects on irradiated CR-39 detectors

One of the most important characteristics of the carbon dioxide treatment is that it’s not very strongly LET dependent (Csige, 1997), which makes its use possible to a very large array of applications, such as neutron dosimetry.

However, the two relevant parameters that influences the detectors sensitization to this treatment are the time and the partial pressure. These parameters were extensively studied by Csige (1997) and Yamauchi et al. (2009). The first one investigated the sensitization effects for incident alpha-particles of 6 MeV, while the second one studied it for five different energies, ranging from 0.52 to 4.94 MeV.

Figure 10 shows their results on the sensitivity S (in this work we labeled as V), namely, the reduced etch rate ratio S-1 of samples treated under 0.6 MPa carbon dioxide, against treatment time.

(34)

Both studies observed the same trend of increasing sensitivity, even though the detectors used were different (Csige used TASTRAK CR-39 sheets from Bristol, England, while Yamauchi used BARYOTRAK from Fukuvi Chemical Industry, Japan).

On the study of sensitivity against the pressure of carbon dioxide, Yamauchi et al. (2009) reported a monotonous increment with the pressure, showing a saturation behavior above 0.2 MPa, especially for lower incident energies, as seen in Figure 11a. On the other hand, Csige (1997) has reported an increasing trend of sensitivity on detectors treated up to 0.9 MPa.

Figure 10: Increase of sensitivity on detectors irradiated with alpha-particles of 0.54, 1.68, 2.78, 3.83 and 4.94 MeV due to 0.6 MPa CO2 treatment against treatment time. Plots for 6 MeV are according to Csige (1997) (Source: Yamauchi et

al., 2009)

Figure 11: Changes in sensitivity with treatment gas pressure. The treatment time was a. 3 days, and b. 1 day (Source: a. Yamauchi et al, 2009, b. Csige 1997).

(35)

As there were no such studies for fast neutrons, direct comparisons to the results that have been found in this work could not be made.

4.2 Ultraviolet treatment

After having its sensitization properties first reported for Lexan polycarbonate films, by Benton (1968), the first one to apply the UV treatment on CR-39 detectors was Hussain, in 1982. In the same year, Wong and Hoberg published an article comparing the effects of this treatment on both Lexan and CR-39 samples. Since then, many studies have been published, most for alpha-particle or fission fragment irradiation, and only one for neutrons (Matiullah and Kudo, 1990).

It has been observed that the exposure of a polymer to ultraviolet (UV) radiation causes chemical and physical changes on its surface, which are dependent on the nature, intensity, duration and the wavelength of the incident radiation. The energy transferred to the polymer by the UV irradiation leads to excitation and ionization of the molecular chains, hence to radical formation, main chain bond scission, and also to crosslinking of polymeric chains. These chemical changes cause deterioration of the physical, mechanical and optical properties of polymers, which in turn affects its bulk and track etch responses.

4.2.1 Chemical and physical changes in UV irradiated PADC detectors

The UV radiation can disengage molecular chains of a polymer by breaking its chemical bonds, which occur through smaller energy transfers than are typically required for ionization (2-3 eV as opposed to 10-15 eV) (Saad et al., 2015), thereby producing free radicals. The radicals so produced can initiate two major reactions, crosslinking and chain scission. It has been shown that the increase on chain scission is related to the increase of hydroxyl groups (Tidjani, 1990; Tse et al., 2006; Jaleh et al., 2017), while crosslinking takes place when free radicals migrate and recombine with other radicals or the main chain. Both reactions occur simultaneously and the dominance depends essentially on the polymer (Tse et al., 2006).

Crosslinking leads to an increase in the molecular weight while chain scission has the opposite effect (Tse, 2007), which is important because it affects the bulk etch rate. Polymers with lower molecular weights will dissolve more rapidly because of their shorter chain lengths, increasing the