Nuclear Track Detectors for Relativistic Nuclear Fragmentation Studies : Comparison with Other Competitive Techniques

The potential of the high resolution nuclear track detector (NTD) CR-39 is examined carefully for the measurement of relativistic nuclear projectile fragmentation cross sections and studies of related processes using the experience of many years of such measurements. The charge resolution and the charge resolving power of CR-39 detectors for the measurements of 158 A GeV Pb projectiles and their fragments are presented. Exposures of target-detector stacks, the chemical etching procedure and the nuclear track measurements are described in detail discussing precautions and possible errors. The procedures discussed are also valid for other NTDs. A comparison with electronic active detectors is also made considering important detection and measurement aspects. An experimental design proposing the co-use of NTDs with in-use active detectors is described.


Introduction
Fragmentation properties of shielding materials can affect protection of astronauts seriously.Radiations have also an adverse effect on microelectronic devices employed in the control systems of high altitude commercial aviation aircrafts and spacecrafts, leading to the failure of such devices [1].Nuclear fragmentation cross sections are crucial to issues of radiation transport through shielding materials and resulting shielding effect [2].Nuclear fragmentation cross sections also give information about collision dynamics of high energy heavy nuclei like lead and uranium.Various techniques [3][4][5][6][7][8] have been employed for studies of nuclear fragmentation and other reactions, including solid state nuclear track detector (NTD) [9][10][11][12][13][14][15][16][17][18].Here, we discuss experimental meth-ods and related problems for the measurement of fragmentation cross sections using NTDs.The present paper is based on the results from several investigations on the methodology of nuclear tracks [19][20][21] and the use of NTDs in nuclear fragmentation studies [22][23][24][25].Present paper addresses two issues: 1) Concise description of experiments to detect fragments of exotic nuclear reaction with CR-39 and other nuclear track detectors and 2) evaluation of experimental methods for the measurement of relativistic charged particles by passive and active detectors.The study of such processes is important for fundamental physics reasons and technical applications in fusion plasma, dosimetry in space and heavy ion therapy.Due to these reasons, the present study is interesting for several scientists in nuclear physics and their application.

High Sensitivity High Resolution Detector CR-39
An NTD is a solid material (normally plastic, glass or a crystal), when exposed to a single charged radiation, a damaged trail of diameter 2 -10 nm is produced which, through chemical etching, can be amplified to microscopically observable conical shapes.The main advantages over other radiation detectors are the detailed information available on individual particles, the persistence of the tracks allowing measurements to be made over long periods of time, and the simple, cheap and robust construction of the detector.A material commonly known and used as an NTD is CR-39.Its chemical name is polyallyl diglycol carbonate (PADC).It is a transparent and rigid plastic material with the chemical formula C 12 H 18 O 7 .Etching is usually performed in solutions of caustic alkalis such as sodium or potassium hydroxide at temperatures of 60˚C ± 30˚C from few minutes to several hours.CR-39 is the most sensitive and the highest resolution NTD.It is sensitive to a wide range of charges down to Z = 1-6e (depending on the Z/β of the incident particle.Its charge resolution of 0.05e for precisely designed exposure and etching experiments.CR-39 was used to search for exotic particles, like Magnetic Monopoles and Strange Quark Matter (SQM) to study cosmic ray composition and for environmental studies [26].

Target-Detector Assemblies and Exposures
Nuclear track detectors can be employed for the measurement of fragmentation cross sections of relativistic projectiles using appropriate target-stack designs.We have employed CR-39 detectors (prepared by Intercast Europe Company of Parma, Italy) for fragmentation studies of relativistic projectiles on various targets.The method of these experiments is the measurement of the beam incident on the target and after the target where are present the survived beam and the fragments or seconddary particles produced in the interaction of the incident beam with the target.The design of target-detector assemblies and exposure geometry are shown in Figure 1.Several such stacks of CR-39 detectors and targets were exposed to 158 A GeV 207 Pb beams at the CERN SPS facility.The target thicknesses were chosen in such a way that multiple interactions were negligible, but still appropriate to produce sufficient fragments.The CR-39 detector plates were 11.5 × 11.5 cm 2 in size and 600 μm in thickness.CR-39 detectors upstream of the targets were used for the determination of the total number of incident Pb ions.The density of lead beam ions was around 1500 ions/cm 2 .The beam then passed through a target of thickness typically half of the mean free path of the Pb ions in the given target.CR-39 foils downstream of the target recorded both the survived Pb ions and their fragments produced in targets.NTD based experiments are very simple compared to those which employ multi wire proportional chambers (MWPCs), ionization chambers (ICs) and Cherenkov detectors (CDs) [27,28].The later experiments necessitate an online data acquisition whereas NTDs provide the possibility of offline data acquisition and are attractive for research groups which do not have direct access to high energy accelerator facilities.

Chemical Etching
Chemical etching is an essential step for NTD experiments.Etching amplifies the nanometer diameter particle trails in NTDs to microscopic dimensions appropriate for observations with optical microscopes.Nuclear track etching involves a number of factors without precisely quantized control.Most important factors are concentrations of active etching species or molecules in etchant, temperature control and stability.The variation in etching behavior of the detector material causes uncertainty in experimental etching results.The variation in the etching temperatures was not more than 1˚C.A standardized process was used during etching of exposed CR-39 detectors.An efficient and stable level of stirring was maintained.Concentration of etchant was kept effectively constant under specific etching conditions by minimizing the water evaporation from the etching solution.Exposed CR-39 detectors were etched in 4N KOH water solution at a temperature of 45˚C for 72 h.An average thickness of about 8 μm was removed from both sides of the detector with an average bulk etch rate of 0.112 ± 0.021 μm/h.The refractive index of the CR-39 was experimentally determined as 1.561 which was later used for the determination of the total height of the etched cones.

Detector Observations
Measurement of track parameters is an extremely slow process if it is done manually.All the track measurements at the PINSTECH laboratory were done manually.Manual measurements are very slow compared with automatic scanning systems, but provide visual insights of track forming particles.Discrimination between real events and background or erroneous events is more trustworthy in manual measurements compared to the discrimination made by an automatic nuclear track scanning.The measurement of cone heights of tracks in three CR-39 foils for the case of each target used, one set of foils for each target used, one set of foils before every target and two after, were measured with an optical microscope manually (more than 6000 etched cones were measured in each foil).It was observed that the cone length increased with increasing ion charges.Measurements of nuclear fragmentation were made with observations of change in cone heights.Zeiss optical microscope with a magnification of 40× was used for these measurements.Cone heights of Pb projectiles and fragments were measured by the top view using the depth measuring system coupled with the microscope.Difference of readings on depth measuring instrument, one when track diameter at the detector surface is focused and other when endpoint of etchable range is focused in the microscope, gives the cone height of a track.Track length measurement resolution or least count of the optical microscope used was ±0.5 μm.Counts in each peak are a measure of production cross section of the corresponding fragment.So, each peak represents production yield of fragments with a specific Z.Well-defined and separated peaks mean precise measurements.It is worth mentioning that detector, beam and etching conditions were same for all plots, but range of etched cone height is different for the case of Al (C) from other three cases.Major differences are difference in brands of KOH used and different chemical etching equipment.Major differences in etching equipment include difference in size and different magnitudes of stirring during etching.

Calibrations
Calibration here refers to the standardization of the particle detection response.Calibration is normally a curve or a relationship between two parameters, one of which is other related to a physical property of the incident particle.Such a measurement curve or relationship represents the detection response of a detector and can be employed to characterize unknown particles.In the present case, the etched cone distribution in Figure 2 was used to calibrate the CR-39 plastic track detector.It is an important step which standardizes response of the detector under a specific set of conditions for the experiment.Etching conditions and precision and repeatability of etching procedures have a strong influence on validity of the calibration.
Figure 3  easured etched cone height and correlated fragment charge.A linear relationship is observed between cone height and corresponding fragment charge Z. Equation of the fitted line is shown in the plot along with regression coefficient and standard deviation, demonstrating excellent fit. Figure 4 shows the reduced etch rate "p" vs "REL" value.The error bars include statistical and systematic uncertainties.The points are the experimental data and the solid line is the best fit to the data points.The fitted equation is valid only in the REL range 3700 to 7100 MeV•cm 2 /gm.This plot can be used to identify unknown particles registered in CR-39 using same etching conditions by determining REL value of the unknown particle corresponding to the measured reduced etch rate "p". Figure 6 shows the charge resolving power (η) of CR-39 detector.Charge resolving power is a measure of the ability of a detector to observe a charge resolved.It is reciprocal of the charge resolution Z  for a specific charge Z and is defined as, her charge means higher energy deposition rate Charge resolving power CR-39 decreases with charge.ig H d d or REL which reflects into enhanced resolving power η.For the present reaction and etching conditions cannot detect or resolve charge below 63.

E x
Figure 7 shows our experimentally measured partial GeV Pb on Cu

Merits
single particle detectors.Every individual ed as a separate event in a NTD for a charge-changing cross sections of 158 A target, as an example.For comparison, cross section measurements of the same projectiles with the same energy on the same target measured by Schiedenberger et al. [3] using an active detector, multi sampling ion chamber (MUSIC).The inset plot shows the comparison of measurement errors.Detection range of the MUSIC detector (Z = 60 -82) is wide than that of CR-39 in the present case (Z = 63 -82).But, in general our measurement error is comparable to that of MUSIC and are slightly better for Z = 76 -82.Our measurements do not allow us to say anything directly about mass changing resolution of fragments as detectors used were only sensitive to charge of fragments and not to their mass.

General
The NTDs are particle is record certain dose or fluence of particles for which events do not overlap.Using the traditional NTD methodology with chemical etching and optical microscopy, a dose of particles up to 10 5 cm −2 can be measured.The present use of AFM can extend the observable dose limit up to 10 8 cm 2 .Important useful features of NTD include small and simple experimental setups, integrated exposure ro capital and operational costs.A method, called fission track dating (FTD), is based on nuclear tracks and is employed to unfold the thermal history of rocks [31].

Specific to Fragmentation
NTDs have several advantages over other tech or reaction studies.A NTD based e (~50 cm) than others: a set up with the MUSIC detector has a length of 2 m [3].So, a NTD experimental setup can easily be coupled to the beam exit point at a high energy accelerator facility.No triggering system for data acquisition not needed in NTD experiments while it is required in electronic experimental setups.Normally, one uses several exposure spills in the case of a NTD target-detector assembly.One spill can be studied as a test case.The procedure of chemical etching can be optimized offline.Incident beam particles and fragments or other particles produced in the reaction are permanently recorded in a NTD without any sophisticated technology.

General
Tedious efforts a energy resolutio related to preci especially uniformity, precautions in exposures and chemical etching.Information about the mass of the incident particles can be deduced with a considerable error.
It lacks real time information about incident particles.No unified and clear physical and mathematical picture of nuclear track formation is available.Nuclear track etching is an essential but still a wild procedure due to the involvement of a number of parameters without quantitative microscopic control.Even microscopic picture of nuclear track etching is well understood.Etch induction time is an example of lacks of understanding.Concentration of active etching species, control on etchant temperature, and variation in etching behaviour of the detector material are major causes of uncertainty and wildness of chemical etching.

Specific to Fragmentation
Apart from NTDs, other detectors in use for high energy nuclear fragmentation are ionization chambers (ICs), tor detectors (SeDs),

Quark Gluon Plasma
of tation en-mplete identification of a parand wire chambers (WCs), semiconduc scintillation detectors (ScDs) and Cherenkov counters (CCs) [27,28].They are online detectors and have a certain superior characteristics.For example, MUSIC is a charge sensitive detector having a smaller number of atoms/cm 2 compared with solid NTDs [29,30].So, in MUSIC there are less secondary hadronic interactions.Much better statistics can be achieved using electronic detectors.Track statistics is limited in NTDs by a certain dose as they record all the particles.So, a fluence of 10 2 -10 4 /cm 2 is a good achievable statistics.It may be noted that achievable statistics depend on diameters of particle tracks and high statistics increase error is statistics due to the overlapping of tracks.

Nuclear Fragmentation versus Fullerene Fragmentation and
There are two possibilities for the fragmentation highly excited nuclei.First is the case when fragmen products depend on magnitude of the excitation ergy induced by the interacting projectile whereas in the second case they depend on magnitude and nature of excitation.The second possibility has some relevance with the case of fragmentation of fullerenes.Fullerene is a cluster of atoms forming a structure with narrow cages whereas nucleus is a cluster of nucleons moving inside in a complex manner.Despite bonding nature of constituents of nucleus and fullerene differs significantly, fragmentation of these two objects may have commonalities.It is difficult to apprehend the nature of excitation of a highly excited nucleus due to involvement of a large number of degrees of freedom some of which still need to be further understood.[32,33].Another interesting outlook realized in this study is the formation of shock wave in relativistic nuclear collision similar to that in fusion plasma in which nuclei collide to hammer each other producing a shocked high density nuclear material which may undergo some oscillations [34] before its fragmentation in somewhat the same way as fullerene under ion bombardment.The shock approach is valid for both central and peripheral interactions due to their penetration into each other, which was initially was proposed by Siemiarczuk and Zielinski in fragmentation of Xe nuclei induced by negative pions [35].The oscillations in the shock are damped due to energy release through evaporation of particles and/or fragmentation [36].Formation of shock is more probable in central collisions case where the nuclear material may undergo through a stage similar to plasma dust crystal [37,38] leading the multi-fragmentation.Due to variance of density of in the collided nuclear material, the formation of dispersive wave can form density varying rings, like Saturn's B ring.Scientifically-based technical chronological details of these rings have been provided by Massod et al. [39] taking the support of the Hubble Space telescope observations of the "spokes".

Co-Use of NTD with Other Techniques
In physics experiments, co ticle means information about its energy, charge mass.In most experiments, limited measurements of par ticle parameters are made and more information is made available by correlating detector observations with the theory and already available information about particles or reactions involved.Every technique has specific merits and demerits, so use of more than one technique can enhance the information about the nuclear reaction and its precision.NTDs remained slightly underdeveloped owing to their passive nature.In a recent study, it was investigated how precision and reliability can be achieved in experiments using NTDs [40].The NTD allow tracking of particles with measurement of their energy and charge.So, despite their passive nature, NTDs could supplement electronic measurements.
Figure 8 shows a schematic proposing the co-use of NTDs with online electronic techniques.Such a design could be useful regarding precision, reliability and widening of scope of the experiment.In the proposed design, the use of three fold ladder is suggested.The central arm will serve as a target as in other experiments with active detectors.The other 2 arms will hold CR-39 samples for a defined time, one upstream and other downstream of the target, respectively for the measurement of projectiles and their fragments.Co-use of active and passive detectors could add confidence and reliability.NTDs are limited to 10 4 -10 6 projectiles/cm 2 .When the defined dose of projectiles is reached, the CR-39 detectors could be removed leaving online electronic experiments in place.Such experiments could help involving researchers from developing countries where lengthy part of track measurements in NTDS is possible.
In some large experiments, this combination was made: for example the MACRO (monopole astrophysics cosmic ray observatory) [41,42] experiment in which NTDs were used as a subdetector.The NTD subdetector was used in the middle and in two sides of its lower structure [43].NTDs are suitable for cosmic ray studies including the searches for exotic particles.They have been employed for such measurements with good precision [44][45][46][47].One half of 32 searches for magnetic monopoles (MMs) were performed with NTDs.Monopole and Exotics Detector at the LHC (MoEDAL) aims to search for MMs at CERN LHC [48].Co-use of NTDs with other techniques [49][50][51][52][53] would enhance their use for cosmic rays and other exotic particle searches for which they are most suitable due to experimental simplicity, light weight and low cost.
Figure 9 is an artistic portraying of the finding reported in paper and related.It also includes segments on the field of radiation detection & measurements and the nuclear safety which are related to the subject of this article.

Conclusion
Selected results of experiments, carried out over many aim of precise measurements of rela-ration of the staff of the hanks are also for the techyears with the main tivistic nuclear projectile fragmentation cross sections and related processes using NTDs, were discussed and we cover some major aspects of nuclear track detection technique.The experimental precautions described here for exposure, chemical etching and track measurements are applicable to experiments in a laboratory with facilities considered essential for NTDs.Possible experimental refinements in nuclear track etching are discussed.
Fragmentation results obtained with CR-39 detectors have been compared with competing active detectors.Issues of charge resolution and charge resolving power of CR-39 were discussed.The results discussed may be useful for the community involved with applications of nuclear instruments and methods, especially in studies of nuclear reactions and cosmic rays.

Figure 2
Figure2shows the etched cone distribution for tracks of lead projectiles (a), and fragments produced in the interaction of Pb ions on stationary CR-39 (b), Al (c) and Cu (d) targets, observed with CR-39 detector.In each plot, each peak can be designated with a charge Z as most of the projectile energy is deposited in a solid (CR-39 here) through Coulomb interaction.In all the three cases of Pb projectile fragmentation in different targets (b, c and d), fragments down to Z value of 63 are observable in CR-39.Counts in each peak are a measure of production cross section of the corresponding fragment.So, each peak represents production yield of fragments with a specific Z.Well-defined and separated peaks mean precise measurements.It is worth mentioning that detector, beam and etching conditions were same for all plots, but range of etched cone height is different for the case of Al (C) from other three cases.Major differences are difference in brands of KOH used and different chemical etching equipment.Major differences in etching equipment include difference in size and different magnitudes of stirring during etching.

Figure 2 .
Figure 2. Etched cone distribution for tracks of lead projectiles (a), and their fragments produced in CR-39 (b), Al (c) and Cu (d) targets, observed with CR-39 detector.

Figure 6 .
Figure 6.Decreasing resolving power of CR-39 with charge Z.

Figure 7 .
Figure 7. Present CR-39 158 A GeV Pb projectile fragmentation cross section measurements in comparison with corresponding online measurements by Schiedenberger et al.

Figure 8 .
Figure 8. Schematic proposing the co-use of NTD with online techniques for the measurement of fragmentation cross sections.

Figure 9 .
Figure 9. Artistic portraying of the finding reported in paper and related.It also includes segments on the field of radiation detection & measurements and the nuclear safety which are related to the subject of this article.In bottom half, global integration scheme, considered the most suitable for the nuclear safety/security, is portrayed.It is an artist-eye view for the broader scope of readers of the scientific literature.Scientific data/results are original or adopted from the scientific literature (most cited in this article in references list).Related pictures are from several digital online/passive resources/search engines, including IAEA Websites, CNN (US), BBC (Britain), ARY TV, Sama TV, Pakistan News Press, PTV, Din News TV, Koh-e-Noor TV, WE TV, Wikipedia, Google, SPD, PAEC, PINSTECH, PIMS, MH (Pakistan), Centre for Ion Beam Applications (CIBA), National University of Singapore (Singapore), University of Manchester, NDA (UK), International Islamic University (IIU), Islamabad (Pakistan), KINPOE, Karachi (Pakistan), NUST, Air University, Bahria University, PAEC Hospital Library, MISI (CDA), PAEC Head Quarters Centre for the Nuclear Safety, PIMS, Islamabad (Pakistan), Military College Jhelum, Jhelum (Pakistan), AS-ICTP, Trieste (Italy), Instituto Tecnológico e Nuclear, Estrada Nacional 10, 2686-953 Sacavém, CICECO, Departamento de Física, Universidade de Aveiro, 3810-193 Aveiro, Portugal, University of Strathclyde, Glasgow G4 0NW, UK, Fermi Lab, NASA, Los Alamos, WM2013 (US), GSI (Germany), Fukushima related Japanese/IAEA Websites and several others.