All contents © R. Jeffrey Wilkes, 1997. Last update: 5/13/97
Detector techniques: TRD, etchable plastics, emulsion
Detector techniques: TRD, etchable plastics, emulsion
Transition Radiation Detectors
Transition radiation is closely related to Cerenkov radiation. The term "transition effect" refers to the perturbation in photon versus electron populations as an EM cascade moves from one medium to another with very different properties (EC, X0). This effect must be taken into account in calorimeters, where the cascade development medium is typically high-Z (Pb or Fe) and the track-sensitive layers are low-Z (plastic scintillators, proportional counters, or emulsions). The transition effect means the ionization measured is not a precise estimator of the ionization in the heavy medium just before the sensitive layer. In general, transition radiation occurs whenever a charged particle passes from one medium to another with different dielectric constant (effective light speed). The result is very weak radiation, on the order of 0.01 photon per transition per charged track for large dielectric coefficient differences. However, by making a stack of very thin layers (eg aluminized mylar) one can build up a measurable signal which is strongly charge-dependent. Thus, like Cerenkov counters, Transition Radiation Detectors (TRDs) can be used to measure both the charge and the velocity of a particle. Common applications are in charge ID for cosmic ray spectrum and composition experiments: the CRN experiment (where the TRD was used for particle and velocity ID) and the Muller-Meyer-Prince electron spectrum experiment, where a TRD was used for proton/electron separation.
etchable plastic detectors
The term "solid state nuclear track detectors" (SSNTDs) includes nuclear emulsions, etchable minerals and plastics. Etchable track detectors are based on the principle that passage of a heavily ionizing cosmic ray track through certain minerals or plastics leaves a trail of radiation damage which make the material locally more vulnerable to subsquent chemical etching. Thus the track leaves an etch pit, whose dimensions (for suitably calibrated etching conditions) can be related to its Z/b , where Z is its electric charge and b its velocity. When the material is immersed in a strong etchant (typically high concentration NaOH solutions are used, often at high temperature), one can define a velocity of bulk etching vB and a velocity of track etching, vT. Since vT > vB for suitable materials, the half angle of the cone-chaped etch pit gives an estimate of the track etch velocity and hence the track charge (if relativistic).
The etched-track technique has a number of advantages. First, it can be applied to fossil rocks and used to observe cosmic ray intensities as a function of time (given rocks, from earth and moon, of different age and burial times). It can be used as a radiation-hard detector for high-intensity environments. It can be used to time-integrate very low fluxes (and is therefore the primary method for home radon measurements; another application was observation of ultra-heavy cosmic ray nuclei via the Long-Duration Exposure Facility (LDEF) flown by NASA in the 1980s). The track detection and measurement technique gives results equivalent to emulsions for many applications and is much simpler. For example, one simple way to map tracks on a plastic plate is to etch the plate until the tracks punch through from both sides. Then the plate can be placed in contact with a sheet of architectural whiteprint paper and exposed to ammonia fumes. The tracks appear as blue spots on the whiteprint. The information content of the etch pit is much simpler than an emulsion track: the mouth of the pit is an ellipse whose major/minor axis ratio gives the track’s zenith angle, and whose depth gives the Z/b information. Automated track measuring systems have been built and used successfully for large detector exposures.
One problem with plastic and mineral detectors is their high threshold. CR-39 is a polycarbonate plastic which is the most sensitive plastci detector, with threshold Z/b ~ 6. Sensitivity can be enhanced with chemical doping, but track fading and temperature sensitivity are increased also. In general, while resolution in Z/b ~ 0.01 has been claimed (for isotope identification), this can be achieved only by keeping the plastic detectors in a constant temperature to within 0.1 deg C during exposure, and protecting them from exposure to oxygen thereafter. The high threshold is of course an advantage in some circumstances, for example on LDEF, where the desired heavy nucleus tracks would be totally wiped out by overwhelming backgrounds of lower Z nuclei, if the material were sensitive to low Zs.
Nuclear Emulsions
Nuclear emulsion refers to photographic emulsion which has been specially sensitized to record tracks of charged particles. Emulsion is a suspension of silver-halide (normally AgBr) crystals in a gelatin medium. Almost unbelievably, despite nearly a century of research by photographic companies, no satisfactory artificial substitute for animal gelatin has ever been found. One authority commented that the photographic process is made possible by cattle’s taste for an occasional bit of spicy greenery, since the trace amounts of sulfur in gelatin are crucial in the solid-state physics of trapping photoelectrons in the halide crystals. Nuclear emulsion differs from simple black and white film emulsion only in its much higher density of AgBr, finer grain size, and additional chemical sensitizers. Emulsion is available from 4 major suppliers: Ilford (UK), Kodak (US), Fuji (Japan) and NIKFI (Russia). However, only Fuji provides emulsion suitable for cosmic ray research at the present time. NIKFI’s situation is presently uncertain, and Kodak and Ilford produce emulsions primarily for the autoradiography market. Autoradiography is a biological technique in which radioactive tracers are introduced into specimens, and then the microscope slide is coated with emulsion, left to accumulate dose, and developed. Thus the emulsion functions as a sort of selective stain displaying takeup regions for whatever radiochemical was used.
Track sensitive emulsions have been available since the 1930s, although the threshold was not brought down to minimum ionizing until about 1947. At that time, Ilford was able to produce an emulsion sensitive to singly-charged relativistic tracks (referred to as "electron-sensitive emulsion"). Immediate application of this emulsion by the Bristol cosmic ray group led to discovery of the pion. Emulsion became the primary tool for high energy nuclear physics for nearly ten years, until bubble chambers became widely available and spark chamber techniques were developed.
The emulsion technique of the 1950s involved stacks of glass-backed plates or "pellicles" (unbacked slabs of nuclear emulsion up to 0.5 mm thick), usually exposed such that beam tracks were parallel to the emulsion plane, so that events could be examined in plan view. Emulsion images are on the scale of fractions of a mm, so high-powered optical microscopes are required for scanning and measurement of tracks. A minimum-ionizing track will have from 20 to 40 silver grains (diameter about 1 micron) per 100 microns. Barkas provides an encyclopedic description of the emulsion technique as of the early 1960s, and Powell’s book contains many illustrations of techniques and results.
Due to the demand from various Japanese cosmic ray and accelerator physics groups, Fuji ET7A emulsion is of extremely good quality (~40 grains/100m m, negligible thermal fog, long fading mean lifetime). Emulsion costs about US$10K per dry liter (ie per liter volume of emulsion after pouring and drying of plates; the gel supplied has about 8X the volume of the dried emulsion).
Modern emulsion experiments almost exclusively use the "emulsion chamber" technique first employed by the Minnesota group and later used by J. Lord’s group in the late 1950s then thoroughly developed by University of Tokyo groups in the 1960s and 70s. In this approach, thin layers of emulsion are coated on both sides of a thin plastic baseplate. Polystyrene can be easily prepared to accept emulsion but is rather brittle; acrylic has better mechanical properties, but the method of making emulsion stick to acrylic is a trade secret known only to Fuji Corp. The plates are exposed perpendicular to the beam, and thus provide a sample of the track pattern at various points along the beam direction. The two sided plates make track identification simple, since soft background tracks will not punch through the baseplate. Double coating is essential due to mechanical as well as tracking considerations, because emulsion shrinks as it dries and is very hygroscopic, so a single-sided plate would curl up with every variation in humidity.
Emulsion chambers are usually constructed in several sections. First comes a primary ID section, containing thicker emulsion layers to allow grain density counting, perhaps interleaved with layers of CR-39 and lower-sensitivity emulsion (for example, Fuji ET6B emulsion has a threshold of Z~3). This is followed by a target section where emulsion tracking layers are interleaved with a suitable target material depending upon the experiment. Finally comes a calorimeter section, in which emulsion plates and x-ray film (used to locate high energy cascades, and thus serving as "triggers" for event finding) are interleaved with layers of Pb. Such arrangements permit identification of the primary particle, observation of the event vertex, and estimation of the primary energy.
The advantage of emulsion chambers over other cosmic ray techniques is their large acceptance and foolproof character. It is extremely rare to get a bad batch of emulsion, straightforward precautions in developing processes can prevent catastrophic losses, and emulsion is unharmed by hard balloon landings or most environmental hazards (again assuming straightforward precautions like waterproof containers, thermal insulations, etc). Because emulsion chambers provide many tracking layers in a very thin detector, their acceptance angle can be very large, typically up to slope 2 (60 deg).
Disadvantages lie primarily in the time-integrating nature of emulsion detectors: they cannot be turned off except by developing the emulsions. Furthermore, a great deal of hand scanning and measurement is normally required. Recently considerable progress has been made in automating data reduction, at least in terms of developing computer aided scanning and measurement systems. A detailed description of the emulsion chamber technique is given in Erik Olson’s recent thesis.
Bibliography
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