Tissue Equivalent Proportional Tissue Equivalent Proportional Counter Design Options Les Braby Nuclear Engineering Texas A&M Slide 2 For evaluation of an unknown radiation field for health physics purposes, have to choose detector parameters that will provide the appropriate evaluation of the radiation For use in space you also need to consider size, weight, reliablity, crew time commitment, and data communications options Slide 3 Need to select • Detector boundary (solid wall or wall-less) • Site size and shape (sphere or cylinder) • Physical size (count rate, multi cell system) • Control of electric field and gain (resolution) Many of these characteristics are interrelated, and also impact system size and reliability There is no specific way to select the optimun combination of parameters Slide 3 Generally a detector design is a compromise between preferred characteristics and practical requirements Lets consider the options and how they effect both the utilization of the resulting data and the adaptability of the detector to the space radiation environment Slide 4 First consider the boundary of the simulated site We have a choice of solid wall or some approximation to wall-less • Solid wall causes wall effect distortion of f(y) • Magnitude of distortion dependson type and energy of radiatio • Partly compensates for difference between y and L due to long range delta rays • Consequences of distortion depend on use of data (no effect on measured dose, increase in average quality factor) Consequences of (no effect on measured quality factor) Slide 6 • Wall-less design increases size and complexity of detector system • Use of electric field lines, plastic grids, or wire spiral to define boundary of site adds complexity • Making detection site a part of a larger, homogeneous, medium requires a gas volume that is (at least) several times the detector diameter, with a TE liner inside the stainless steel vacuum chamber Generally detector volumne and weight are limiting factors in space applications so there is a strong preference for use of solid wall Slide 7 Site Size and Shape For radiation protection and environmental studies there is no “correct” site size or shape Some consequences of site inclue . Starters and stoppers if site is too large relative to incident radiation . Can detect lower values of y, above fixed electronic noise, in larger sites (Epsilon is larger) . More sigma ray loss from smaller sites Slide 8 Particles with very short range (less than 10 mu) are probably not a significant factor in the space radiation environment Large simulated site sizes can be used Difference between y and L increases with decreasing site size Use of two or more different (small) site sizes may provide information on primary particle velocity Slide 9 Consequences of site shape include • Chord length distribution and effect on analyzing radiation quality • Mean chord length for sphere and “square” cylinder is the same • Maximum chord length for random tracks through a square cylinder is 1.414 times the maximum chord length for a sphere of same diameter • Chord length distribution for a beam of radiation depends on orientation of cylindrical detector, but is same as fro random tracks fo a sphere Slide 10 • Complexity of features required to achieve uniform gas gain • Cylinders generally simpler than spheres • Spheres often require slightly larger vacuum chambers For solid walled detectors the difference between cylindrical and spherical designs are relatively minor Simplification in calibration and data analysis may be worth the slight increase in size, weight, and development time of spherical detectors Slide 11 Physical Size (this is related to site size by the gas density) Count rate is proportional to the cross sectional area of the detector in a given radiation field At low dose rate need large detectors to get statistically significant data in an acceptable time Large events, which typically occur at low rates, can be responsible for a significant fraction of the absorbed dose, so they generally drive the detector size requirements Q is large for large events so rapid evaluation of dose equivalent requires even larger detectors slide 12 If f(y) is known the required detector size can calculated A single detector large enough to provide the desired sensitivity may be too large to meet size and weight constraints A large number of small detectors can have the same area (count rate) as one large detector Because area increases r2 and volume increases as r3, the small detectors take less space than the large detector Slide 13 So far this is only practical for cylindrical detectors If you try to make individual detectors too small it is very difficult to prevent electric field fringing in a significant part of the detector volume Need to find a compromise based on selected method for maintaining electric field uniformity 25 one cm diameter detectors have the same area as one 5 cm diameter detector but have only 1/5 of the volume Slide 14 Probably about ½ the volume after including detector walls and amplifiers Diagram of detectors with labels: Cover, TE Front Plate, TE Back Plate, Electronics Diagram with labels a, b, c, d Slide 15 Control of Electric Field We need a constant electric field along node in order to have constant gas gain This means we need to prevent fringing in a cylindrical detector and to compensate for changing distance between cathode and anode in a spherical detector Slide 16 Solutions for cylindrical detectors are relatively simple . Best is to use field tubes; length = cylinder radius, which doubles volume of detector . Alternative is field shaping electrodes add about 10% to detector volume but makes detector volume slightly uncertain design with labels: v1v2v3 Slide 17 Solutions for spherical detectors tend to generate other problems . Use helical gridUse helical grid Diagram of Tissue - equibalent spherical proportional counter with label Tissue equivalent plastic, Lucite, Telfon, Aluminum, Brass, Steel, Rubber- O-ring. From Rossi Fig.IV.I5 Typical walled proportional counter employed in microdosimetry. Grid can vibrate making it microphonic Slide 18 . Use a field shaping electrode at each end diagram with labels: soft soldered flange, 0.020 in. copper wall, to vacuum valve, wire support sealed into insulator with aradite, 33 m diameter anode wire (tungsten) (5.x10-4 C), scale 0 to 1 in., insulator, to E>H>T> socket, stepped tube supporting insulator This flattens the ends and effective volume is not known exactly Slide 19 . Use multiple field shaping electrodes diagram with labels:v5 v1 v2 v3 v4 v4 v3 v2 v1 Worked well as a grid-walled detector (Braby and Ellett 1972) but has never been used as a solid-walled detector Should have well defined volume and low microphonics Slide 20 Conclusions . Most applications require solid walled detectors – consider grid walled ones for special experiments . The benefit of a spherical detectors chord length distribution may override the added weight and complexity of the spherical . Cylindrical, multichamber designs may be required to obtain adequate count rate in very limited detector volumes