cr-39 detectors are widely used as neutron dosimeter in complex radiation fields such as particle accelerator radiation environment , space stations in low earth orbit ( leo ) , etc . , because of their insensitivity to low linear energy transfer ( let ) radiations and convenient to expose , process and analyze . in an accelerator radiation environment , especially in a positive ion accelerator of intermediate energy , neutron is the dominant radiation produced during the interaction of ion with the target . this is due to the interaction of projectile ions with the elements of surrounding materials forming a composite system whose mass , energy and number of nucleons depend on those of the projectile and the target material . this composite system goes to an excited state with a number of allowed decay channels to produce neutrons depending on the excitation energy of the system . the energy and yield of the emitted neutrons depend on the type and energy of the projectile and type of the target . however , in case of medical applications such as boron neutron capture therapy ( bnct ) higher neutron yield is required , which is a binary radiotherapy method used for treatment of brain tumour . in this regard , the accelerator based neutron sources ( abns ) are preferred to provide required flux of neutrons necessary for treatment purpose . the commonly used projectiles for abns are proton ( h ) and deuteron ( h ) with different targets such as deuteron ( h ) , tritium ( h),li , be , etc . owing to the increasing demand of more powerful accelerators for various applications such as industrial , medical or research purposes and also increase in the number of workers in these radiation fields , the techniques and methodologies for neutron dosimetry are continuously being improved . it is important to mention that , measurement and/or estimation of neutron dose in such radiation environments are critical for radiation protection of occupational workers during normal operating as well as accidental situations . neutron dose estimation using cr-39 detectors are usually performed by the following approaches : ( 1 ) correlation of track density with established calibration factors obtained from the standard neutron sources,(2 ) generation of the neutron spectrum from track parameters and then multiplying with the dose conversion coefficient , ( 3 ) measurement of optical absorbance and then correlating the changes in absorbance with neutron dose . each method has its inherent advantages and limitations , but can always serve as complementing to other methods if any of the techniques fail under typical radiation environments . let spectrometry method using cr-39 detectors is also another way of estimating neutron dose , which is useful in situations when the information about neutron energy distribution and the radiation components are not available . in this approach , the discrimination of different radiation components ( such as proton , carbon and oxygen ) due to neutron interaction in cr-39 is not required . furthermore , this let spectrum is crucial for determining the energy deposition in microscopic level and to estimate the absorbed dose ( dlet ) and dose equivalent ( hlet ) . since the composition of cr-39 is similar to that of human tissue , it is reasonable to consider that the dose delivered in cr-39 is equivalent to that in tissue . however , different radiation components do have different contribution to the total dose depending on their let values . in this work , cr-39 detectors were used to measure the let spectrum and to estimate the neutron dose for the be ( p , n ) reaction at different proton energies from 424 mev . the be ( p , n ) reaction is chosen because of its importance for producing fast neutrons at relatively higher yield which is suitable for bnct application . the cr-39 detectors ( 12 mm  12 mm  1.5 mm , intercast , parma , italy ) were irradiated to neutrons generated from the interaction of protons with a thick be target at the 6 m irradiation port above the analyzing magnet of barc - tifr pelletron accelerator facility . the irradiation was performed along the beam direction at a distance of 1.75 cm from the 6 mm thick be target . the projectile ( proton ) energies used in this study were 4 , 8 , 12 , 16 , 20 and 24 mev . the detectors were exposed for 1 minute duration at 70 na beam current which was found to be sufficient to produce statistically significant number of tracks . a current integrator was attached to the target to measure the total accumulated charge on the target from which the number of protons hitting the target was found out . beam parameters during irradiation after irradiation , the initial thicknesses of the detectors were measured using a precision thickness gauge ( model : ft3 , hanatek , east sussex , uk ) with resolution of 0.1 m . then the neutron induced latent tracks produced in cr-39 detectors the images of developed tracks were captured by manually selecting undamaged frames using an optical microscope ( model : axio scope a1 , carl zeiss , germany ) at a microscopic magnification of 200x . all the 2-dimensional track parameters , such as track area , diameter , major and minor radii , grey level , feret length , perimeter , etc . , were automatically obtained by the software , out of which the major and minor radii of all the counted tracks were considered to determine the etch rate ratio ( v ) which is the ratio of track etch rate ( vt ) to bulk etch rate ( vb ) . the v was calculated from the major and minor radii using the formulation given below . where  is the cone angle of the track , a and b are major and minor entrance radii of tracks , t is the thickness of removed surface from each side of the detector due to etching . from the distribution of v of the tracks , the v spectra were then converted to let spectra using the calibration curve reported in the literature which correlates the v values to let in water . from the let spectra , the absorbed dose ( dlet ) and the dose equivalent ( hlet ) were obtained for be ( p , n ) reaction at different proton energies from 4 - 24 mev . the cr-39 detectors ( 12 mm  12 mm  1.5 mm , intercast , parma , italy ) were irradiated to neutrons generated from the interaction of protons with a thick be target at the 6 m irradiation port above the analyzing magnet of barc - tifr pelletron accelerator facility . the irradiation was performed along the beam direction at a distance of 1.75 cm from the 6 mm thick be target . the projectile ( proton ) energies used in this study were 4 , 8 , 12 , 16 , 20 and 24 mev . the detectors were exposed for 1 minute duration at 70 na beam current which was found to be sufficient to produce statistically significant number of tracks . a current integrator was attached to the target to measure the total accumulated charge on the target from which the number of protons hitting the target was found out . beam parameters during irradiation after irradiation , the initial thicknesses of the detectors were measured using a precision thickness gauge ( model : ft3 , hanatek , east sussex , uk ) with resolution of 0.1 m . then the neutron induced latent tracks produced in cr-39 detectors the images of developed tracks were captured by manually selecting undamaged frames using an optical microscope ( model : axio scope a1 , carl zeiss , germany ) at a microscopic magnification of 200x . all the 2-dimensional track parameters , such as track area , diameter , major and minor radii , grey level , feret length , perimeter , etc . , were automatically obtained by the software , out of which the major and minor radii of all the counted tracks were considered to determine the etch rate ratio ( v ) which is the ratio of track etch rate ( vt ) to bulk etch rate ( vb ) . the v was calculated from the major and minor radii using the formulation given below . where  is the cone angle of the track , a and b are major and minor entrance radii of tracks , t is the thickness of removed surface from each side of the detector due to etching . from the distribution of v of the tracks , the v spectra were then converted to let spectra using the calibration curve reported in the literature which correlates the v values to let in water . from the let spectra , the absorbed dose ( dlet ) and the dose equivalent ( hlet ) were obtained for be ( p , n ) reaction at different proton energies from 4 - 24 mev . the cr-39 detectors ( 12 mm  12 mm  1.5 mm , intercast , parma , italy ) were irradiated to neutrons generated from the interaction of protons with a thick be target at the 6 m irradiation port above the analyzing magnet of barc - tifr pelletron accelerator facility . the irradiation was performed along the beam direction at a distance of 1.75 cm from the 6 mm thick be target . the projectile ( proton ) energies used in this study were 4 , 8 , 12 , 16 , 20 and 24 mev . the detectors were exposed for 1 minute duration at 70 na beam current which was found to be sufficient to produce statistically significant number of tracks . a current integrator was attached to the target to measure the total accumulated charge on the target from which the number of protons hitting the target was found out . after irradiation , the initial thicknesses of the detectors were measured using a precision thickness gauge ( model : ft3 , hanatek , east sussex , uk ) with resolution of 0.1 m . then the neutron induced latent tracks produced in cr-39 detectors were developed by chemical etching ( 6.25 n naoh , 70c , 6h ) . after etching , the final thicknesses of the detectors were measured . the images of developed tracks were captured by manually selecting undamaged frames using an optical microscope ( model : axio scope a1 , carl zeiss , germany ) at a microscopic magnification of 200x . all the 2-dimensional track parameters , such as track area , diameter , major and minor radii , grey level , feret length , perimeter , etc . , were automatically obtained by the software , out of which the major and minor radii of all the counted tracks were considered to determine the etch rate ratio ( v ) which is the ratio of track etch rate ( vt ) to bulk etch rate ( vb ) . the v was calculated from the major and minor radii using the formulation given below . where  is the cone angle of the track , a and b are major and minor entrance radii of tracks , t is the thickness of removed surface from each side of the detector due to etching . from the distribution of v of the tracks , the v spectra were then converted to let spectra using the calibration curve reported in the literature which correlates the v values to let in water . from the let spectra , the absorbed dose ( dlet ) and the dose equivalent ( hlet ) were obtained for be ( p , n ) reaction at different proton energies from 4 - 24 mev . figure 1 shows the microphotographs of an unirradiated cr-39 and the neutron induced recoil tracks produced by be ( p , n ) reactions at different proton energies . as shown in the figure , number of tracks was found to be less in case of 4 mev protons while more number of tracks appeared in case of 24 mev protons indicating higher neutron yield at higher projectile energies . microphotographs of developed recoil tracks produced by neutrons from ( a ) 4 , ( b ) 8 , ( c ) 12 , ( d ) 16 , ( e ) 20 and ( f ) 24 mev protons with be target along with ( g ) background tracks as the energy of the proton increases , the tail end of the neutron spectrum spreads towards higher energy contributing to higher neutron yield , thereby forming more number of tracks in the cr-39 detector . nevertheless , the tracks were found to be of different shapes and sizes depending on the recoil angle and energy of the ions within the detector . the track density ( number of tracks per unit area ) in cr-39 is plotted versus projectile ( proton ) energy in figure 2 . the track density presented in figure 2 was obtained after subtracting background track density ( which is about 1800 tracks cm ) from the total number of tracks . it is observed from the figure that , the track density increased almost linearly with the proton energy . the projectiles ( proton ) of different energies from 4 - 24 mev were bombarded on a thick be target to produce neutrons figure 3 shows the variation of the track major radii to the minor radii at all the proton energies . as seen from figure 3 , the tracks near the 45 line represent the circular tracks whereas the tracks above the line are the elliptical tracks . circular tracks were found to be more than the elliptical tracks for all energies except 24 mev protons . the ratio of circular to elliptical tracks were found to be about 1.91 at 4 mev and decreased upto about 0.87 at 24 mev . the tracks near the solid line at 45 are circular and rest tracks are elliptical with different degree of ellipticity using the equation 1 and 2 given above , the v value was obtained from major and minor radius of each track from which the v spectra were generated for all the detectors . figure 4 shows the unit v spectra obtained for all the detectors irradiated with neutrons produced by be ( p , n ) reaction at all the proton energies used in this study . the unit spectrum was generated by dividing the track density in each bin by the total track density . unit etch rate ratio ( v ) spectra for neutrons produced by be ( p , n ) reactions at different proton energies . each spectrum is generated per unit track density the let spectra from 12 kev/m to 330 kev/m obtained from the v spectra are shown in figure 5 . as observed from figure 5 , the track density per proton at each let interval was found to be lowest at 4 mev protons and was found to be increasing with proton energy . however , a few overlapping regions were observed towards higher let values where the statistical fluctuations appeared due to the less number of track density in higher let bins . this observed fluctuation was mainly due to the recoil carbon and oxygen ions having less number of counts . however , their contribution to dose is significant because of their high let values . the larger contribution towards lower values of the let spectra was due to the neutron induced recoil protons within the detector . let ( linear energy transfer ) spectra for neutrons produced by 9be ( p , n ) reactions at different proton energies the dose characteristics ( dlet , hlet ) of secondary particles due to neutrons were determined from the measured let spectra using the formulations where dn / dl is the number of tracks per unit area in a let interval and l is the value of let . the absorbed dose ( dlet ) and dose equivalent ( hlet ) per proton obtained from let spectra is presented in table 2 . as can be seen form table 2 , the absorbed dose and dose equivalent per incident proton was found to be increasing with increase in proton energy . the microdosimetric distributions of absorbed dose and dose equivalent are presented in figure 6 and 7 respectively . however , the absorbed dose and the dose equivalent estimated in the present work have large fluctuations beyond let of about 200 kev/m due to statistical uncertainty in the measurement of vt / vb and corresponding let values . absorbed dose ( dlet ) and dose equivalent ( hlet ) per proton obtained from let spectra for different proton energy microdosimetric distributions of absorbed dose as a function of let microdosimetric distributions of dose equivalent as a function of let the let spectra from 12 kev/m to 330 kev/m were measured for the neutrons generated from the interaction of proton with a thick be target at different proton energies between 4 and 24 mev . the track density was found to increase almost linearly with proton energy indicating higher neutron yield at higher projectile energy . similarly , the absorbed dose and the dose equivalent per incident proton were also found to be increasing with the proton energy . the dose values obtained in this study can be useful for personal neutron dosimetry in the accelerator environment .
cr-39 detectors , widely used for neutron dosimetry in accelerator radiation environment , have also been applied in tissue microdosimetry by generating the linear energy transfer ( let ) spectrum . in this work 
 , the neutron dose has been estimated via let spectrometry for 9be ( p , n ) reaction which is useful for personnel monitoring around particle accelerators and accelerator based therapy facilities . 
 neutrons were generated by the interaction of protons of 6 different energies from 424 mev with a thick be target . 
 the let spectra were obtained from the major and minor radii of each track and the thickness of removed surface . from the let spectra , 
 the absorbed dose ( dlet ) and the dose equivalent ( hlet ) were estimated using q - l relationship as given by international commission on radiological protection ( icrp ) 60 . the track density in cr-39 detector and 
 hence the neutron yield was found to be increasing with the increase in projectile ( proton ) energy . 
 similar observations were also obtained for absorbed dose ( dlet ) and dose equivalents ( hlet ) .