There are several possible variables.

Gamma energy:

The gamma energy will determine the amount of electronic build up in the sample with thickness. More build up will present a higher dose to the exit surface.

Source to surface distance:

The amount of electronic build up in air or in the source will affect the entrance surface. Consider an absorber to control the electronic build up between source and entrance surface.

Sample preparation method:

if sample was prepared on a substrate the surfaces could have differing structures.

Sample mounting method, in particular, material on or near the exit surface:

A sample mounted on a substrate or near another surface will be subject to electron and photon backscatter.

These variables will have interdependence, thus requiring more detail for evaluating the effects.

Polymers are generally make poor gamma shields, but the hydrogen is important in neutron shielding,

thanks Dr. Joseph for you appreciated answer.

the sample is CR-39 detector, irradiated with gamma energy 661keV from Co60 source.

the sample dimensions is 1x1 cm and thickness 0.5 cm, mounted on air at a certain distance from the source. nothing touch the sample. no possibility for any backscattered electrons from the surrounding.

661 keV from Cs-137?

The dose at the front surface will be lower than the back surface. The difference is about 20% according to my estimate from tables in ICRU 57. The ICRU 57 tables assume on phantom measurements, so I do not know if this estimate is valid for your geometry. If you have access to an extrapolation chamber you could measure the dose difference.

How important is the dose difference to the expected effects? 

Thanks my dear.

this is a journal reviewer comment. I studied the exit surface because the effect is higher. and he wants me to study the entrance surface as well and compare. he is sure that the entrance surface is more affected by gamma. the samples is damaged now cos of some destructive measurement and I don't know how to prove that the exit face is the most affected. I am searching for any theoretical model or a reference for the response to the reviewer, I cant find such a thing.

Dear Ashraf,

I do not think that it should make any difference whether you consider front or back surface of CR-39 after irradiating to high-energy radiation such as gamma rays. When these rays pass through the plastic detector and make tracks they are throughout the thickness. But they are revealed better on etching. I strongly feel that the number of tracks formed should be almost the same on both the sides. Thank you, Regards.

P D Sahare

Prof. Sahare,

thanks for sharing. your opinion may be right. since nearly all the articles I read in such field of gamma and CR-39 didn't tell any difference between the two faces. logically there is a difference since some of its energy is dissipated in ionizing the target or releasing secondary electrons that initiate a shower of ionizing processes up to the opinion of prof Joseph. no reference for me but your opinion is agreed with the published articles. the reviewer insist that there is a difference in effect between the upper and lower faces I dont know how to respond without reference. 

Ashraf

Note my point on 

Source to surface distance:

The amount of electronic build up in air or in the source will affect the entrance surface. Consider an absorber to control the electronic build up between source and entrance surface.

The photo and Compton scattered electrons will impart a larger dose to the front surface. The dose distribution from the energetic electrons in the Cr-39 would be the same as from a pure beta source; most energy deposited in the front surface. See my article on the 

Angular Dependence of Shallow Dose from Energetic Electrons

J L Alvarez, available on RG for dose from photo and Compton electrons and their angular dependency.

Likely, the reviewer is aware of effects on CR-39 where no provision was made to shield photo and Compton scattered electrons. The experimental setup will determine the electronic build up.

To understand gamma radiation in simple way, Only lead metal (very thick can stop gamma radiation to pass through. For polymers, it passes through. Hydrogen is abstracted randomply and also C-C bond, C-F bond can be broken to form radicals. For polymer samples, the radical on the surface can get destroyed. But many are buried inside matrix which can be used later by for chemical reaction outside. Attenuation is normally used for variation of radiation intensity.   

Thanks prof. Alvarez, Sahare and Samui, for you positive cooperation and good answers. 

When comparing the effect of gamma and ion beam on polymers, is it good to compare gamma in Gy and ions fluence in Cm-2.

The reviewer said it is meaningless and these units are different and cannot be used in comparing the induced effects. Is that right and how to respond, most published articles use these two units.

instead of Gy, you can use Sv - it is also J/kg.

What are you reporting that requires units? If you are reporting the gamma beam energy it is kerma, free-in-air. The unit is Gy. If you are reporting the dose to the polymer it is energy deposited and the unit is Gy.  To report dose to the polymer you will have to calculate the energy deposited using assumptions on the kerma distribution in the polymer or you will need a method to measure it. Sv is inappropriate as it is a unit of risk. 

Ion fluence assumes you have a measurement or estimate. The unit should be that used to measure or estimate. The unit cm-2 seems appropriate.