It is a simple matter of different sensitivity / efficiency of detection. Apparently, LR-115 has a higher efficiency than CR-39 for radon. See, for example: Nidal Dwaikata et al., "CR-39 detector compared with Kodalpha film type (LR115) in terms of radon concentration" in Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 574, Issue 2, 1 May 2007, Pages 289–291

Thank you Dr. 

Thank you very much Dr. Radu A.  Vasilachi. 

Kindest Regards

Time of etching of LR.115 IS lower than of Cr.39 where the first has 0.5 but the second has nearly 7 hour

Radu and Giueseppe have given BEST Answers to Question. Let me add my Observations: We preferred LR-115 Koda Pathe Film of France in our Experiments as it has a narrow Window of Detection for Radon Alphas. CR-39 is better detector, more sensitive but that is its Disadvantage also, as it can detect Alphas of all energies, in addition to Radon/Radium Alphas. LR-115 film is easy to count, as on Chemical etching HOLEs are developed, easy to count by Spark Counter also. As Dr Ali states: LR film can be etched easily with aq. NaOH in lesser time. 

Radu,Giuseppe and Singh Virk gave complete answers to your questions.

I would like to point out that I worked with Cr-39 till 1985 (last century-30 years ago), as a passive detector for neutrons , to be used in personnel neutron dosimetry. The dosimeter was based on boron pellets ( n,alpha reaction).Of course for this application, electrochemical etching was the best solution ( diue to the size of the individual tracks).

Even today the personnel dosimeters for fast neutrons in Israel are based on ECE of Cr-39.