Dear Joseph, in RELEC/Vernov experiment - last experiment of  SINP - http://smdc.sinp.msu.ru/index.py?nav=relec -  we can detect gamma- ray with time resolution: 15 mcs in event mode 

@Joseph, Please could you give a hint, are you looking for count rates (cps) or frequencies = energy of gamma quants?

Thank you all for your interesting responses. 

30 Billions of eV that is real a challenge for EGRET!

: a wave length the order of 10^(-17)  m !

Greetings 

JFL

The highest energy gamma rays are observed with ground based instruments, namely MILAGRO and now HAWC. I would say that we should reach something close to 100 TeV, but I would have to check for sure.

By the ay, EGRET has mong been superseded by the Fermi observatory, that reach close to 1TeV in space. Voir http://fermi.gsfc.nasa.gov/ssc/

I am afraid that the Compton gamma Ray Observatory was reentered over the Pacific quite a few years ago having done a great job. As has been mentioned we now have Fermi in space but the highest energies are detected on the ground from large air shower arrays. The largest air shower detected is from a particle of around 4x 10**20 eV but this is thought to be from a cosmic ray particle rather than photon. There is a distance limit on high energy particles or photons propagating through space due to interactions with the microwave background.

Rather than answering your question in frequency, it would be more realistic to answer in energy, simply the frequency is so high that gamma ray behave more like a particle (photon) than a wave. The CGRO and other satellite experiment can detect gamma ray only up to several 100s GeV or < 1TeV = 1.E12 eV. Higher than that, the flux is so low that satellite instruments loss detection power or their discrimination power to separate gamma from much higher flux of cosmic rays. The Ground gamma ray telescope can detect gamma photons interaction with atmosphere via indirect measurement. The highest energy of gamma ray  of those experiments can reach approximately 1.E14 eV, in terms of frequency ~ 2.4E28 Hertz.

as we know that gamma ray has high frequency so, gamma ray behaves like photon than wave and it has more penetrating

Thank you Mohammad Ayaz Ahmad, and all others for their clear answers.

I appreciate your time and explanations.

Gamma Ray Astronomy is real challenging new frontiers for young people.

Joseph Lemaire