Based on the subject areas in your profile, I am assuming that you are asking about particle radiation or the type that may affect health physics or a particle detector of some type.  Some of the above entries also are good answers, but it depends on what you need.  Here I am assuming that you are asking about so-called cosmic rays, which originate from outer space (out of the solar system), but some from the sun as well. 

At sea level, the nominal value is 10,000 cosmic radiation hits per square meter per minute. That is a decent beginning if you are doing simple particle detection.  Most of the hits will be from muons (mu+ and mu-) and electrons/positrons. Since it is only a nominal value, it is only useful as an order-of-magnitude check.  Also, please refer to any of the following references:

Particle Data Group:

http://pdg.lbl.gov/, and more specifically

http://pdg.lbl.gov/2014/reviews/rpp2014-rev-cosmic-rays.pdf

Also, there are some good references out there, but the one I use is:

Peter K.F. Grieder, "Cosmic Rays at Earth"

Additionally,

http://pdg.lbl.gov/2014/reviews/rpp2014-rev-radioactivity.pdf, look under the heading "Cosmic Ray Background Radiation" in particular.

Andrew

Your question is ambiguus:

do you mean: in outer space, or on earth due to extraterrestrial radiation

In europe the airlines have a good database for assessing the extraterrestial radiation contribution depending on your position on or above the earth at any time, taking into account solar activity/flares as well. In fact in the Netherlands all of the dosimetry of flying staff of all commercial airlines is based on that model. After each flight the GPS data of that flight is turned in and some software calculates to within 20% the doses for each and all of the staffmembers on board. This software is in place for at least a decade. If this is the type of information you are after, I can look up several references [on my computer at home].

If instead you want to know the dose contributions somewhere outside the lower athmosphere, I cannot help you further.

Extraterrestrial radiation is electromagnetic radiation which originates outside the earth or its atmosphere, as in the sun or stars. In different locations in the earth the average daily dose irradiation on a horizontal surface can be measured. For a more accurate indication of how much energy is falling on a solar panel titled at an angle from the horizontal, a series of calculations must be done either by using software programs or manual. For more information you can visit this web site:

http://www.uprm.edu/aret/docs/Ch_4_Solar_resource_and_solar_thermal.pdf 

My remark was not about the radiation source but about where humans/solar panels may be exposed:

astronauts or humans on earth. In the latter case I can help you further.

You mention solar panel. We use them both in space and on earth for decades.

hello,

If you do experimental work, i think you have to take a measurement without any identifiar source that you know and then you make your experimental work. At the end you extract this measure from the result obtained. I hope that my answer  give you some help.

I can't give you the code as I am undercontract, but a reference you might find helpful is:

K. O’Brien, The theory of cosmic-ray and high-energy solar-particle transport in the atmosphere, in The Natural Radiation Environment VII, Seventh International Symposium on the Natural Radiation Environment (NRE-VII), J.P. McLaughlin, S.E. Simopoulos, and F. Steinhäusler, Eds., ISBN: 0-08-044137-8, Elsevier: Amsterdam, Boston, Heidelberg, London, New York, Paris, San Diego, San Francisco, Singapore, Sydney, Tokyo; pp. 29-44 (2005).

@Keran   Does your code/reference take into account variation of dose rate over time? I.e. during solar flares ? and along specific orbits?

If the application is, for example. about protecting semiconductor sensors in outer space in certain specific orbits, then the peak dose rate may be at least as important as the cumulative dose.

Dear Dr. Borm,

The  Boltzmann equation is solved for incident cosmic rays and incident protons accelerated by first-order diffusive shock acceleration by a CME.  The boundary conditions are the local interstellar spectrum as modulated by the solar wind for cosmic rays, and by the SEP spectrum as it changes over time.  Changes in solar modulation over periods of minutes can be taken into account.

   I have not applied it to Van Allen belt radiation.

I guess you are referring for the measurement of background (or natural) radiation levels at a place. If you are only interested in the measurement of levels you can use any active or passive dose/dose rate measuring instruments. Like using a sensitive radiation survey meter (having scintillator or semiconductor detector) is sufficient for general purpose radiation level measurement. Alternatively many data are available in literature especially for the places with high background radiation.

There are many applications/papers available for calculating this. Of course it depends very much on which radiation you're interested in (low energy, high energy, protons or neutrons etc.). For most high energy radiation several spectra/fluxes for different particles are provided in the below mentioned paper and references therein.

If you want to see the secondary radiation produced from extraterrestrial in the atmosphere things are more complicated, if needed I can give references for this as well where radiation levels are calculated with location and time dependence taken into account. So let me know if you need this.

Sorry forgot the link to the paper: 

https://www.researchgate.net/publication/1814602_Cosmic-Ray_Background_Flux_Model_based_on_a_Gamma-Ray_Large-Area_Space_Telescope_Balloon_Flight_Engineering_Mode

Article Cosmic-Ray Background Flux Model Based on a Gamma-Ray Large ...