Different portable devices for gamma ray measurement used in field are calibrated for different unit.Portable scintillometer generally gives output in micro-Rontgen per hour, Gieger Muller couter in count per second and presently available portable gamma ray spectrometres in ppm.  For lithological mapping, I would suggest to use portable gamma ray spectrometres which is capable to give you uranim, thorium and potassium values in ppm.

I agree with Mr Sinha's reply. Also, depending on the conditions you have in the field, perhaps you can use a portable HPGe detector, that can give you the whole spectrum of gamma emitting radionuclides in your sample. For that you will need the detector itself, a lap top computer with some gamma acquisition software and a possibility to take samples which are to be measured. A bit of work perhaps, but the results are very precise and detailed. Also if you take samples of soil or rock you can derive a transfer factor to the outcrops.

There are several instruments out there. For geology, we just purchased the RS-230 gamma-ray spectrometer from Radiation Solutions, Inc.(Ontario, Canada). My students and I love it. It has direct readout, in assay mode, of K%, U ppm, and Th ppm. If you are measuring a stratigraphic section in outcrop, or describing a core, then you can convert the assay data into API (American Petroleum Institute) units and generate a gamma-ray log of your outcrop/core to compare with a borehole gamma-ray log (SGR). The same unit has options for mapping when merged via Bluetooth with GPS (although I have not yet tried it), plus RS-Analyst software for storing and analyzing the actual spectrum (1024 channel) of a sample. 

I am a geologist and I utilize the RS-230 gamma-ray spectrometer (Radiation Solutions, Canada). According to Mr Richard Busch, I find this unit versatile and simple to use, so I am really satisfied with it. I have direct experiences using this device in outcrop gamma-ray logging and mapping for heavy minerals detection. For mapping I merge the RS-230 with a GPS bluetooth very cheap and functional (HOLUX M-241 Bluetooth GPS logger). Then, after downloading all data recorded (radioactivity values, GPS coordinates, etc.), I export them on ArcGis for build a radioactivity map.

I note that the RS230 has a BGO detector and this unit allows for determination of K,U, and Th in PPM: this unit looks field ready.   I sense that the HPGe detectors can give a fuller spectrum of radionuclides, but this sort of unit must be much more expensive and delicate.    In my search I need to consider price, but this information is apparently difficult to get on the internet and quotes need to be requested.  

I agree that the RS-230 is a good choice for field use.

The critical issues are detector volume and stability.

A small detector crystal volume requires longer count times to achieve adequate statistics. The paper by Leif Lovborg and Erik Mose 'Counting statistics in radiation assaying with a portable spectrometer, Geophysics 52(4) 555-563, 1987 is a useful reference.