Overview:

Predominantly used as a method for elucidating the surface composition of a planet or celestial body via monitoring the gamma ray emission caused from collisions of cosmic rays on surface material.

Abilities:

Can be used to formulate the presence of certain elements, most particularly, Aluminium, Calcium, Iron, Magnesium, Oxygen, Potassium (long lived isotopes), Silicon, Thorium, Titanium and Uranium. With addition of a suitable neutron spectrometer surface abundance of hydrogen can also be measured, and also the rare earth elements Gadolinium and samarium.

Simply by following some simple chemistry to extrapolate the presence of these elements back to their original compounds a simple idea of the surface makeup of a body can be obtained.

The coupled use of neutron spectrometry and gamma spectrometers to find water is one of the major reasons for use of this instrument type, though planetary atmospheres of any appreciable thickness render the neutron spectrometer useless for detecting ground based neutrons as these are readily intercepted by the atmosphere. However the gamma ray spectrometer is of a limited use in characterizing the composition of the atmosphere, and can be useful tools in hot Jupiter atmospheres.

Another consideration is the fact that atmospheres themselves mop up most of the incident cosmic rays and so resultant gamma ray emission is some what reduced, and any naturally generated gamma rays (from radioactive processes) are also soaked up in the atmosphere as well.

Methodology

The gamma ray spectrometer is open to the source of emission, that is hull mounted, or behind hull aperture, and the spectrum obtained from gamma ray exposure is screened for particular energy peaks which correspond to particular elements.

One consideration in design methodology is the fact that there are many other sources of gamma rays other than those arising from cosmic ray impacts, so an idea of local background radiation must be taken into consideration. Identification of the elements is somewhat easier because they form narrow discrete peaks on spectral analysis, this is useful as the effect of background noise is less of a problem.

Shielding the spectrometer, or constricting it’s field of view to the target will help reduce random noise generated from else where in the surrounding space, in most instruments the detector sits at the bottom of a massless neutronium cone (film over metal foam) which cuts out most of the ambient noise. Constricting the field of view further, and by articulating the instrument it is possible to generate narrow sweeps of the target regions.

Another consideration is the fact that the number of gamma rays likely to be received from a surface thousands of kilometres away is also rather low, so for greater accuracy cumulative sweeps of areas must be made to derive ‘true’ values for the abundance of the elements. (Larger sensor sizes also help combat the relatively low count rate, and where space affords larger instruments are indeed used)