MTFchart

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Lower MTF limits the resolution of the system as a result of which small details of objects are no longer discernable, and this is wavelength dependent.  In some cases this can be overcome to some extent by decreasing the noise level, which additional to MTF also causes a reduction in contrast, by increasing the light level.

In general, CCD technology has better MTF in NIR because of higher electric fields in the depletion layer, and deep-P diffusions for isolation of the columns in the Interline Transfer (IT) structure (compared with standard CMOS pixel design).  Click here for more information on CCD vs. CMOS in defense applications.

If you try to resort to seeing if, after the absorption of some of the radiation by aluminium, the count rate from the GM_counter differs as to whether the GM-tube is head on (detecting beta? and gamma from the source) and broadside (detecting only? gamma from the source) there is a significant problem. When betas slow down they emit bremsstrahlung (braking radiation) which the GM-tube will also detect.

See the websites https://www.edmundoptics.eu/ and https://www.trioptics.com/ that provide a more detailed explanation of the MTF.

I have noticed there are A LOT of isotopes, and A LOT of them look like Co-60 in the files (having both beta/gamma). How can i find/identify the "real" radiation type of the isotopes?

MTFOptics

Even if the betas originate from an uncovered Co-60 source a GM-tube is often used in the "broadside" position to try and make the GM-tube more efficient at detecting gammas and at the same time the betas would be absorbed by the $\thicksim 75 \,\mu\rm m$ steel cylinder.

The MTF of the image sensor should not be overlooked when comparing image sensor technologies, especially when working in the Near Infra Red (NIR) spectrum where there are big differences.  The MTF of the lens and sensor varies with wavelengths which can be characterized using MTF-curves. The problem is that sensor manufacturers do not always (read “hardly ever”) specify MTF or when they do, it is only at a wavelength where QE is at maximum value (is close to the best case scenario).  The MTF of all sensors at 550 nm is similar, but at longer wavelengths, the MTF of a sensor can be lower compared to other sensors.  For some sensors, for instance, the MTF is reduced by a factor of 2 or 3 at 850 nm.

Besides the spatial frequency, the MTF strongly depends on the lens aperture, the light spectrum, the optical conjugation (e.g. magnification), field position, temperature and last but not least the level of optical correction of the lens itself (its quality so to say).

Cs-137 is a solid gamma emitter at 95% with one easy to recognize energy peak. Just use simple shielding to block the betas. No isotope emits only gammas. Gamma radiation is caused by electron de-excitation. The excitation is caused by alpha, beta, proton or neutron decay.

Modulation transfer function

Talking about the MTF of optics, say a lens, it is the reduction in modulation depth, starting with a modulation depth of hundred percent, due to that optics. The MTF of an optic is usually depicted versus the spatial frequency. Generally speaking the MTF drops with increasing frequency, which frequency is indicated as cycles or line pairs per mm (square-wave signal).

MTFlens

A lot of sealed Co-60 gamma sources have a thin sheet of aluminium above the pellet of radioactive material to absorb the betas. The betas have a lower energy than the gammas. Sealed Stronium-90 have a plastic cap above them.

The mica window at the front of the GM-tube with a thickness of about $5\,\mu \rm m$ absorbs some of the betas given that the energy of the betas ranges from "zero" up to $0.31 \, \rm MeV$.

The MTF can be measured in various ways, either directly or indirectly. In case of direct measurement, a sine- or square-wave signal is used. In case of indirect measurement, a step, line or point source, of in principle infinite steepness is used. From the spatial light distribution after passing the optics, which will no longer be infinite, one can obtain the MTF via Fourier transform.

MTFimage quality

Mtf cameraexplained

“The modulation transfer function is a measure of the transfer of modulation (or contrast) from the subject to the image. In other words, it measures how faithfully the lens reproduces (or transfers) detail from the object to the image produced by the lens.“

Mtf cameranikon

The ideal alpha source for what you want is Pu-239. With 70% at 5.157 MeV it will give you a good knee on the curve to determine sensitivity. But it's a special nuclear material so very difficult to come by. The substitute is Am-241, but it has 84% at 5.476 MeV so your knee is going to sit a little higher and your tube won't be as sensitive to the lower energies. Th-232 is also an option, it has 78% at 4 MeV which would be a better option than Pu-239 but it also has a half life of about 14 billion years so you're going to need a crap ton of it to get anywhere, so not ideal.

Cobalt-60 (Co-60) is known as a "gamma" source. It appears on youtube-videos that only emits gamma ray, since beta sensetive Geiger Mueller Tubes do not give a higher count than only gamma sensetive tubes.

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The problem with trying to differentiate directly between betas and gammas is that the design of the GM-tube is such that an ionising radiation of any type will produce approximately the same size output pulse.

So while MTF is widely accepted as an important factor with lens selection, it is often not considered with the sensor/camera selection.  MTF provides an indication of the sharpness of the image or image quality and is determined by both the lens AND the sensor.

Further Information: The source is a plastic disk, it should not shield the betas, since a Sr-90 source of the same construction lights up the Geiger Counter pretty much.

MTF cameratest

This questions is about radioactive Isotopes. My current problem is, how can i "pick" isotopes, that only emit a specific type of radiation.

The Optical Transfer Function (OTF) is an important specification of lens performance. The OTF describes both the amplitude and the phase of a signal. In most cases the former, most times indicated as is Modulation Transfer Function (MTF), suffices. Modulation, as it is a measure of the modulation depth (contrast) of an optical signal. The MTF is for that reason sometimes also called Contrast Transfer Function (CTF).

Experienced camera manufacturers will have measured the MTF of the implemented sensor for the full wavelength range and can make the appropriate recommendations for your application.

The main problem is that gammas are produced when an excited nucleus, produced as a result of alpha or beta emission, loses energy to go (towards) its ground state. So gamma sources are going to produce alphas and betas and the best you can do is to "eliminate" the alphas or betas; absorption being one way.