Intensified gamma Spinthariscope / Fluoroscope.

Version 4.0

By Leslie Wright

I designed and built the following device to view Gamma scintillations in real time, and as an imager for low level x-ray sources.

Overview:

The following is a highly sensitive instrument, for viewing Beta, and Gamma scintillations, in real time.

Scintillations caused by Gamma rays are very weak compared with those generated by Alpha particles, since Alpha particles normally have energies of around 3-7 MeV. This means of course, that low energy Gamma/x-rays (20KeV-100KeV) would not be visible using a conventional Spinthariscope, since comparatively few photons would be emitted per event.

To deal with this problem, a device was constructed that amplifies the scintillations to a level that can be seen with the naked eye, or recorded with an ordinary still/video camera (something that cannot be accomplished with an ordinary Spinthariscope). Not only that, the scintillations are viewable in dimly lit conditions, without the need for the eyes to become dark adapted, as is normally the case with Spinthariscopes, and thus lends itself well to things such as class demonstrations etc.

The device comprises of a Gd₂O₂S:Tb Scintillator, coupled to a stack of three generation-1 image intensifier tubes. Each scintillation is amplified approximately 40,000 times!

The device is capable of detecting very low energy x-rays (~20KeV), as well as higher energy photons, and will easily detect cosmic rays.

It is also capable of x-ray imaging, functioning as a highly sensitive Fluoroscope. Real-time x-ray images can be recorded with energies as low as 20KeV, and beam currents as low as 50µA

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Side view of the Fluoroscope/Scintilloscope, complete with battery pack.

Takes 4x AAA which should give you an idea of scale.

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Light tight input window.

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Output screen.

Features:

3 stacked, high gain, gen1 image intensifier tubes.

Built in high voltage PSU.

Automatic brightness control.

Operating voltage: 6.5-6.8v will run fine off of a 6v battery.

Current: 30-50mA

Resolution Centre: 28-32 line pairs/mm

Resolution Edge 25-30 line pairs/mm

Distortion max: 21% (Pincushion distortion. May be corrected with a suitable lens.)

System gain: ~40,000

Sensor diameter: 23mm

Scintillator material: Gd₂O₂S:Tb

Scintillator thickness: 80µm (34mg/cm²)

Output screen: 25mm diameter. P20 phosphor centred at 560nm

Note: The image produced at the output screen is inverted.

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Above a diagram of a single stage intensifier tube, with x-ray detector phosphor.

distortion

Image distortion map.

Due to the curvature of the inner surfaces of the photocathode, and phosphor screen, the output image suffers from pincushion distortion. This is inherent in all Gen1, and some Gen2 image intensifiers, as well as x-ray intensifiers.

This can be corrected in this case with a single very short focus lens.

If images are to be recorded with a digital camera, pincushion distortion may be removed with external programs such as Gimp (Linux or Windows), using the wideangle.c plugin.

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Above.
An x-ray image of a capacitor. The distance between the fluoroscope and x-ray source is 15cm.
The x-ray source voltage is 30kV, and the beam current is a mere 50µA !
The image is somewhat noisy at these low beam currents, the speckles you see, represent individual x-ray photons!
This imager produces high quality images at beam currents greater than 200µA.
Note the pincushion distortion of the image.

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Above, the image after processing with Gimp, using the wideangle.c plugin.

Beta/Gamma Spinthariscope/Scintilloscope

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Above.
An image of the scintillations produced by placing a sample of Pitchblende in front of the screen.

The image was taken with an unmodified Phillips SPC900NC webcam, using the software bundled with it. I recommend using the SPC900NC for group demonstrations etc, as it has excellent low light capability.

It should be noted when viewing with the eye, that the scintillations are quite dim, and a darkened room is required to see them.
However, there is no need for the eye to become dark adapted as with a conventional Spinthariscope (which normally takes around 15 minutes!), and the scintillations can be viewed the moment the lights go out.
A suitable lens is required to view the scintillations, as they are tiny and need magnifying. I find that the eyepiece lenses removed from cheap Chinese Rifle scopes, are excellent for this job.

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Above.

An image of the Gamma (60keV) scintillations from a 0.9µCi Am²⁴¹ source.

This is a particularly weak source, and demonstrates how effective the device is at imaging low level sources.

It is possible to video the scintillations, at a sufficiently low frame rate (5-10 fps).

It should be possible to image using relatively low levels of Gamma radiation, by videoing the scintillations, splitting the image into individual frames, and then adding the frames together with image processing software such as ImageJ.

I have tried this with the 0.9µCi Am²⁴¹ source, but it requires a lot of processing power and hard-disk space, as the source is so weak. I will update the page once I have decent and easily repeatable results.

X-ray Fluoroscope

The device functions exceptionally well as a fluoroscope. The image is bright and crisp and either stills, or video can easily be taken. Nothing beats looking at the output screen with the eye, at a live moving x-ray image!

Below are just a few stills I have taken with the device.

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Above an x-ray image of a printed circuit board.

Captured with a webcam.

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Above

An image of a shell.

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Above.

An image of the front portion of a Weasel skull.

Intensified gamma Spinthariscope / Fluoroscope.

By Leslie Wright

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