The origins of neutron cameras

Neutron cameras are almost as old as neutron diffraction itself, and are simply a variation of the even older photographic techniques used from the discovery of X-rays. Indeed neutron cameras are just X-ray cameras with a component (usually ⁶LiF) to convert neutrons into ionising particles and X-rays, which are then converted into light using an X-ray scintillator (usually ZnS). A neutron Polaroid film camera was used with a scintillator from the beginning of ILL in the early 1970's, and even earlier elsewhere.

The idea of using a video camera instead of film goes back to Arndt, U.W. & Ambrose, B.K. (1968) in Cambridge (UK) "An Image Intensifier – Television System for the Direct Recording of X-ray Diffraction Patterns", IEEE. Trans. Nucl. Sci. NS-15, 92-94. This was also proposed for neutron detection by Arndt, U.W. & Gilmore, D.J. (1975) "A Neutron Television Camera Detector", Brookhaven Symposia in Biology, 27. VIII 16.-VIII 23.

Neutron CCD cameras were then developed at ILL and elsewhere for tomography and other imaging applications. They consisted of the usual neutron scintillator screen reflected in a mirror and imaged by a sensitive lens and camera, often via an image intensifier. An attempt was made by A. Heidemann in the 1990's to replace the ILL Polaroid neutron camera by a CCD camera based on the AUDINE Project. This proved too large for routine use, but a cheaper compact version developed by A. Hewat in 2006 was widely adopted.

How can an inexpensive Neutron Camera compete ?

Neutron intensities are low and neutrons are difficult to detect because they are non-ionising. They do however interact with the nuclei of atoms, and nuclear fission and decay can produce ionising particles that can easily be detected. This is the basis of all neutron detectors, using strongly neutron absorbing nuclei such as ³He, ⁶Li, Gd etc. However, position sensitive neutron detectors are usually expensive, costing many tens of thousands of dollars.

Expensive neutron cameras typically use a large CCD, perhaps coupled to a photo-multiplier using a tapered fibre bundle. The scintillator screen is focused onto the CCD or photomultiplier using a large aperture lens; the larger the CCD, the larger the lens. Clearly such hardware is expensive, so is there a cheaper solution ? The answer is "yes, for some applications"; it very much depends on the design objectives. We will attempt only to indicate some of the issues without going into detail, or using specialist terms.

• Neutron and X-ray Scintillators
  Although thermal neutron energies are low, the energy released by prompt fission following neutron capture by nuclei such as ⁶Li is high; the resulting fission products produce as many as 160,000 light photons when absorbed by a common x-ray scintillator such as ZnS. NeutronOptics cameras use the same high resolution ⁶LiF/ZnS neutron scintillators as much more expensive cameras.

  X-rays are converted to visible photons by X-ray scintillators such as CaWO4 and other heavy metal oxides. Much work has been done to find efficient X-ray scintillators to minimise radiation exposure from medical examinations, and NeutronOptics cameras use the most sensitive medical x-ray scintillators.

• Resolution
  

  The resolution of consumer cameras has increased dramatically, driven by marketing and consumer demand. But the resolution of a neutron or x-ray camera depends mainly on the resolution of the scintillator, which decreases with thickness, since light is emitted along the capture path. The efficiency of a scintillator increases with thickness, so thickness is a compromise between resolution and efficiency. Geometric magnification can increase resolution with an x-ray point source, but for neutrons it is difficult to obtain much better than 0.1mm (100µ) resolution even with thin scintillators.

• Optical Efficiency
  The efficiency with which photons are collected depends on the ratio of the effective area of the CCD, enlarged perhaps by an optic fibre bundle, to the area of the scintillator screen. So smaller screens or larger CCDs have an advantage. Light gathering power also depends on the f-number of the lens i.e. the ratio of the focal length of the lens to its diameter. Classical 35mm camera lenses with focal lengths of 50mm start at f1.4, but since efficiency depends on solid angle, an f1.0 lens will be twice as efficient.

  Unfortunately large aperture lenses become expensive, and a 50mm lens also implies a large camera. NeutronOptics cameras use a smaller "half inch" CCD, so that only a small lens is needed, but it can still be large enough to allow f1.0. As well, the focal length of this small lens can be reduced to 8mm or less, making for a very compact camera. The depth of focus of a large aperture lens is also small, but only the plane of the scintillator need be in focus.

• The Camera CCD
  Consumer cameras use colour CCDs, with filters covering three pixels at each point. Light is absorbed by the filters and less light is collected by smaller pixels. Ordinary cameras also use "mega-pixel" CCDs, which are good for resolution, but bad for efficiency because the pixels are tiny. High efficiency CCDs, such as those used by NeutronOptics, have no filters and a relatively small number of large pixels to maximise light gathering. Arrays of on-chip micro-lenses are used to gather light even from the areas between pixels. Our Sony "super-HAD" CCD actually has a second micro-lens for each pixel to maximise light gathering from low f-number lenses, which deliver light at low incident angles. HAD (Hole Accumulation Diode) CCDs use an extra accumulation layer to drain-off thermally generated electrons and reduce thermal noise, most notable at low light levels.

• The Camera Electronics
  The NeutronOptics Camera uses an electronic shutter that allows long exposures, with charge integration on-chip to reduce random electronic noise. Exposures can be several seconds, or even minutes with Peltier cooling. On the other hand, it is convenient to output a standard video signal that can be displayed on an ordinary TV monitor, or input to a computer using a simple USB video frame grabber. The camera electronics then scans the integrated CCD image at PAL video rates of 25 frames/second, even though the image itself is only updated after integration over n-frames, e.g. with n=250 for a 10 second exposure.

• Dynamic Range
  

  The dynamic range of the NeutronOptics camera is normally limited to 8-bits by the USB frame grabber supplied; this corresponds to the range of levels that can be detected as distinct by the human eye. The camera itself is capable of a dynamic range of 60 dB (10-bits) which might be achieved with a more expensive frame grabber such as the basic Ellips Santos or National Instruments PCI-1410. A higher dynamic range would allow integration over longer times for strong beams that would otherwise saturate.

  You can also obtain 10-bit or better images from NeutronOptics cameras by automatically integrating a number of shorter exposures using ImageJ acquisition. But very large intensity ranges are not physically meaningful; 16-bits corresponds to measuring only 10 photons alongside 655,000, and 160,000 photons are produced by each neutron.

Advantages of the NeutronOptics Camera Design

NeutronOptics cameras are inexpensive, compact, have good efficiency and resolution, and are very easy to use; essentially they are "plug and play". Frankly, we don't know how to design a better neutron camera without spending much more money. It is possible to buy more expensive cameras, and for some applications that is justified. For example, although the ~100µ resolution of the compact NeutronOptics camera is similar to that of more expensive cameras, and near the usual limit for neutrons, it would be possible to improve efficiency by using electron or photon multipliers; NeutronOptics itself proposes a relatively inexpensive, electron-multiplier option (EMCCD). But although multiplying the light produced by a single neutron might permit its detection, counting the same neutron with multiple photons doesn't help with statistics or dynamic range. The compact NeutronOptics camera produces reasonable images with 10 second exposures at 10⁴.n.cm^-2.sec^-1, or only 1 neutron/second per 100µ resolution pixel.

More expensive neutron detectors are of course justified for some applications, such as neutron tomography, and crystallography, where the highest sensitivity and dynamic range is an advantage, and orders of magnitude cost increases are acceptable. NeutronOptics can provide advanced imaging cameras for neutron radiography, and the new CYCLOPS "4&pi" 16-CCD thermo-electrically cooled, image-intensified Laue diffractometer at ILL Grenoble probably represents the current state-of-the-art in neutron CCD cameras. Here is a short streaming video illustrating the astonishing power of such a machine, even though it is located on a low-flux guide with a 10⁷.n.cm^-2.sec^-1 white thermal beam.