Applied Photonics Ltd (APL) has a long-standing association with the UK nuclear industry which for some of the APL team dates back to the 1980s. The founder and MD, Andrew I Whitehouse (AIW), has been involved with the UK nuclear industry since 1987 when he was funded by UKAEA (Winfrith) as a PhD student at Dept. of Physics, Swansea University. He later went on to work for BNFL Research & Technology at their Sellafield site (1992 – 1998). APL’s software engineer / physicist, Phil Evans, worked for BNFL Research & Technology at Sellafield (1988 – 1998). Although APL is an independent, privately owned company, its roots are firmly based in the work AIW and colleagues (Dr James Young and Dr Iain Botheroyd) conducted during his time at Sellafield during the 1990s. APL’s first contract was to provide a LIBS in-situ characterisation service to the then owners of the UK’s fleet of seven AGR and one PWR nuclear power stations, British Energy. This resulted in the first reported use of LIBS inside a nuclear reactor pressure vessel. During the following 20+ years, APL has conducted a number of unique applications of LIBS at various nuclear sites in the UK and overseas. This section of our website summarises this work.
The two general types of LIBS instrument of relevance to our nuclear work differ in the method used to transmit the laser beam to the sample and return the plasma light to the spectrometer. As may be visualised from the LIBS schematic diagram
, the most simple method is to use a telescope arrangement and line-of-sight access to the sample. The distance between the telescope and the sample can range from 10s of cm (e.g. a laboratory bench-top LIBS instrument) to 10s of metres (a Stand-Off LIBS instrument). Another method is to use one or more fibre-optic cables to transmit the laser beam and plasma light and this is known as Fibre-Optic LIBS (FO-LIBS). The length of fibre-optic cable can range from typically a few metres to approaching 100 metres. More recently, a variation of FO-LIBS was developed by APL where a compact, (gamma) radiation-hard, passively Q-switched Nd:YAG laser is incorporated in a remote LIBS probe and one or more fibre-optic cables are used for transmitting plasma light to the remotely located optical spectrograph. A LIBS instrument of this design is installed at a hot cell facility at the Sellafield site and is discussed later in this section of our website.
Fibre-Optic LIBS (FO-LIBS)
APL has considerable experience in the design and deployment of FO-LIBS for various applications in the nuclear industry. APL has used multiple-fibre and single fibre versions of FO-LIBS but has found that a single-fibre design is adequate for most applications and has the advantage of offering a simple, relatively low-cost design for the remote probe and fibre-optic cable (or umbilical). APL’s single-fibre FO-LIBS design will now be described. The laser beam is launched into the optical fibre via launch optics at the proximal end of the fibre-optic. The fibre-optic cable is terminated at the distal end with a probe containing two lenses which collimate and focus the laser beam to a small spot on the sample surface. These lenses also image the laser-induced plasma on the distal end of the fibre-optic cable so that plasma light is transmitted back to the launch optics (i.e. the optical system used to inject, or launch, the laser beam into the fibre-optic cable) using the same fibre-optic cable used to transmit the laser beam. Within the launch optics, the plasma light is separated from the path of the laser beam for subsequent transmission to an optical spectrometer. The majority of APL’s previous applications of FO-LIBS utilised this single-fibre, twin-lens probe design. The following diagram illustrates a typical single-fibre, twin-lens probe FO-LIBS instrument.
|Schematic illustration of a single-fibre FO-LIBS instrument
||Sectional view of a typical single-fibre
(approx. 25 mm diameter, 100 mm length)
The single-fibre, twin-lens probe can be made to be reasonably compact using relatively small diameter lenses (e.g., typically 20 mm diameter but we have built probes as small as 6 mm diameter by 25 mm in length) and so is suitable for applications where physical access to the sample is limited. The probe is a simple and rugged design, can be made highly resistant to ionising radiation and, together with the single fibre-optic cable, is a relatively low-cost component of the FO-LIBS instrument and so may be regarded as a consumable item. This is advantageous in some nuclear industry applications where the probe and umbilical are at risk of becoming contaminated with radioactive material and so may need to be disposed of as radioactive waste rather than being re-used in a subsequent application.
One of the important limitations of a fibre-optic LIBS instrument is the relatively low peak power densities that are achievable at the sample surface. This can make it difficult or even impossible to analyse some materials using a FO-LIBS instrument. This limitation is exacerbated when the sample material has low opacity to the laser wavelength (e.g. optically transparent materials such as certain types of glass, plastic, etc). FO-LIBS is also an invasive technique since the probe needs to be located either in direct contact with or very close to the sample. The design, advantages and limitations of this and other types of FO-LIBS instrument are described in more detail later.
Stand-Off LIBS (ST-LIBS)
The optical system used in the basic design of ST-LIBS instrument consists of a laser beam expander (two or more lenses) which is designed to first expand the laser beam from typically less than 10 mm at the output of the laser head to nearer 50 mm (or sometimes considerably more) at the output aperture of the ST-LIBS instrument. The beam expander is also designed to have an adjustable focus which is compatible with the requirements of the application. The plasma light is collected by the final lens in the laser beam expander and then separated from the laser beam path using a dichroic mirror. A single lens system is then used to image the plasma light on to the input end of a fibre-optic cable which is connected to a spectrometer. All of the ST-LIBS applications described in this section of this report utilised this basic design of instrument which is shown schematically in the following figure.
An ST-LIBS instrument is able to achieve significantly higher peak-power densities at the sample surface than is possible with a FO-LIBS instrument and can in principle therefore be used to analyse materials that a FO-LIBS instrument cannot, e.g., gases, liquids, optically transparent materials. An ST-LIBS instrument only requires optical access to the target material and so therefore offers a truly non-invasive materials analysis technique and so no components of the ST-LIBS instrument are required to enter the radioactive environment where the sample is located.
Schematic illustration of a basic design ST-LIBS instrument
In its simplest form an ST-LIBS instrument uses a relatively simple optical design, but such a design can suffer from the following two limitations: i) the wavelength range of plasma light which may be detected is limited by the optical transmission properties of the dichroic mirror and, ii) the plasma light collection efficiency is limited by the f# of the final lens used in the laser beam expander. It also requires line-of-sight access to the sample and the optical properties of the materials which are in the beam path between the ST-LIBS instrument and the sample (e.g. a radiation shield window) will have a significant impact on the performance of the instrument. The design, advantages and limitations of this and other types of ST-LIBS instrument are discussed in more depth later in this section of our website.