Radiation measurement apparatus and method

An apparatus for measuring radiation includes a plurality of detectors (2), each detector (2) including: a scintillating material (4) for emitting light in response to incident radiation (6), and a photodetector (8) for receiving light emitted by the scintillating material (4) and outputting an electrical pulse in response to light received from the scintillating material (4), wherein a parameter characterising the electrical pulse is related to an energy associated with the incident radiation (6); and a power supply (10) for supplying power to a plurality of the photodetectors (8). The apparatus reduces the volume of hardware to be transported to the measurement location and therefore provides particular advantages for scanning pipelines and other structures located deep underwater.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an apparatus and method for measuring radiation, and relates particularly, but not exclusively, to an apparatus and method for scanning a structure to detect changes in density between different parts of the structure. The apparatus and method of the invention have particular benefits for use in an undersea pipeline inspection apparatus.

Description of the Related Art

It is known to use gamma radiation for scanning structures, for example to obtain information about the density within the structure or to identify flaws such as cracks or corrosion in a structure. This is particularly useful for inspecting pipes subsea, where it is not always possible to inspect the pipe from the interior. Gamma scanning is also used for obtaining information about other industrial structures such as distillation columns and the like.

An apparatus for scanning structures such as a pipeline or process vessel using gamma radiation is described in GB 2496736 A. This apparatus comprises a source of gamma radiation and an array of detectors. The apparatus is capable of being arranged with the structure to be scanned, such as a pipeline, positioned between the source and detectors so that radiation emitted by the source can pass along a plurality of paths through a portion of the structure to the detectors. The number of detectors in the array may range from fewer than 10 up to more than 100, e.g. up to 150, depending on the application. To obtain high resolution data, a large number of detectors are used, closely spaced from one another. The detectors are arranged in the form of an arc centred on the structure to be scanned. In operation, the source and array of detectors are arranged in fixed relationship with respect to each other, and are rotated around the structure to be scanned. In this way, information about the density of the structure along a plurality of paths is obtained, enabling a high resolution density map of the structure to be calculated. This technique has similarities with medical imaging techniques such as x-ray tomography.

When deploying this gamma scanning techniques in a subsea environment, there are additional challenges which do not arise with land-based measurements. When operating subsea at a depth of 1000 metres the pressure is 100 atmospheres. For each additional 1000 metres of depth below sea level, the pressure increases by a further 100 atmospheres. The apparatus must be able to withstand this pressure. Furthermore the apparatus must be compact for deployment subsea using submarines capable of operating at the required depth. It is challenging to package all the required components of the apparatus into a sufficiently small volume. In order to deploy this technique at even greater depths, it is necessary to meet ever more stringent requirements, particularly regarding the size of the apparatus.

A typical detector for detecting gamma radiation comprises a scintillating crystal and a photodetector. Gamma rays entering the scintillation crystal interact with the scintillating material to produce photons in the visible and/or ultra violet region. These scintillation photons are detected using a photodetector, typically a photomultiplier tube (PMT), which outputs an electrical pulse. The electrical pulse provides information about the number and energy of the incident gamma photons. Counting the number of electrical pulses corresponding to gamma rays transmitted from the source to the detector, through the structure being scanned, enables differences in the density of different parts of the structure to be detected.

Integrating a large number of photodetectors into an apparatus for use subsea is one of the many challenges of designing such an apparatus. Photomultiplier tubes are preferred due to their high sensitivity to low light levels. However, photomultiplier tubes comprise vacuum tubes which must be sealed against the high pressure encountered at depth. Photomultiplier tubes also require high voltages (˜1 kV) for biasing the dynodes of the photomultiplier tube and these high voltages must be effectively isolated for subsea operation. The power supply for each photomultiplier tube must also be very stable because the gain or calibration of photomultiplier tubes is very sensitive to changes in the high voltage biasing voltage.

BRIEF SUMMARY OF THE INVENTION

Preferred embodiments of the present invention seek to overcome one or more of the above disadvantages of the prior art.

According to the present invention there is provided an apparatus for measuring radiation, comprising:

a plurality of detectors, each detector comprising:a scintillating material for emitting light in response to incident radiation, anda photodetector for receiving light emitted by the scintillating material and outputting an electrical pulse in response to light received from the scintillating material,wherein a parameter characterising the electrical pulse is related to an energy associated with the incident radiation; and

a power supply for supplying power to a plurality of said photodetectors.

By providing a power supply for supplying power to a plurality of the photodetectors, the overall volume of the apparatus is reduced. This reduction in volume of the apparatus is critical to deploying the apparatus at greater depth subsea. Reducing the number of power supplies also reduces the electrical isolation requirements.

The apparatus may comprise a plurality of power supplies, wherein each power supply is arranged to supply power to a respective plurality of photodetectors.

By providing a plurality of power supplies, it is possible to power a greater number of photodetectors than that allowed by the maximum power output of a single power supply.

The apparatus may further comprise at least one capacitor for stabilising the voltage supplied by the power supply.

Advantageously this prevents an event at one photodetector from affecting the biasing voltage applied to another photodetector, thereby maintaining a stable power supply to each photodetector. In one embodiment, the capacitor is connected between the terminals of the power supply.

The apparatus may further comprise at least one data acquisition part for receiving electrical pulses output by a respective detector and counting a number of said electrical pulses having a value for said parameter for a predetermined range. The data acquisition part may be configured: to count a respective number of electrical pulses having a value for said parameter within each one of a plurality of sampling ranges; to identify a value of interest for said parameter based on the counted numbers of electrical pulses in said sampling ranges; to determine a measurement range centred on said value of interest; and to count a number of electrical pulses having a value for said parameter within said measurement range.

This feature enables the number of electrical pulses corresponding to particles (e.g. photons) of radiation detected by each one of the detectors to be counted, taking into account the fact that each photodetector may have a different gain, since it is no longer possible to individually tune the gain of each photodetector by adjusting the power supply when two or more photodetectors are powered by each power supply. Additionally by counting a number of electrical pulses having a value for said parameter within said measurement range, a single number is retained for the measurement along each path, rather than a complete spectrum, which reduces the data storage and transmission requirements of the apparatus.

Said data acquisition part is configured to count said respective numbers of electrical pulses successively.

Although scanning through a plurality of sampling ranges takes more time counting electrical pulses in all sampling ranges simultaneously, as would be the case if using a multichannel analyser, the advantage is that less hardware is required, thereby saving space.

Preferably, said value of interest of said parameter corresponds to the full energy of the incident radiation.

That is, the measurement range is centred on or covers the photopeak produced by deposit of the full energy of the particles (e.g. photons) of radiation in the scintillating material.

In a preferred embodiment, said radiation is gamma radiation.

In a preferred embodiment the photodetector is a photomultiplier tube.

The apparatus may further comprise a source of radiation.

The apparatus may be a subsea apparatus. For example, the apparatus may be a subsea apparatus suitable for use at a depth of 1000 m. The apparatus may be a subsea apparatus suitable for use at a depth greater than 1000 m.

The plurality of detectors may be arranged in an arc around a space adapted to receive a structure to be scanned.

The plurality of detectors and said source may be arranged on opposite sides of said space, and the apparatus may further comprise means for rotating said plurality of detectors and said source around a structure to be scanned, in fixed spatial relation to each other.

According to a second aspect of the invention, there is provided a method for measuring radiation, using an apparatus as defined above, comprising:

counting a respective number of electrical pulses having a value for said parameter within each one of a plurality of sampling ranges;

identifying a value of interest for said parameter based on the counted numbers of electrical pulses in said sampling ranges;

determining a measurement range centred on said value of interest; and

counting a number of electrical pulses having a value for said parameter within said measurement range.

Said respective numbers of electrical pulses may be counted successively.

The method may further comprise the step of positioning the apparatus at a subsea measurement location, prior to carrying out above steps at said location. For example, the subsea measurement location may be at a depth of up to 1000 m. The subsea measurement location may be at a depth of 1000 m or greater.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An apparatus for measuring radiation according to an embodiment of the present invention is illustrated byFIGS. 1 and 2. The apparatus comprises a plurality of detectors, although only one is shown inFIG. 1. With reference toFIG. 1, each detector2comprises a scintillating material4, in the form of a scintillating crystal4, for emitting light in response to incident radiation6, and a photodetector8, in the form of a photomultiplier tube8, for receiving light emitted by the scintillating material4and outputting an electrical pulse in response to light received from the scintillating material4. The electrical pulse output by the photodetector8is related to an energy associated with the incident radiation6. The electrical pulse output by the photodetector8is received by a respective data acquisition part12, comprising a pulse height analyser, for counting the number of photons of gamma radiation detected by the detector2. The photodetector8is powered by a power supply10. In this embodiment the power supply10is a high voltage power supply, as required for powering the photomultiplier tube8.

The radiation source is a gamma radiation source, such as137Cs which has a characteristic emission at 661.7 keV and sufficient penetrating power for scanning dense structures such as subsea pipelines. A scintillating material suitable for detecting gamma photons at this energy is BGO (bismuth germanate). However, other sources and scintillating materials may be used as is known in the art.

With reference toFIG. 2, a single power supply10supplies power to a plurality of photomultiplier tubes8connected in parallel. A capacitor14is connected across the terminals of the power supply10for stabilising the voltage to the photodetectors8. The negative terminal of the power supply10is connected to ground. A 220 μF high voltage capacitor14has been found to be sufficient for stabilising the voltage when up to seven photomultiplier tubes8are connected in parallel to a single power supply10. This is a surprising result, since conventionally each photomultiplier8would be powered by a dedicated power supply. This has previously been considered necessary for maintaining voltage stability for biasing the photomultiplier tube in order to avoid fluctuations in gain.

For simplicity, only one power supply10is shown inFIG. 2. However, for scanning a pipeline or similar structure, a large number of detectors2is preferable for obtaining high resolution data with a reasonable scanning time. In one embodiment of the apparatus, 95 detectors2are arranged closely spaced along an arc of a circle on one side of the structure to be scanned, with the radiation source positioned on the other. The apparatus includes means for positioning the source and detectors2in close proximity to the structure and for rotating the source and detectors2around the structure, without changing their relative positions. To power such a large number of photodetectors8, several power supplies10may be used, each one supplying power to a plurality of photodetectors8.

The number of photodetectors8which can be connected to a single power supply10is limited by the maximum power which can be drawn from the power supply10. Photomultiplier tubes generally have a low input resistance for optimal operation. However, it has been found that increasing the input resistance of the photomultiplier tubes8has the advantage of reducing their power draw, thereby enabling more photomultiplier tubes to be connected to the same power supply, and that any deterioration in performance is small enough to be outweighed by this advantage.

It has been found that up to at least seven photomultiplier tubes can be powered by a single power supply without any significant deterioration in performance. This reduction in hardware represents an important saving, particularly in reducing the space occupied by the power supplies when a large number of detectors are used. For example, in an apparatus comprising 95 photodetectors8, the number of power supplies required is reduced from 95, as would be used according to the prior art, to just 14 when the photodetectors8are grouped in groups of six or seven, each group powered by a single power supply10.

The detector2is sensitive not only to the number of gamma photons detected, but also their energies. The amplitude of the electrical pulses output by the photomultiplier tube8depends on the energy of the photons received from the scintillating material4. Since the amount of light produced by the scintillating material4is proportional to the amount of gamma ray energy absorbed in the scintillating material4, it follows that the amplitude of the electrical pulses output by the photomultiplier tube8depends on the gamma ray energy absorbed by the scintillating material4. The data acquisition part12, comprising a pulse height analyser, includes circuitry for discriminating between pulses of different amplitude and for counting the number of electrical pulses having an amplitude within a specified range.

FIG. 3shows an example energy spectrum20for scintillation events detected by one of the photomultiplier tubes8. This high resolution spectrum was obtained using a multi-channel analyser (MCA), and is shown by way of example only. In the present embodiment, data is acquired using a pulse height analyser operating in scanning mode at a lower resolution, as will be described below. InFIG. 3, the vertical axis shows the number of events detected (i.e. the number of electrical pulses output by the photomultiplier tube8), and the horizontal axis shows the channel number of the multi-channel analyser (MCA) to which the pulses were assigned. Each channel corresponds to an amplitude range of the detected electrical pulses.

The gamma ray source used for obtaining the spectrum20shown inFIG. 3is137Cs, which has a characteristic emission at 661.7 keV. However the energy spectrum20shown inFIG. 3Ais not a single narrow peak because of the way the gamma photons interact with the scintillating material4. The conversion to visible photons made by the scintillating material4depends on whether the gamma photons are completely absorbed or randomly scattered by the scintillating material4. The peak22towards the right of the spectrum20(around channel number510) is called the photopeak and is due to interaction processes in the scintillating material4in which the full energy of a gamma photon is deposited in the scintillating material4, for example the photoelectric effect. The photopeak22appears as a Gaussian due to intrinsic energy broadening within the detector2itself. The mean and standard deviation of the Gaussian photopeak22give information concerning the energy of the gamma photons and the detector resolution respectively. The signal24to the left of the photopeak22is due to processes which deposit only a part of the energy of a gamma photon in the scintillating material4, such as Compton scattering, which lead to a smooth distribution at energies lower than the photopeak22. Background radiation also contributes to the spectrum20at low energies.

The only useful information comes from the photopeak22. Changes in the height of the photopeak22give information about changes in the density of the structure through which the radiation has travelled. When measuring gamma radiation, one would typically focus on this part of the spectrum, performing what is commonly called ‘windowing’.

The position of the photopeak22with respect to channel number depends on the gamma photon energy and photodetector gain. In the case of a photomultiplier tube, the gain depends on temperature, photomultiplier tube biasing voltage and the intrinsic properties of the particular photomultiplier tube used. A problem with using photomultiplier tubes is that the manufacturing process cannot guarantee a consistent gain for all tubes. This means that given the same boundary conditions (voltage, temperature etc.) different photomultiplier tubes would provide signals for the photopeak centered on different channels (i.e. electrical signals corresponding to the full gamma photon energy are output with a different amplitude by each photomultiplier tube). In the prior art, this effect would be compensated by tuning the respective high voltage power supply of each photomultiplier tube, but this is only possible in cases in which the photomultiplier tubes are independently powered. In the present invention, this is not possible because groups of photodetectors8are each powered by a single power supply10.

This problem is partly avoided by grouping photomultiplier tubes8having similar intrinsic gain together. That is, photomultiplier tubes8belonging to a group powered by a single power supply10are selected from all those available by identifying those having a similar gain under identical conditions (temperature, voltage). The voltage output of the power supply10is then adjusted to broadly optimise the average gain of all the photomultiplier tubes8in the group for the specific application. However, this does not completely remove the problem as each photomultiplier tube8will still generate pulses corresponding to the photopeak22at different channel numbers.

In the present embodiment, this problem is solved by using a software tool capable of adapting a measurement range28to the photopeak22for each photodetector8. This technique is illustrated byFIG. 3. Instead of collecting the detailed spectrum20shown inFIG. 3, a pulse height analyser is used in scanning mode to count a number of electrical pulses having an amplitude within a specified sampling range or window26(FIG. 3A). The sampling window26is scanned (stepped) along the spectrum to count the number of electrical pulses in each successive sampling range26. The collection time interval for each sampling range26is maintained constant while moving the window the necessary steps to move throughout the whole spectrum (move-count-move etc.). From the number of counts recorded for each sampling range26, it is possible to locate the photopeak22and to define its mean position and width. A final measurement range or window28is then defined (FIG. 3B). The measurement range28is preferably centered on the mean of the photopeak22and is wide enough to cover the whole photopeak22, typically between two and three times the standard deviation of the photopeak22. This sequence is repeated for each of the photodetectors8in the apparatus, i.e. 95 times in the present embodiment. The entire sequence may be repeated each time a measurement is made, or whenever measurement conditions (e.g. position or temperature) change.

This technique, involving scanning through the spectrum using a sampling window26is slow but effective for identifying the position of the photopeak22for each photomultiplier tube8. Importantly, it compensates for the inability to individually tune the power supply10for each photomultiplier tube8, such that it is possible to use one power supply10to power two or more photodetectors8. By using a single channel pulse height analyser and scanning through the spectrum, considerable space is saved compared to using a multi-channel analyser which requires much more circuitry. It has been found that around64sampling windows26are sufficient for identifying the position of the photopeak22with suitable precision. Rather than storing the entire spectrum20, a single count output from the measurement range28is output.

Although the present embodiment is based on gamma photon detection using photomultiplier tubes, it will be appreciated that the present invention may be applied to other types of radiation (e.g. neutrons) and other types of photodetectors (e.g. avalanche photodiodes) in situations where conventionally each photodetector would be powered by an individual power supply. In the present invention, a number of photodetectors are powered by a single power supply. It has been found that the problem of maintaining a stable photodetector gain can be overcome by using a stabilising capacitor, by grouping photodetectors having similar intrinsic gain properties, by adjusting the power draw of the photodetectors, and/or by using the detection process described above to identify detection events corresponding to the photopeak.