GB2261110A - Gamma ray detector - Google Patents

Abstract

A PET radiation detector includes a pair of amplifying gaps (3, 21) to enhance the electron yield from a TAME gas, BaF2 gamma detector, together with a gate electrode (17) to inhibit passage of charge to the detector electrodes and reverse passage of ions to the crystal. A further reverse-biased gap may be positioned adjacent the crystal to prevent charge build-up thereon. Shield electrodes (23, 25) prevent gate switching signals causing spurious responses in the detector circuit.

Description

Radiation detectors This invention relates to radiation detectors and, in particular, to detectors for gamma radiation.

In our new radiation detector, SllkeV gamma rays are converted into UV photons which are then detected in TMAE vapour in a multiwire counter structure which functions as a position sensitive photomultiplier.

According to the present invention there is provided a radiation detector comprising a scintillation crystal, means to convert optical radiation into electrical charge carriers and detector means to detect the generated charge carrie.rs wherein a gap provided with means to inhibit the passage of charge carriers is positioned between said crystal and said detector means The invention will be particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic section of a prior art positron camera; and Figure 2 shows a schematic layout of a camera in accordance with a specific embodiment of the present invention; Figure 3 is a section through a practical embodiment of the invention; and Figures 4 and 5 show details of the appratus of Figure 3.

Referring now to the drawings, Figure 1 shows a prior art design. Incident gamma radiation causes a BaF2 crystal 1 to scintillate, generating ultra-violet photons. The UV photons convert in a gas space 3 adjacent to the crystal and the resulting handful of electrons are amplified in a high electric field applied between a conductive mesh 5 on the crystal surface and the cathode 7 of a multiwire proportional counter (MWPC) 9.

The signal is transferred into this section and further amplified on the anode wires 11. Some form of read-out is built into the MWPC section.

We found that this structure was very unstable and started to breakdown after 20 minutes or so, due to the charging of the crystal surface by the positive ions returning from the avalanche in the MWPC. However, by installing a protective gap 13 (Figure 2) with a reverse bias immediately against the crystal face. This gap (preferably 0.5 - l.Omm wide) sacrifices a little signal for a very much enhanced stability. We have found that with lO0V of reverse bias the modified counter will run all day without showing charging effects.

In the prior art positron camera a severe practical problem is caused by the very high ratio of-single, counts to coincidence (i.e. useful) counts (up to -50:1). This overloads the gain elements of the MWPC and causes serious deadtime losses in the read-out system. We found that two further modifications which improve this situation significantly. Firstly, the initial parallel amplifying gap 3 which now follows the crystal barrier gap is separated from the MWPC by a wide gap 19 (-30mm). (This on its own further enhances the stability of the counter.) In order to do our fast coincidence we would now like to take a trigger signal from the back of this gap. However, as this would demand too much gain from one gap we insert a further gap 21 and take the trigger signal from its rear face.

Secondly the fast coincidence with the other detector is performed while the electron cloud drifts towards a MWPC 9 which delivers a final burst of gain and performs the read-out.

Roughly in the middle of the drift region 15 is an electronic gate 17 operated by the coincidence circuit. This ensures that only "good" events trigger the MWPC and the read-out system.

This simultaneously enhances the stability of the counter and dramatically reduces the pile-up in the read-out electronics.

This gate has been carefully designed with shield electrodes 23,25 to minimise the interference it can cause in the read-out electronics.

With these modifications our tests to date have been able to demonstrate a quantum efficiency of 20% and a spatial resolution of 6mm fwhm with a time resolution of 4ns fwhm. The efficiency is three times that of the lead system and the time resolution 1/3. This means a factor of 9 in sensitivity and a factor of 27 in signal to noise ratio. The predicted maximum data-rate rise from 2kHz to 20kHz under comparable conditions.

A practical embodiment of the detector is shown in Figure 3. This comprises a barium fluoride crystal detector 31 comprising twelve tiles of BaF2 mounted on a stainless. steel frame 33. Next there is a series of wire planes. The first plane 35 consists of 50pm diameter wire at a pitch of 500pm and is spaced 0.5mm from the BaF2 crystal. The second plane 37 consists of lOOpm diameter wire at lmm pitch and is spaced 3.0mm from the first plane. The third plane 39 also consists of lOOpm diameter wire at lmm pitch and is spaced. 9.0mm from the second plane.A gate 41 comprising 100pom wires at lmm pitch is positioned 20.0mm from the third wire plane and has first and second metallic mesh screens 43,45 positioned one on either side. Following a further gap of 13.2mm, is a back end detector 47 which looks like a conventional multi-wire proportional counter with orthogonal cathodes. The cathodes consist of 50pm wire at 2.0mm pitch and the anode/cathode plane comprises 20pm anode wires and lOOpm cathode wires at 4.0mm pitch. X-and Y-coordinates are obtained with with delay lines 49,51. The detector is mounted within a sealed enclosure comprising aluminium honeycomb-structure sheets 53 mounted on a stainless steel frame 55-.Heater pads are provided to keep the enclose at a constant temperature of 600C at which the TAME vapour pressure is 4.5mB. The honeycomb aluminium sheet is structurally strong but is substantially transparent to the gamma photons.

In operation, a 5llKeV gamma photon is trapped by the BaF2 crystal which emits a flash of 190no ultra-violet radiation.

The UV radiation is absorbed by TAME gas within the camera chamber and is photo-ionised creating electrons.

A voltage source V1 is connected between the first and second planes, creating a field of 300V/mm which permits the creation of further electrons. A gain of a level generated by this field in a wider gap would be unstable, so a second amplification gap connected to a further voltage source V2 has a lower field strength of about 150V/mm. The two gaps will together provide sufficent electrons for a detectable signal.

With a positron camera, there are two detectors, one on either side of the source. The positron emission event is characterised by coincidence of signals in the two detectors. A signal is therefore taken from the third plane and fed into an amplifier Al and discriminator D. The signal from the corresponding amplifier A2 on the other detector is similarly extracted and examined for coincidence. When events are detected by both third planes within a time window of 5ns, two events one on each end and they are in time coincidence.

Between the third plane and the multi-wire detectors is placed a gate electrode which is normally biased +20V on alternate wires. It is a flat plane of wires, but with alternate wires connected to two bus bars so that if one wire is up, the next wire is down. As long as this voltage is applied, the gate acts as a barrier to the passage of electrons between the third plane and the detectors. When coincidence occurs, the gate electrodes are brought to zero volts whilst the triggering electrons are still in process of passing through the drift field between the third plane and the gate. A window of about 200ns is available for this decision and action.

When the bias is removed from the gate wires, electrons can pass and will drift to the multi-wire and give rise to a normal avalanche around an anode wire, induce a signal on the cathode, thus permitting a readout of the X- and Y-coordinates.

The effect of the gating is that the back end of the counter operates only at the coincidence rate whereas the front end generates electrons at a much higher rate corresponding to single events. This is an improvement of greater than 100:1, which has a corresponding effect on the signal/noise ratio of the detected signal. It also relieves congestion at the detector.

Another advantageous effect of the gate electrode is that it also acts as a barrier to the reverse transmission of positive ions from the multi-wire region.

Yet a further advantage of this arrangement is that the actual generation of positive ions is reduced since a large signal is produced only after coincidence has opened the gate rather than continuously as with a conventional arrangement.

A problem with the fast switching of the gate electrode is that, with the level of signal and impedance of the multi-wire detector, spurious signals could be induced in the detctor circuit. To prevent this a copper mesh screen is provided on either side of the gate, so the gate electrode. The screens are connected to potential sources appropriate to maintain the electron drift field. A metallic shield on the frame completely encloses the gate. AC continuity is maintained by surface mount capacitors bridging connecting gaps, thereby creating a shielding cage round the gate electrode.

A further problem is that of positive ions drifting back to the front end, since BaF2 is an almost perfect insulator. This problem is addressed by two measures. Firstly a metallic wire is wound round the crystal. With 25pm wire at 250pm pitch there is still 90% spare area, but there is an electrode to trap the ions and also to reduce the length of the discharge track enormously.

Secondly, in the front of this crystal is provided a reverse biased gap 0.5mm wide. so that the +ve ions are not driven to the crystal. When they arrive at the first plane, 0.5mm in front of the crystal, that is the most negative point and they don't go any further. The disadvantage is that any UV light which is converted in the first 0.5mm gap is effectively lost.

The thickness of the BaF2 crystal may be increased to improve sensitivity. This is, however, a trade-off against resolution with a practical maximum of 16mm.

Claims (9)

Claims

1. A radiation detector comprising a scintillation crystal, means to convert optical radiation into electrical charge carriers and detector means to detect the generated charge carriers wherein a gap provided with means to inhibit the passage of charge carriers is positioned between said crystal and said detector means.

2. A radiation detector as claimed in claim 1 wherein said means to inhibit the passage of charge carriers comprises a reverse-biased grid immediately adjacent said scintillation crystal.

3. A radiation detector as claimed in claim 2 wherein said grid is spaced 0.5 to l.Omm from said scintillation crystal.

4. A radiation detector as claimed in claim 3 wherein said fbi as is of the order of lOOV.

5. A radiation detector as claimed in claim 1 including a succession of charge amplifying gaps across which different fields are applied.

6. A radiation detector as claimed in claim 5 including a pair of charge amplifying gaps the first of which is of the order of 3 millimetres in width and the second of which is of the order of 9 millimetres in width.

7. A radiation detector as claimed in claim 6 wherein fields of the order of 300V/mm and 150V/mm are applied across said respective gaps.

8. A radiation detector as claimed in claim 1 wherein said means to inhibit the passage of charge carriers comprises a gate electrode placed intermediate said crystal and said detector means.

9. A method of detecting radiation comprising using a scintillation crystal to generate optical radiation, using said optical radiation to create an electrical charge by means of a photo-ionising medium, multiplying said charge in an electrical field, causing said multiplied charge to drift across a gap between field-creating electrodes, deriving a signal from said charge carriers and using said signal to control a gate electrode positioned in the drift path of said charge carriers.