Detector configuration with semiconductor photomultiplier strips and differential readout

Abstract

A detector configuration that combines a plurality of elongated semiconductor photo-multiplier sensor strips coupled to a scintillator crystal block with a differential readout that will enhance the time resolution. This is permitted due to a reduction of electronic noise due to reduced cross talk and noise in the ground. In addition, the dead area is minimized and thus the efficiency of the photodetector is enhanced.

Claims

1. A detector configuration, comprising: a scintillator crystal block comprising a plurality of elongated scintillator elements arranged in a matrix of rows and columns; a plurality of semiconductor photomultiplier sensor elements optically coupled to said scintillator crystal block, wherein said sensor elements comprise elongated strips and wherein said sensor element has a width (w) in a direction perpendicular to a length direction, said width (w) being at least two times smaller than a width of a scintillator element to which said sensor element is optically coupled; and readout means electrically coupled to said plurality of sensor elements, wherein said detector configuration is adapted to differentially couple each said sensor element to said readout means.

2. The detector configuration according to claim 1, wherein each said sensor element comprises its own anode connection and its own cathode connection, and wherein said detector configuration is adapted to differentially couple said anode connection and said cathode connection to said readout means.

3. The detector configuration according to claim 1, wherein at least two of said sensor elements do not share a common electrode.

4. The detector configuration according to claim 1, wherein said readout means comprises an amplification stage, wherein said detector configuration is adapted to differentially couple each said sensor element to said amplification stage.

5. The detector configuration according to claim 1, wherein a number of said sensor elements optically coupled to said scintillator crystal block is larger than the square root of the number of said scintillator elements in said scintillator crystal block.

6. The detector configuration according to claim 1, wherein a length (1) of said sensor element is at least five times larger than a width (w) of said sensor element.

7. The detector configuration according to claim 1, wherein said sensor elements extend from one edge of said scintillator crystal block to an opposite edge of said scintillator crystal block.

8. The detector configuration according to claim 1, wherein said plurality of semiconductor photomultiplier sensor elements comprise a first subset of semiconductor photomultiplier sensor elements optically coupled to said scintillator crystal block at a first side surface of said scintillator crystal block, and further comprises a second subset of semiconductor photomultiplier sensor elements optically coupled to said scintillator crystal block at a second side surface of said scintillator crystal block, wherein said second side surface is opposite from said first side surface.

9. The detector configuration according to claim 1, wherein said sensor elements each comprise a plurality of semiconductor avalanche photodiodes.

10. The detector configuration according to claim 1, wherein said sensor elements each comprise a plurality of semiconductor photomultiplier cells arranged in a row, wherein adjacent cells in said row are electrically connected, wherein each cell comprises a plurality of semiconductor avalanche photodiodes.

11. The detector configuration according to claim 1, wherein said sensor elements each comprise a first set of electrical connection elements for electrically coupling said sensor element to said readout means and a second set of electrical connection elements for electrically coupling said sensor element to said readout means, wherein said first set of electrical connection elements and said second set of electrical connection elements are positioned at opposing ends of said sensor element along a length direction thereof.

12. The detector configuration according to claim 11, wherein said readout means is adapted to determine a time difference of signals received from said first set of electrical connection elements and said second set of electrical connection elements, and to determine a location of a detection event along a length direction of said sensor element based on said time difference.

13. A detection method, comprising the steps of: providing a detector configuration with a scintillator crystal block comprising a plurality of elongated scintillator elements arranged in a matrix of rows and columns, a plurality of semiconductor photomultiplier sensor elements optically coupled to said scintillator crystal block, wherein said sensor elements comprise elongated strips, and wherein said sensor element has a width (w) in a direction perpendicular to a length direction, said width (w) being at least two times smaller than a width of a scintillator element to which said sensor element is optically coupled and a readout means electrically coupled to said plurality of sensor elements; and differentially coupling each said sensor element to said readout means.

14. The detection method according to claim 13, wherein said detector configuration is a detector configuration according to claim 1.

Description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(1) The features and advantages of the present invention will become best apparent from a detailed description of preferred embodiments in conjunction with the accompanying drawings, in which:

(2) FIG. 1a is a schematic perspective view of a conventional detector configuration employing an array of silicon photomultiplier pads coupled to an array of scintillating crystals;

(3) FIG. 1b is a schematic cross-sectional view illustrating the electrical connection of the conventional detector configuration of FIG. 1a via a common ground electrode;

(4) FIG. 2 is a schematic perspective view of a detector configuration according to an embodiment of the present invention;

(5) FIG. 3 is a front view showing the readout connections of the silicon photomultiplier strips of the detector configuration in FIG. 2; and

(6) FIG. 4 shows the differential readout connections of the detector configuration according to the present invention.

(7) FIG. 2 is a perspective schematic view of a detector configuration 10 according to the present invention. The detector configuration 10 may be used in a time-of-flight detector in a particle physics experiment, but may also be used as a gamma detector in medical, military and security applications, such as in the field of positron emission tomography (PET).

(8) The detector configuration 10 comprises a scintillator crystal block 12 formed of a plurality of elongate scintillator elements 14 arranged in a matrix of rows and columns. FIG. 2 shows a configuration with 16 scintillator elements 14 arranged in four rows and four columns. However, this is a mere example, and scintillator crystal blocks with any number of rows and columns may be employed. Adjacent scintillator elements 14 may be optically separated from one another by means of an interposed reflective foil or an air gap.

(9) Typical sizes for medical PET detectors are 3 mm×3 mm×15 mm. The 3 mm×3 mm can be larger or smaller depending on the size of the SiPM cell employed. The length (15 mm) needs to be as large as possible to increase sensitivity of the device; however, a greater length degrades the time (and energy) resolution.

(10) The scintillator elements 14 shown in FIG. 2 are cuboid in shape with a square cross section. However, the invention is not so limited, and depending on the applications and the manufacturing constraints scintillator elements 14 with a rectangular cross section or other shapes may be employed as well.

(11) The detector configuration shown in FIG. 2 further comprises a first sensor array 16 physically and optically coupled to a first end surface 18 of the scintillator crystal block 12, and a second sensor array 20 coupled to an opposite second end surface 22 of the scintillator crystal block 12 in a way that each of the scintillator elements 14 is optically coupled to the first sensor array 16 at a first end thereof and is optically coupled to the second sensor array 20 at a second opposite end thereof.

(12) In the schematic drawing of FIG. 2, the first sensor array 16 and the second sensor array 20 are shown detached from the scintillator crystal block 12. However, this is for illustrative purposes only, and it should be understood that during operation and for readout and analysis the first sensor array 16 and the second sensor array 20 are placed and mounted onto the respective end surfaces 18 and 22 of the scintillator crystal block 12 so to establish an optical contact between the scintillator crystal block 12 and the sensor arrays 16, 20.

(13) The first sensor array 16 and second sensor array 20 are generally identical to one another, apart from the fact that the second sensor array 20 is placed onto the second end surface 22 in an orientation that can be rotated by 90° with respect to the orientation of the first sensor array 16 placed onto the first side surface 18. The perpendicular orientation of the sensor arrays 16, 20 enhances the spatial resolution of the detector configuration 10.

(14) The first sensor array 16 and the second sensor array 20 each comprise a plurality of silicon photomultiplier strips 24 that are arranged in parallel and adjacent to one another. In the configuration shown in FIG. 2, the first sensor array 16 and the second sensor array 20 each comprise a number of 16 silicon photomultiplier strips 24, and hence each of the scintillator elements 14 will be in optical contact with four silicon photomultiplier strips 24 of the first sensor array 16 and four silicon photomultiplier strips 24 of the second sensor array 20. But this configuration is a mere example, and both the first sensor array 16 and the second sensor array 12 may comprise a smaller or a larger number of silicon photomultiplier strips 24.

(15) In the configuration shown in FIG. 2, the silicon photomultiplier strips 24 extend over the entire length l of the scintillator crystal block 12 from one edge thereof to an opposite and thereof.

(16) In general, the size of a SiPM cell (or strip) is limited in size. This is because the dark count rate increases (linearly) with the area and also the electrical capacitance of the cell increases linearly with area The increased dark count rate leads to problems, since a dark count occurring just prior to the event of interest destroys the timing. The increased capacitance just makes it increasingly difficult to design fast electronics. However, strips, if they are long, may give an advantage since they could be considered as transmission lines.

(17) As an example, the strips 24 may have a length of 15 mm and a width of 0.75 mm.

(18) Each silicon photomultiplier strip 24 is a photodiode run at a high gain such that a primary electron generated by an incident photon by means of the photoelectric effect initiates an avalanche or Geiger discharge. To limit the discharge from spreading over the whole device, each silicon photomultiplier strip 24 is subdivided into small pixels of a limited area with the voltage supplied through a limiting resistor. Each pixel corresponds to an avalanche photodiode, as is generally known from the prior art. The charge of the signal generated by a single avalanche photodiode undergoing a Geiger discharge is given by the capacitance of the pixel (diode) times the over-voltage applied. The overvoltage is the voltage above the breakdown voltage and typically amounts to several volts. The generated charge is typically in the range of 10.sup.6 electrons.

(19) The silicon photomultiplier strip 24 may in general be formed of a plurality of square photomultiplier cells arranged in a row, wherein adjacent cells in the row are electrically connected. In this way, the strips 24 may be formed of square silicon photomultiplier cells that are readily commercially available from a number of suppliers.

(20) The electrical connections and readout of the detector configuration 10 will now be explained in further detail with reference to FIG. 3. FIG. 3 is a front view of the first sensor array 16, and shows 16 adjacent silicon photomultiplier strips 24a to 24p. Each of the strips 24a to 24p has a first set of connection elements 26a to 26p at a first surface side thereof, and a second set of connection elements 28a to 28p formed at an opposite surface side thereof Each of the first set of connection elements 26a to 26p and the second set of connection elements 28a to 28p comprises both an anode connection and a cathode connection.

(21) The electrical connection of the connection elements 26a to 26p and 28a to 28p to the readout means is illustrated in greater detail in FIG. 4, which shows a schematic view along the side surface of the first sensor array 16. FIG. 4 is a cutout, which, for ease of illustration, only shows five silicon photomultiplier strips 24a to 24e with corresponding connection elements 26a to 26e. One of the connection elements 26a to 26e is the cathode element, whereas the opposite connection element 26a to 26e is the respective anode element. As can be taken from FIG. 4, the anodes and cathodes of the individual silicon photomultiplier strips 24a to 24e are not directly electrically or physically connected. Hence, each of the silicon photomultiplier strips 24a to 24e has its own and separate set of anodes and cathodes 26a to 26e. As can be taken from FIG. 4, the anode and cathode electrodes 26a to 26e of each of the silicon photomultiplier strips 24a to 24e are connected to the inputs to corresponding front-end amplifiers 30a to 30e via respective pairs of electrical connections 32a to 32e. Such a readout connection is known as fully differential.

(22) Corresponding anode and cathode elements, front-end amplifiers and electrical connections are provided at the opposite ends of the silicon photomultiplier strips 24a to 24e.

(23) In contrast to the state of the art as explained with reference to FIG. 1b above, the differential coupling of the silicon photomultiplier strips 24a to 24e to the front end amplifiers 30a to 30e avoids a common ground connection. If an avalanche in generated in the silicon photomultiplier strip 24b, the corresponding charge will be locally supplied via the electrical connections 32b to the corresponding front end amplifier 30b only, and there is no current injected into the ground. The discriminator (not shown) coupled to the front end amplifier 30b senses the difference between the plus and minus inputs (rather than measuring the signal with respect to the ground), which results in the reduction of jitter and an improved timing.

(24) As an additional advantage, the elongate silicon photomultiplier strips 24 transport the readout signal to the side edges of the detector, and thereby allow an easy access to both the anode and the cathode electrode. Compared with a conventional pad geometry as shown in FIG. 1a, this reduces the amount of electrical connections in the sensor arrays 16, 20 and hence the dead area, thereby enhancing the efficiency of the photodetector.

(25) If an avalanche is triggered in one of the silicon photomultiplier strips 24a to 24h, independent charge signals can be collected from the first set of collection elements 26a to 26p and the second set of connection elements 28a to 28p. The time difference between the signals allows the derivation of the position of the hit along the strip, whereas the time of the hit may be computed from the sum of the detection times (i.e. the average) of the signals. Due to the independent measurements at both ends of the strips 24, jitter introduced by the electronics and time to digital converters can be reduced by a factor √{square root over (2)}.

(26) Similarly, the time difference of signals measured with the first sensor array 16 and the second sensor array 20 allows the determination of the position of the hit along the length of the corresponding scintillator element 14, in a direction z perpendicular to the plane xy of the sensor arrays 16, 20. This determination reduces parallax errors and also can improve the time resolution.

(27) One of the problems conventionally associated with silicon photomultipliers is their comparatively high dark count rate (DCR). A dark count is a random firing of a silicon avalanche photodiode. If a dark count happens a short time before the light pulse of interest, both pulses will be merged together and the time attributed to the event will be early due to the electronics firing early on this dark count. Moreover, after a dark count signal the electronics take some time to recover, and during this recovery time the time resolution of the electronics is degraded. In a conventional silicon photomultiplier system, it is hence difficult to determine whether a timing is accurate or is an early timing due to a dark count pulse or a delayed timing caused by electronics recovery time.

(28) The inventors found that the effect of dark counts can be minimized by decreasing the width w of the silicon photomultiplier strips 24 such that each scintillator element 14 is optically coupled to a plurality of silicon photomultiplier strips 24. In the configuration shown in FIG. 2, a width w of the silicon photomultiplier strip 24 is four times smaller than a width and length of the scintillator elements 14, and hence each of the scintillator elements 14 is coupled to four silicon photomultiplier strips 24 of the first sensor array 16 at one side surface thereof, and is further coupled to four silicon photomultiplier strips 24 of the second sensor array 20 at the opposite side surface 22 thereof. Reduction of the area of the silicon photomultiplier strips 24 reduces the number of dark counts. At the same time, each of the strips 24 allows to make an independent time measurement. An increase in the number of strips therefore allows an improvement in the timing by taking an average of independent measurements, and further allows early pulses caused by dark counts to be discarded. The inventors found this configuration particularly advantageous in applications in which the time of arrival of a first photon needs to be determined.

(29) The detailed description of the preferred embodiments and the figures merely serve to illustrate the invention and the advantageous effects it achieves, but should not be understood to imply any limitation. The scope of the invention is to be determined solely by means of the appended claims.

(30) A Detector Configuration with Semiconductor Photomultiplier Strips and Differential Readout

REFERENCE SIGNS

(31) 10 detector configuration 12 scintillator crystal block of detector configuration 10 14 scintillator element of scintillator crystal block 12 16 first sensor array 18 first side surface of scintillator crystal block 12 20 second sensor array 22 second side surface of scintillator crystal block 12 24 silicon photomultiplier strip 24a-24p silicon photomultiplier strips 26a-26p first set of connection elements of silicon photomultiplier strips 24a-24h 28a-28p second set of connection elements of silicon photomultiplier strips 24a-24h 30a-30d front end amplifiers 32a-32d electrical connections 100 conventional detector configuration 102 scintillator crystal block of detector configuration 100 104 scintillator element of scintillator crystal block 102 106 sensor array 108 sensor pads of sensor array 106 108a-108e sensor pads 110 side surface of scintillator crystal block 102 112 common anode of sensor pads 108a-108e 114a-114e cathodes of sensor pads 108a-108e 116a-116e front end amplifiers 118a-118e electrical connections 120 link connection