A position-sensitive radiation detector has a plate-like scintillator and a photomultiplier tube array that are connected each other. At least one of scintillator surfaces of a radiation incidence-side and a connection-side to the photomuliplier tube array is provided with plural grooves. This structure enables the detection of the depth of a scintillation point in the scintillator and the detection of the scintillation point in horizontal direction with high resolution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a high-resolution position sensitive radiation 
detector, which can also detect the position in depth of radiation 
absorption in a scintillator. 
2. Prior Art 
A conventional position-sensitive radiation detector has been disclosed in 
which independent scintillators in a columnar form are bound together in a 
mosaic and connected to a position-sensitive photodetector (Japanese 
Patent Application Laid-open No. 237081/1986, Japanese Patent Application 
Laid-open No. 271486/1986). Another device is known in which a monolithic 
plate scintillator is connected to a position-sensitive photo-detector 
such as a photomultiplier tube (PMT) array, or the like for use as a 
position-sensitive radiation detector which can also detect the position 
in depth of radiation absorption in a scintillator (J. G. Rogers et al., 
Phys. Med. Biol. (1986), Vol. 31, No. 10, pp. 1061 to 1090). 
FIG. 1 shows a conventional radiation detector in which a 
position-sensitive photo-detector is connected to a plate-like 
scintillator. In this drawing, reference numeral 1 represents the 
scintillator, reference numeral 2 represents the position-sensitive 
photo-detector, reference numerals 3 and 4 represent gamma rays, and 
reference numerals 5 and 6 represent scintillation light. 
In this figure, the positions upon which the gamma rays 3 and 4 are made 
incident are detected by making use of the distribution positions of 
scintillation light outputted from the scintillator 1, for example, by 
calculating the center of gravity of the outputted light distribution. The 
radiation absorbed positions A and B in a vertical direction (the depth of 
scintillation points) are also detected by using the extent of 
distribution of the outputted light. 
However, the type of position-sensitive radiation detector which uses a 
mosaic-shaped BGO (Bismuth Germanate) cannot obtain the information on the 
depth of the radiation absorption in a scintillator, that is, the 
information on the depth of the scintillation point. Furthermore, 
assembling such a detector requires much labor and is costly. Although the 
radiation detector device shown in FIG. 1 can obtain the information on 
the depth of the radiation absorption in a scintillator, the light is 
distributed over a very large area, particularly when the scintillation 
light is emitted in an upper portion of the scintillator. This causes the 
deterioration of position resolution. The position resolution R is given 
by R .alpha..sigma./.sqroot.N, where the spread of light distribution is 
represented by .sigma., and the number of emitted photons is represented 
by N. Therefore, in the detector shown in FIG. 1 in which the spread of 
light output distribution .sigma. is large, the position resolution R 
deteriorates. 
To obtain the information on the depth of the light scintillation point in 
the scintillator it is helpful to reduce to positional error called a 
parallax error, which is important for applications to positron CT and so 
on. The occurrence of the parallax error .DELTA. is illustrated in FIG. 2. 
When two radiation rays are diagonally made incident on the scintillator 1 
with the same incident direction and position, are absorbed at the 
different positions A and B, and scintillation light is emitted from 
positions A and B, these two radation rays are detected as if these two 
rays have different incident positions (the parallax error .DELTA.). (If 
the scintillator 1 is sufficiently thin, that is, a width t is small 
enough, these two radiation rays are substantially detected as the same 
ones with the same incident position O.) If the information D on the 
absorption depth is obtianed, this type of error can be eliminated. 
SUMMARY OF THE INVENTION 
An object of the present invention is, therefore, a position-sensitive 
radiation detector that can obtain information on the depth of the 
scintillation point in a scintillator. 
Another object of the present invention is a radiation detector having an 
improved position resolution. 
A further object of the present invention is a radiation detector that can 
be easily assembled and manufactured. 
The position-sensitive radiation detector according to the present 
invention comprises a scintillator having a horizontal and a vertical 
extent and having a top surface and a bottom surface, the surfaces 
extending horizontally and defining therebetween the vertical extent of 
the scintillator, a plurality of regularly aligned reflection means 
extending from at least one of the top surface and the bottom surface of 
the scintillator for reflecting light emitted from a scintillation point 
in the scintillator, position-sensitive photo-detector optically coupled 
to the scintillator and disposed along the bottom surface of the 
scintillator for detecting light emanating from the bottom surface of the 
scintillator, and a scintillation position computer for determining a 
scintillation position in both horizontal and vertical direction. 
Other and further objects, features and advantages of the invention will 
appear more fully from the following description taken in connection with 
the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will now be described with reference 
to the accompanying drawings. 
FIG. 3 and 4 illustrate position-sensitive radiation detectors according to 
embodiments of the present invention, wherein reference numeral 11 
represents a scintillator, reference numeral 12 represents a 
position-sensitive photo-detector, reference numerals 13, 14, 16, and 17 
represent grooves, and reference numeral 15 represents a light guide 
region. 
Referring to these figures, the grooves 13, 14 or 16 and 17 are provided in 
the scintillator 11 which is originally a plate-like BGO block by, for 
example, milling it from both top surface 37 and bottom surface 38. As 
embodied herein, top surface 37 is the radiation incidence side of 
scintillator 11. That is, it is the side of scintillator 11 upon which 
gamma rays, or other desired rays of radiation, are incident. Bottom 
surface 38 is the connection side of scintillator 11. That is, the 
scintillator 11 is optically coupled to the position-sensitive 
photo-detector 12 through the bottom surface 38. The top surface 37 and 
the bottom surface 38 extend along the horizontal extent of scintillator 
11. 
In accordance with the present invention, a plurality of regularly-aligned 
reflection means are provided from at least one of the top surface and the 
bottom surface of the scintillator for reflecting light emitted from a 
scintillation point in the scintillator. As embodied herein the reflection 
means are grooves, such as 13, that extend from the top surface 37, the 
bottom surface 38 or from both surfaces 37 and 38 of scintillator 11. 
FIG. 3(b) is an enlarged view of the groove 13 of FIG. 1. The groove 13 is 
defined by two opposing walls 34 and 36 formed in the scintillator 11. The 
bottom of the groove 13 is defined by a bottom face 35. The walls 34 and 
36 establish an interface between the scintillator 11 and the air in the 
groove 13. If desired, the walls 34 and 36 can be provided with a 
reflecting coating or, alternatively, groove 13 can be filled with a 
reflective substance. The purpose of the walls 34 and 36 is to establish 
an interface whereby the light emitted from the scintillation point in the 
scinitillator 11 is selected in the manner described below. 
Because of a difference in refractive index between air layers formed in 
the grooves and a scintillator material or existence of reflective 
substance disposed in the grooves, the emitted light is reflected by the 
groove walls 34 and 36. As a result of this, the same effect of narrowing 
the spatial distribution of output light as that obtained by a device in 
which the scintillators are arranged in a mosaic can be obtained. 
Furthermore, the information on the depth of the radiation absorption can 
be obtained as described below in detail. 
The grooves may be provided in the scintillator in such a manner that, as 
shown in FIG. 3(a), the upper grooves 13 and the lower grooves 14 are 
aligned opposite each other (pattern A), or as shown in FIG. 4, the 
grooves 16 and 17 are arranged alternately (pattern B). It is desirable 
for the groove wall surfaces after cutting has been completed to be nearly 
mirror surfaces. 
Furthermore, by enclosing reflective substance such as, for example, 
BaSO.sub.4 in the grooves, the efficiency of reflection can be further 
increased. Thereby, a complete separation between adjacent scintillator 
segments can be achieved. In the pattern shown in FIG. 3(a), a light guide 
region 15 is provided between the upper grooves and the lower grooves in 
the scintillator 11. 
FIG. 5 illustrates a method of detecting light emanating from the 
scintillator and computing the radiation incidence position according to 
the radiation detector shown in FIG. 3(a), wherein reference numerals 20 
to 27 represent multi-anodes. 
Referring to this figure, when the light is emitted at a position A in the 
upper portion of the scintillator 11, spatial distribution of light is 
widened in the light guide region 15 through downward light propagation, 
thereby light outputs are obtained from a plurality of lower segments. As 
a result, the light response function (LRF) has a relatively large 
distribution spread as shown by a dashed line in the FIG. 5. 
When the light is emitted at a position B in the lower portion of the 
scintillator 11, light output is mainly obtained from a single lower 
segment, as a result of which, the LRF is narrow as illustrated by a solid 
line in the figure. 
Therefore, by detecting, for example, the number of wire anodes of 
multi-anode PMT whose corresponding outputs exceed a given threshold 
value, the radiation incidence horizontal position and the scintillation 
depth can be determined. For reference, anode outputs from anodes 20-27 
corresponding to the light output distribution from the scintillator are 
shown in Table 1. If each output of the multi-anode PMT corresponding to 
each anode exceeds the threshold value the value of "1" is given, and if 
it does not exceed the threshold value the value of "0" is given. As an 
example, the scintillator includes grooves having a 4 mm pitch and the 
pitch of the wire anodes is also 4 mm. 
TABLE 1 
______________________________________ 
(Upper light emission) 
(Lower light emission) 
#20 . . . #27 #20 . . . #27 
______________________________________ 
0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 
0 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 
0 0 0 1 1 1 0 0 0 0 0 0 1 0 0 0 
0 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 
0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 
0 0 0 0 0 1 1 0 0 0 1 1 1 0 0 0 
0 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 
0 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 
0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 
0 0 1 1 1 1 0 0 0 0 0 1 1 1 0 0 
0 0 1 1 1 0 0 0 0 0 0 1 0 0 0 0 
0 0 1 1 1 0 0 0 0 0 0 1 0 0 0 0 
0 1 1 1 1 1 0 0 0 0 1 1 1 0 0 0 
0 0 1 1 1 0 0 0 0 0 0 1 0 0 0 0 
0 1 1 1 1 0 0 0 0 0 1 1 1 0 0 0 
0 0 1 1 1 0 0 0 0 0 0 1 0 0 0 0 
0 0 1 1 1 1 0 0 0 0 1 1 1 0 0 0 
0 1 1 1 1 0 0 0 0 0 1 1 0 0 0 0 
______________________________________ 
Judging from Table 1, a case where "1" appears in sequence occurs more 
frequently when the light is emitted in the upper portion in comparison to 
that when light is emitted in the lower portion. With making use of this 
characteristic, it can be determined from which of the upper portion and 
the lower portion of the scintillator the light is emitted when the 
radiation is absorbed. The position in the horizontal direction can be 
determined by the center of gravity of the wire-anode outputs. As shown in 
FIG. 5, the output from each of anodes 20-27 passes along a line 46 to a 
scintillation position computer 47. The computer determines, in the manner 
described above, the scinillation position in the horizontal direction and 
in the vertical direction (depth) in the scintillator 11. 
Using the foregoing apparatus in a PET (Positron Emission Tomography) 
device with a small ring diameter, the deterioration of resolution in the 
radial direction in the periphery can be restrained. 
FIG. 6 illustrates a method of position detection by means of the radiation 
detector shown in FIG. 4. When the light is emitted at a position A in the 
upper portion of the scintillator 11, in the downward propagation the 
light is equally distributed to two segments in the lower portion of the 
scintillator 11. On the other hand, when the light is emitted at a 
position B in the lower portion of the scintillator, the light emanates 
from one segment in the lower portion of the scintillator. 
The distribution of the LRF is shown by the solid line when the light is 
emitted in the upper portion of the scintilator, and is as shown by the 
dashed line when radiation is emitted in the lower portion of the 
scintillator. 
The calculated positions of the center of gravity should shift each other 
by a half of the groove pitch, d. Therefore, by making the width of bins 
40-45 to be d/2 and disposing the bins 40-45 in such a manner as 
illustrated in FIG. 6, each bin alternately gives the detected position 
with the scintillation point in the upper portion and that with the 
scintillation point in the lower portion, thereby the information (1 bit) 
on the scintillation depth can be obtained. For example, bins 40, 42 and 
44 provide detection of scintillation points in the lower portion while 
bins 41, 43 and 45 provide detection of scintillation points in the upper 
portion. 
The importance of obtaining information on the depth will now be described 
with reference to a radiation tomography device. 
FIG. 7 illustrates the occurrence of a detection error in a radiation 
tomography device, wherein reference numeral 31 represents a scintillator 
and reference numeral 32 represents a center line of the scintillator. 
As the angle of incidence of radiation made upon the scintillator becomes 
large, a positional error called a parallax error occurs that is related 
to the depth of absorption. Many radiation tomography devices have the 
detecting device disposed annularly and detect incidence positions of 
gamma rays with various angles of incidence. The detected results are 
accumulated as data on the line l.sub.1 -l.sub.2 and l'.sub.1 -l'.sub.2, 
thus, generating projection data. The incidence angles of gamma rays can 
be obtained from the collimater direction in the case of single gamma ray, 
or by connecting the positions which are detected at the same time in the 
case of positron annihillation gamma rays (two gamma rays emitted in the 
opposite directions). When a gamma ray is made incident on the 
scintillator diagonally, the gamma ray is detected with a certain width 
P.sub.1 -P.sub.2 on the scintillator center line and is recorded also with 
a certain spread C'.sub.1 -C'.sub.2 on the projection data compared with 
those in the parpendicular incidence. 
Therefore, the measurement accuracy of incidence position of gamma rays 
when they are diagonally made incident deteriorates as the incidence angle 
becomes large, or as the thickness of the scintillator becomes greater; in 
other words, the position resolution deteriorates. If the scintillator is 
made thin, the detection efficiency of gamma rays deteriorates. 
In these cases, according to the present invention, by obtaining the 
information on the depth of absorption of gamma rays, the illustrated 
width P.sub.1 -P.sub.2 can be decreased, thereby the position resolution 
for the diagonally incident gamma rays can be improved. 
Although the foregoind description is devoted to the position-sensitive 
radiation detectors with horizontally one-dimensional structure which can 
determined the radiation incidence position (i.e., the scintillation 
position) in the horizontal one dimension, the horizontally 
two-dimensional position detection can be realized with the present 
invention by employing a lattice-shaped groove. FIGS. 8 and 9 illustrate 
two-dimensional position-sensitive radiation detectors accoridng to 
embodiments of the invention, wherein reference numeral 51 reprsents a 
scintillator, reference numeral 52 represents a position-sensitive 
photo-detector, reference numerals 53, 54, 56 and 57 represent grooves and 
reference numeral 55 represents a light guide region. Structures of the 
two-dimensional radiation detectors in FIG. 8 and 9 correspond to those of 
the one-dimensional radiation detectors in FIG. 3 and FIG. 4, 
respectively. 
The grooves 53, 54, 56 and 57 of the lattice shape provided in the 
scinillator 51, wherein each lattice-shaped groove has regular intervals 
in two horizontal directions orthogonal each other. The position-sensitive 
photo-detector 52 is optically coupled to the scintillator 51. Operations 
of determining the radiation incidence position with the detection of the 
scintillation depth in these two-dimensional radiaiton detector are same 
as the operations in the respective one-dmensional radiation detectors 
except for the dimensional difference. 
Although the scintillator in the plate-like shape is describe in the above 
embodiments, it may be in any desired shape, such as a columnar shape, for 
example. Furthermore, the grooves in the scintillator may be provided for 
either one of the radiation incidence or the connection sides of the 
scintillator as an alternative to grooves provided in both sides. 
According to the present invention, the position in the scintillator upon 
which radiation is made incident can be detected with excellent accuracy. 
Furthermore, as the information on the depth of the scintillation point in 
in the scintillator can be obtained, the positional resolution can be 
improved, and assembling and manufacturing of the scintillator is 
simplified as a result of which, the production cost can be reduced. 
It will be apparent to those skilled in the art that various modifications 
and variations can be made in the radiation detection device of the 
present invention without departing from the scope of spirit of the 
invention. Thus, it is intended that the present invention cover the 
modifications and variations of this invention provided they come within 
the scope of the appended claims and their equivalents.