an embodiment of the present invention is hereafter described with reference to the accompanying drawings . fig1 shows a photomultiplier according to this invention . fig1 ( a ) shows an enlarged perspective view of a shield electrode 26 as shown in fig1 , and fig1 ( b ) and 12 ( c ) show enlarged perspective views of modifications of the shield electrode as shown in fig1 ( a ). the mutually corresponding components as shown in fig1 and 1 are represented by the same references . the photomultiplier 1 according to this invention as shown in fig1 includes a focusing electrode 25 , the shield electrode 26 , a flat plate electrode 6 and an electric insulator 31 on the flat plate electrode 6 , which are provided between the photocathode 4 and a first dynode 7 . the focusing electrode 25 functions in the same manner as the first focusing electrode 52 as shown in fig1 so that photoelectrons primary electrons emitted from the photocathode 4 when light impinges on the photocathode are converged toward the first dynode 7 . the bottom 39 of the focusing electrode 25 is provided with an opening 30 in which the shield electrode 26 is inserted . the focusing electrode 25 is located at predetermined distances from the electric insulator 31 and the flat plate electrode 6 by stem pins 32 . the shield electrode 26 is provided so that elastically reflected electrons from the first dynode 7 are prevented from impinging on the first dynode again . as shown in fig1 ( a ), the shield electrode 26 comprises a top 34 having an upper opening 28 , a side portion 35 partially shaped as a truncated cone , and a bottom 36 having a lower opening 29 . each of the top 34 and bottom 36 of the shield electrode 26 is in the form of a disk . the upper and the lower openings 28 and 29 are concentric to the top 34 and the bottom 36 respectively . the shield electrode 26 is located in such a position that the primary electrons emitted from the photocathode 4 surely pass through the upper opening 28 of the shield electrode toward the first dynode 7 and secondary electrons elastically reflected from the first dynode do not return to the photocathode through the upper opening 28 but are captured by the inside surface of the shield electrode . as shown in fig1 , the top 34 of the shield electrode 26 is located closer to the photocathode 4 than to the first dynode 7 . in order to allow the primary electrons from the photocathode 4 to surely pass through the upper opening 28 of the top 34 of the shield electrode 26 , the upper opening should preferably be located in a position where the cross section of the orbit of the primary electrons converged by the focusing electrode 25 is most constricted . for that purpose , the lower edge of the truncated - cone - shaped part of the side portion 35 of the shield electrode 26 is located in nearly the same plane as the bottom 39 of the focusing electrode 25 , and the top 34 of the shield electrode is located below the upper edge of the side portion 38 of the focusing electrode 25 . in order to place the shield electrode 26 in the above - described manner , the axial length of the side portion 38 of the focusing electrode 25 is made longer than that of the truncated - cone - shaped part of the side portion 35 of the shield electrode 26 . the diameter d1 of the upper opening 28 of the shield electrode 26 is set so as to limit the effective area of the light incidence portion of the photocathode 4 . if the diameter d1 of the upper opening 28 is smaller , the elastically reflected electrons return to the photocathode with a smaller probability , and at the same time the photoelectrons emitted from the photocathode 4 when the light impinges on the photocathode reach the first dynode 7 with a smaller probability , so that it is impossible to measure a main pulse current accurately or with high sensitivity . if the diameter d1 of the upper opening 28 is larger , the photoelectrons emitted from the photocathode 4 impinge on the first dynode 7 with a higher probability to increase the main pulse current , and at the same time the elastically reflected electrons from the first dynode 7 return to the photocathode with a higher probability to increase a residual pulse current , so that it is impossible to detect only the main pulse current accurately . accordingly , the diameter d1 of the upper opening 28 is designed such that the passage of the photoelectrons emitted from the photocathode 4 is not so much hindered and the return of the elastically reflected electrons to the vicinity of the photocathode is effectively suppressed . when the diameter of the actual light incidence portion of the photocathode 4 for the effective area of the portion is set at about 5 mm , for example , the diameter d1 of the upper opening 28 is set at 77 to 67 % of the diameter of the actual light incidence portion of the photocathode for the effective area of the portion or set at about 3 . 5 mm . the diameter d3 of the lower opening 29 of the shield electrode 26 is set such that the photoelectrons having passed through the upper opening 28 are not prevented from impinging on the first dynode 7 . for example , the diameter d3 of the lower opening 29 is set so that the periphery of the lower opening is located on an imaginary truncated - cone - shaped surface extending from the periphery of the upper opening 28 to that of the opening 24 of the flat plate electrode 6 . in that case , the diameter d3 of the lower opening 29 is set at about 8 mm . the diameters d2 and d4 of the top 34 and bottom 36 of the shield electrode 26 as shown in fig1 ( a ) are set at about 7 mm and about 31 mm , respectively . the height h1 of the truncated - cone - shaped part of the side portion 35 of the shield electrode 26 is set at about 21 . 35 mm . each of shield electrode 71 and 73 as shown in fig1 ( b ) and 12 ( c ) may be provided instead of the shield electrode 26 as shown in fig1 ( a ). the shield electrode 71 as shown in fig1 ( b ) is in a disk form and has an opening 72 at the center thereof . the shield electrode 73 as shown in fig1 ( c ) is in the form of a truncated cone and has openings defining the top 74 and bottom 75 thereof . as well as the shield electrode 26 shown in fig1 ( a ), the shield electrodes 71 and 73 as shown in fig1 ( b ) and 12 ( c ) are located in such a position that the primary electrons emitted from the photocathode 4 surely pass through the shield electrode toward the first dynode 7 and the secondary electrons elastically reflected from the first dynode do not return to the photocathode . for that purpose , each of the shield electrode 71 and the top 74 of the shield electrode 73 is located closer to the photocathode 4 than to the first dynode 7 . the present invention is not limited to the shapes of the shield electrodes 26 , 71 and 73 as shown in fig1 ( a ), 12 ( b ) and 12 ( c ), but may be embodied using other shield electrodes having different shapes . the shield electrodes 26 , 71 and 73 as shown in fig1 ( a ), 12 ( b ) and 12 ( c ) are preferably made of a metal , and more preferably made of such a metal having large work function as tin and copper . further it is preferable that the inside surface of each of the shield electrodes 26 , 71 and 73 is not mirror - polished , but made porous in order to efficiently capture the secondary electrons reflected from the first dynode 7 . each of the shield electrodes 26 , 71 and 73 , as well as the focusing electrode 25 , is located at predetermined distances from the electric insulator 31 and the flat plate electrode 6 by stem pins 33 . the flat plate electrode 6 supports the focusing electrode 25 and the shield electrode 26 , 71 or 73 with the stem pins 32 and 33 and electrically separates the photocathode 4 form the first dynode 7 to twelfth dynode 18 and an anode 19 . the photocathode 4 , the focusing electrode 25 , the dynodes 7 to 18 and the anode 19 are connected to corresponding connection pins k , g , dy1 , dy2 , dy3 , dy4 dy5 , dy6 , dy7 , dy8 , dy9 , dy10 , dy11 , dy12 and p through the stem pins 32 and 33 and lead wires not shown in the drawings . each of the shield electrodes 26 , 71 and 73 is connected to a connection pin g1 which is connected to the seventh dynode 13 , for example . the flat plate electrode 6 is connected to the connection pin dy1 for the first dynode 7 . potentials as shown in table 1 are applied to the electrodes of the photomultiplier 1 from an external circuit which is entirely the same as the external circuit 37 as shown in fig2 . the potential of - 2 , 500 v is applied to the photocathode 4 . the anode 19 is kept at the ground potential . each of the shield electrodes 26 , 71 and 73 is kept at the same potential of - 1 , 200 v as the seventh dynode 13 . as the potential on each of the shield electrodes 26 , 71 and 73 is higher than that on the photocathode 4 , the electric field intensity near the photocathode 4 is heightened . table 1 shows the concrete values of the voltages applied to the electrode of the photomultiplier 1 and those of the potentials of the electrodes of the photomultiplier based on the photocathode . the state of the connection of the connection pins k , g , g1 , dy1 to dy12 , p and the external circuit is the same as that shown in fig2 . when the photomultiplier 1 as described above is used in the apparatus as shown in fig4 to count photons instead for the conventional photomultiplier 51 ( in order to simplify the descriptions , it is supposed that the shield electrode 26 shown in fig1 ( a ) is provided in the photomultiplier ), the weak light such as fluorescent light , which is in the spe state , is irradiated upon the photocathode 4 of the photomultiplier 1 by the pulse generator 62 , the optical fiber 63 and the filter 64 . as a result , the photoelectrons or primary electrons are emitted from the photocathode 4 , and converged by the focusing electrode 25 to accurately enter the upper opening 28 of the top 34 of the shield electrode 26 and impinge on the secondary electron emission surface 22 of the first dynode 7 through the upper opening , the lower opening 29 of the bottom 36 of the shield electrode and the opening 24 of the flat plate electrode 6 . at that time , secondary electrons are emitted from the secondary electron emission surface 22 of the first dynode 7 . among the secondary electrons , those which are ordinary secondary electrons in the region a of the energy distribution as shown in fig8 directly proceed to the second dynode 8 to be multiplied by the second dynode 8 to twelfth dynode 18 so that a main pulse current mp 2 which is the same as that shown in fig9 ( a ) is outputted form the anode 19 . further , some of the secondary electrons in the region b of the energy distribution as shown in fig8 directly proceed to the second dynode 8 so that a main pulse current which is the same as that shown in fig9 ( b ) is outputted from the anode 19 . the others of the secondary electrons in region b of the energy distribution shown in fig8 that is , backscattered electrons proceed toward the photocathode and are reflected or absorbed by the bottom 36 of the shield electrode 26 as shown by orbits l4 and l5 in fig1 , or are reflected or absorbed by the inside surface of the shield electrode 26 to be captured by the shield electrode even though having passed through the lower opening 29 of the shield electrode as shown by orbits l1 and l2 in fig1 . even if the backscattered electrons return to the first dynode 7 , a residual pulse current resulting from the backscattered electrons is not practically measured as described above . among the secondary electrons , elastically reflected electrons tend to return to the vicinity of the photocathode 4 but are reflected or absorbed by the bottom 36 of the shield electrode 26 as shown by the orbits l4 and l5 as well as the backscattered electrons , not to return to the first dynode 7 , or the elastically reflected electrons pass through the lower opening 29 of the shield electrode as shown by the orbits l1 and l2 in fig1 and are thereafter reflected or absorbed by the inside surface of the shield electrode 26 not to reach the first dynode 7 again . even if the elastically reflected electrons pass through the lower and upper openings 29 and 28 of the shield electrode 26 and return to the vicinity of the photocathode as shown by an orbit l3 in fig1 , the electrons cannot reach the first dynode 7 through the upper opening 28 of the shield electrode 26 again . if the elastically reflected electrons reach the first dynode 7 again , a residual pulse current ap 2 would be outputted from the anode 19 as shown in fig9 ( c ). however , in this invention , the shield electrode reflects or absorbs the elastically reflected electrons to capture them , or to make it impossible for the elastically reflected electrons to reach the first dynode 7 again even though returning to the vicinity of the photocathode 4 , so that the probability that the residual pulse current ap 2 is outputted from the anode 19 is very low . since the potential on the shield electrode 26 is kept higher than that on the photocathode 4 , the probability that the secondary electrons are absorbed by the shield electrode is heightened . the output pulse signal from the photomultiplier 1 , which is the pulse current , is amplified by the amplifier 65 and then supplied to the constant fraction discriminator 66 which removes the pulse current of a noise such as a dark current based on a predetermined theshold value lld to detect only a light pulse current , as described above . when the pulse current not lower than the threshold value lld is outputted from the photomultiplier 1 , the constant fraction discriminator 66 supplies the stop siganl stop to the time - to - amplitude converter 60 and then the converter measures the time tt from the generation of the start signal stt to that of the stop signal and supplies the measured time to the computer 58 through the multichannel analyzer 59 . the frequency α of single photon for the time tt is accumulated as photon counting data in the computer 58 , so that when a piece of data , that is , the time tt is supplied from the time - to - amplitude converter 60 to the computer , the frequency of single photon for the time tt is increased by one . the single photon sp is repeatedly irradiated upon the photomultiplier 1 . at every time of the irradiation , the time tt which elapses from the generation of the start signal stt to that of the stop signal stop is measured , and the frequency α of single photon for the time tt is determined by the computer 58 and outputted to the plotter 47 . fig1 shows the result of the measurement of the output from the photomultiplier 1 . it is apparent from fig4 that the full width at half - maximum fwhm2 of the frequency of single photon for the main pulse current mp 2 is about 200 to 300 picoseconds . the residual pulse current ap 2 is detected with its generation probability of 0 . 13 %, in about 8 to 10 nanoseconds after the main pulse current mp 2 is detected . by comparing the result of the measurement of the output from the photomultiplier 1 as shown in fig1 with that of the measurement of the output from the conventional photomultiplier 51 as shown in fig7 it is found out that the full width at half - maximum fwhm2 of the frequency of single photon for the main pulse current mp 2 from the photomultiplier 1 is about a half of that fwhm1 of the frequency of single photon for the main pulse current mp 1 from the conventional photomultiplier 51 , and the detected time from the generation of the main pulse current mp 2 to that of the residual pulse current ap 2 is about a half of that from the generation of the main pulse current mp 1 to that of the residual pulse current ap 1 . the probability of the generation of the residual pulse current ap 2 is about one - thirtieth of that of the residual pulse current ap 1 . the shield electrode 26 having the openings of prescribed sizes according to this invention is provided in such a position in the photomultiplier 1 that the primary electrons emitted from the photocathode 4 and converged by the focusing electrode 25 surely pass through the shield electrode toward the first dynode 7 and the elastically reflected electrons emitted from the first dynode toward the photocathode are captured through reflection or absorption by the shield electrode , so that the probability of the generation of the residual pulse current ap 2 is greatly reduced . further , the potential ( which is kept equal to that on the seventh dynode , for example ) on the shield electrode 26 is higher than that on the photocathode and therefore the electric field intensity near the photocathode is higher than that in the conventional photomultiplier 51 , so that the probability of the absorption of the elastically reflected electrons by the shield electrode 26 is heightened thereby to reduce the probability of the generation of the residual pulse current ap 2 further . as the electric field intensity near the photocathode is made higher , the transit time of the electrons from the photocathode to the first dynode 7 is shortened to about 4 . 5 nanoseconds whereas that of the conventional photomultiplier is about 8 nanoseconds , and therefore the former shortens the transit time of the electrons to nearly a half of the latter . as a result , the detected time from the generation of the main pulse current mp 2 to that of the residual pulse current ap 2 is nearly reduced to a half of that of the conventional photomultiplier 51 . further , the fluctuation of the main pulse current mp 2 with time is reduced so that the full width at half - maximum fwhm2 of the frequency of single photon for the main pulse current is decreased to about a half of that of the conventional photomultiplier 51 . in fig1 , the time convolution of the time characteristic or fluorescent light damping curve cv o ( t &# 39 ;) ( as shown in fig3 ( b )) of actual fluorescent light and the time fluctuation curve h ( t &# 39 ;- t ) of the main and the residual pulse currents mp 2 and ap 2 which are photon counting data obtained through the use of the photomultiplier 1 is shown in the form of fluorescent light damping data cv 2 ( t ). since the time fluctuation represented by the time fluctuation curve h ( t &# 39 ;- t ) and the frequency of single photon for the residual pulse current ap 2 are reduced in the photomultiplier 1 , the fluorescent light damping data cv 2 ( t ) are closer to the actual fluorescent light damping curve cv o ( t ) as shown in fig3 ( b ) than the fluorescent light damping curve cv 1 ( t ) shown in fig7 . according to the shield electrode 26 of this invention , the generation of the residual pulse current ap 2 is effectively suppressed . further , as the potential on the shield electrode 26 is higher than that on the photocathode 4 , the generation of the residual pulse current ap 2 is more effectively suppressed and the time fluctuation of the main pulse current mp 2 is effectively suppressed . as a result , the time point of generation of single photon sp as shown in fig3 ( c ) can be detected with high accuracy . still further , when the photomultiplier 1 is used to measure the actual fluorescent light from a sample , the fluorescent light damping curve cv o ( t ) as shown in fig3 ( b ) can be accurately detected and therefore accurately measure the life of the fluorescent light . fig1 and 15 show partial views of photomultipliers 40 and 44 which are modifications of the photomultiplier 1 . in the photomultiplier 40 as shown in fig1 , a focusing electrode 41 is provided instead of the focusing electrode 25 of the photomultiplier 1 as shown in fig1 . the axial length of the side portion 42 of the focusing electrode 41 is shorter than that of the side portion of the focusing electrode 25 . the bottom 43 of the focusing electrode 42 is located in nearly the same plane as the top 34 of the shield electrode 26 . the same potential of - 2 , 360 v as that on the focusing electrode 25 is applied to the focusing electrode 41 . in the photomultiplier 40 as shown in fig1 , photoelectrons or primary electrons emitted from the photocathode are converged toward the center line of the photomultiplier by the focusing electrode 41 so that the cross section of the orbit of the electrons is more constricted than that in the photomultiplier 1 as shown in fig1 . therefore , the primary electrons emitted from the photocathode 4 of the photomultiplier 40 are caused to more accurately proceed to the first dynode 7 through the upper opening 28 of the shield electrode 26 and the diameter of the effective area of the photocathode 4 , which is 5 mm in the photomultiplier 1 , can be increased to about 7 mm to produce the output signal with higher sensitivity . in the photomultiplier 44 as shown in fig1 , a light - incident plate 45 and a photocathode 46 , which differ in form from the light - incident plate 3 and photocathode 4 of the photomultiplier 1 as shown in fig1 , are provided . the light - incident plate 45 differs in the form of the inside curved surface from the light - incident plate 3 and is larger in the curvature of the inside curved surface than the light - incident plate 3 . the radius of curvature of the inside surface of the light - incident plate 3 is 55 mm , while that of curvature of the inside surface of the light - incident plate 45 is 25 mm . therefore , the curvature of the inside surface of the light - incident plate 45 is about twice as much as that of the inside surface of the light - incident plate 3 . the radius of curvature of the photocathode 46 provided along the inside curved surface of the light - incident plate 45 is set at 25 mm . as a result , the curvature of the photocathode 46 is about twice as much as that of the photocathode 4 . since the curvature of the photocathode 46 is made larger , the cross section of the orbit of photoelectrons or primary electrons emitted from the photocathode is more constricted toward the center line of the photomultiplier 44 to accurately guide the electrons to the upper opening 28 of the shield electrode 26 . as a result , the effective area of the photocathode 46 can be increased to produce the output signal with higher sensitivity . when the apparatus as shown in fig4 is used for each of the photomultipliers 40 and 44 as shown in fig1 and 15 , to perform measurement , photon counting data are obtained with a low probability of generation of a residual pulse current and a small time fluctuation as well as the photomultiplier 1 as shown in fig1 . according to the present invention , a shield electrode having an opening of prescribed size is positioned between a photocathode and a dynode to direct primary electrons from the photocathode to the dynode and capture secondary electrons emitted from the dynode toward the photocathode , so that the probabiltiy of the generation of a residual pulse current is greatly reduced and the accuracy of analysis of photon counting data can be very much heightened . table 1______________________________________ applied voltage applied voltageelectrode ( v ) ( v ) ______________________________________photocathode 4 - 2 , 500 0focusing electrode 25 - 2 , 360 139 . 5shield electrode 26 - 1 , 220 1 , 279 . 1flat plate electrode 6 - 2 , 035 465 . 1first dynode 7 - 2 , 035 465 . 1second dynode 8 - 1 , 895 604 . 6third dynode 9 - 1 , 687 813 . 9fourth dynode 10 - 1 , 570 930 . 2fifth dynode 11 - 1 , 453 1 , 046 . 5sixth dynode 12 - 1 , 337 1 , 162 . 7seventh dynode 13 - 1 , 220 1 , 279 . 1eight dynode 14 - 1 , 105 1 , 395 . 3ninth dynode 15 - 990 1 , 511 . 6tenth dynode 16 - 815 1 , 686 . 0eleventh dynode 17 - 640 1 , 860 . 5twelfth dynode 18 - 290 2 , 209 . 3anode 19 0 2 , 500 . 0______________________________________