fig1 illustrates by way of example a circuit comprising a detector , constituted in this embodiment by two silicon pin photodiodes 1 and 3 , together with other circuit elements forming a complete meter . in this particular embodiment of the invention the other elements comprise a capacitor 4 in series with the diode 1 , a preamplifier 5 which is arranged to receive the combined output of the diodes 1 and 3 , an amplifier 6 , a threshold detector 7 , and a counter and timer 8 , the elements 5 to 8 constituting , in this embodiment , a means for indicating the rate of pulses which exceed a threshold . the counter may be coupled to a processing circuit 9 , which in known manner may produce denoting the dose rate and the total dose and provide an alarm signal if either the dose rate or the dose exceeds a threshold . these signals may control respective sections of a display device 10 ( such as a liquid crystal display ). considering first the diode 1 , which is preferably a silicon photodiode having an intrinsic layer between appropriately doped p and n regions , it will be understood that photon radiation incident on the diode can generate energetic electrons from photoelectric and compton interactions with the silicon crystal lattice . such electrons then lose energy by interaction with the crystal lattice and can generate electron - hole pairs within the intrinsic layer . the electrons thus generated are separated by the inherent field within the device to form a charge pulse which can be amplified by the preamplifier 5 and the amplifier 6 . in this embodiment of the invention , if the charge pulse is in excess of said threshold as determined by the threshold detector 7 , the pulse may be counted . although a device employing only a single pin diode in a self - biassing mode has some utility , it exhibits a response which is substantially non - uniform with variation in radiation energy and it is preferably to provide compensation by means of at least one further diode in order to obtain a combined output which is substantially uniform with variation of radiation energy . for this purpose the diode 1 , which may be a bpx61 pin diode , is provided with means for attenuating incident radiation , preferably constituted by a brass cap , typically of the order of 2 . 5 millimeters thick . the cap is schematically illustrated in fig1 by the dashed line 2 . the provision of such a cap would normally result in a large underestimate for radiations of energy less than , for example , 150 kev . to compensate for this the output from a smaller diode 3 is added to that from the larger diode 1 . the diode 3 , which may be a bpx65 pin diode of which the glass lens is removed to improve the low energy response , may have , for example , approximately 14 % of the effectively sensitive area of the diode 1 . in addition , a capacitor 4 may be disposed electrically in series with the diode 1 so as to reduce the signal amplitude from the diode 1 . in this manner the falling output , from the larger diode , combined with the rising output , from the smaller diode , produces a response which is substantially uniform with respect to radiation energy . for the particular examples of diode as stated above , the capacitor may be a 180 pf capacitor . fig2 illustrates by way of example the physical arrangement of a meter 11 which is electrically arranged according to fig1 . the diodes 1 and 3 may be incorporated into an integrated circuit 12 , which contains all the detection , amplifying and processing stages of the device . the larger of the two diodes is covered by the brass shield 2 . the two diodes should be mounted at least 6 mm apart to reduce shadowing by the brass shield 2 of the unshielded diode 3 from low energy radiation . the surface of the integrated circuit in which the diodes are incorporated faces away from the body of the wearer . the shield may comprise a disc which is mounted centrally over the diode 1 and is of significantly larger area than the active area of the diode 1 . this configuration has in essence the same properties as the shield described with reference to fig1 . the display 10 , is preferably mounted in the top face of the meter for the sake of good visibility . if desired an audible alarm device 13 ( driven by the circuit 9 which is incorporated in the integrated circuit 12 ) may be disposed adjacent the display 10 and may be triggered by the alarm signal . a battery 14 , preferably of sufficient capacity to last at least one year , may be disposed in a recess at the bottom of the meter . a detector as described may readily be arranged to provide a pulse output which at a suitable threshold is substantially independent of photon energy and angle of incidence for photons of energy greater than approximately 40 kev . such a device has a sensitivity suitable for employment of the device in an active personal dosemeter . a detector as described is capable of providing improved energy response and polar response with respect to known energy compensated geiger - muller tubes and offer further the advantages of robustness , consistency , cheapness of ease of manufacture and a more accurate background response , in addition to avoiding any need for high voltage electrical supplies