INT1327/sb AELO78 EXPLOSIVE INITIATOR BACKGROUND OF THE INVENTION [0001] This invention relates to an explosive initiator which lends itself for use in a system in which the initiator can be identified and tracked. [0002] It is important, for security reasons, to be able to control the use of explosive material. As it is relatively hard to ignite an explosive composition without making use of a suitable detonator, substantial attention has been focussed on identifying and controlling the use of individual detonators. [0003] Diverse control techniques have been described in the specifications of international applications Nos. PCT/AU2006/000204, PCT/AU2008/000651, PCT/AU2006/000203, PCT/AU2009/000721, PCT/AU2007/001957, PCT/US2006/018935 and PCT/N02008/000294. These techniques do not necessarily provide a solution for identifying and tracking an individual detonator in a contactless manner. [0004] In one approach an identifier such as a barcode is attached to a detonator. This type of marking is however easily removed or overcome. Also, an electronic detonator typically includes a microprocessor or a similar component which is capable of storing an identity number for the detonator. The identity number does identify the detonator but, usually, a connection must be made to the detonator to read the identity number. An object of the present invention is to provide an explosive initiator which can be identified remotely with relative ease. 6610124 1 (GHMa11ers) P91921.AU INT1327/sb 2 AELO78 SUMMARY OF THE INVENTION [0005] The invention provides an explosive initiator comprising: a housing which is made from a non-electrically conductive material; a thermal time delay component and at least one explosive composition inside the housing; and a radio frequency identification device within or secured to the housing; wherein the time delay component is enclosed in a thermal barrier so that the housing is protected against melting or burning until such time as the at least one explosive composition is initiated. [0006] As used hereinafter the expression "RFID" denotes a radio frequency identification device. This type of device is known in the art and for this reason is not described in detail herein. Embodiments of the invention may use a passive transponder which, in response to an interrogating signal of a particular kind, emits a unique signal, which may be encoded, which determines the identity of the device. [0007] The RFID may be attached to an inner or outer surface of the non-electrically conductive material, i.e. the RFID may be inside or outside the housing. Preferably the RFID is embedded wholly or partially in the non-electrically conductive material. [0008] As used herein "non-electrically conductive" means that the material in question is an electrical insulator and thus does not exhibit a capability of conducting electrical current. By securing the RFID to the non-electrically conductive material the influence of an electromagnetic screening capability, which would be exhibited by a conductive material, may be avoided. [0009] The housing may be of any suitable shape and form and preferably is tubular with an open end and a blind end. 6610124 1 (GHMa11ers) P91921.AU INT1327/sb 3 AELO78 [0010] The housing may include a portion which is made from a metallic material and the time delay component is positioned wholly inside this portion. A remainder of the housing, to which the RFID is secured in the manner described, is then made from a non-electrically conductive material. [0011] A thermal insulator which may form a thermal barrier between the time delay component and at least part of the housing may be made from any appropriate material which is capable of withstanding high temperatures for a period of several seconds. A suitable material is a high alumina ceramic. [0012] The explosive composition can take on any appropriate form and may include a primary explosive, or a secondary explosive, or an appropriate combination of a primary explosive and a secondary explosive. The invention is not limited in this respect. [0013] The time delay component is preferably located at an open end of the tubular housing. An appropriate connector or seal may be attached to this end to allow the time delay component to be ignited using a suitable technique. For example the time delay component may be ignited by means of energy from a shock tube. In one embodiment of the invention the time delay component is ignited by actuating an electrical element which is exposed to the time delay component. Leads to the electrical element may extend through an appropriate seal fixed to the housing. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of an explosive initiator. Figure 2 is cross-sectional view of an explosive initiator according to another embodiment. 6610124 1 (GHMa11ers) P91921.AU INT1327/sb 4 AELO78 Figure 3 is cross-sectional view of an explosive initiator according to yet another embodiment. DETAILED DESCRIPTION [0014] Figure 1 of the accompanying drawings illustrates from one side and in cross-section an explosive initiator 10, also referred to as a detonator, according to one form of the invention. [0015] The initiator includes an elongate tubular housing 12 which has an open end 14 and a blind end 16. [0016] A secondary explosive 20 is packed into the tubular housing, adjacent the blind end 16. This is followed by a quantity of primary explosive 32. The natures of the primary explosive and of the secondary explosive, the quantities thereof placed in the housing and the manner in which such placements are effected, are aspects which are known in the art and, for this reason, are not further described herein. [0017] A time delay element 34 is positioned inside a holder 36 located within the housing 12 adjacent one end of the primary explosive 32. The time delay element is a thermal preparation which, upon ignition at an end 38, burns at a predictable rate with a burn front moving in a longitudinal direction of the element towards an interface 40 between the element and the primary explosive 32. When the burning front reaches the interface the primary explosive is ignited and this is followed by ignition of the secondary explosive. [0018] An RFID 44, of a kind known in the art, is embedded wholly within the material of the housing 12. The RFID is notionally shown only for, in practice, it can take on a plurality of different forms. 6610124 1 (GHMa11ers) P91921.AU INT1327/sb 5 AELO78 [0019] The RFID is used to identify the detonator in a unique manner. The RFID should be associated with the detonator in a way which prevents the RFID from being removed from the detonator while leaving the detonator intact. To this end it is preferred if the RFID is embedded wholly inside the material from which the housing 12 is made. This housing is made from a non-electrically conductive material so that radio signals to, and from, the RFID are not subjected to electromagnetic screening as would be the case if the housing were made from an electrically conductive material. Suitable non-electrically conductive materials for forming the housing are paper, cardboard, non-conductive carbon fibre, ceramic and a suitable plastic or polymeric material. [0020] The delay element 34 may for example have a lead-based composition, be formed from a "green" material or a nano-material such as porous silicon, or be polymer-based. Although composition-dependent, a common characteristic of time delay elements is that when the elements burn, temperatures of over 1000 LC can be generated. This can be over a time period of several seconds, say up to 9 seconds. The materials referred to hereinbefore, from which the housing 12 is made, are not able to withstand such temperatures. Also, when the delay element 34 burns, it generates gases which can influence the rate of flame propagation through the delay element. For a predictable delay time it is important that the gases are contained and do not vent through a containing body. To address these problems, the holder 36 is made from a thermal insulator such as a high alumina ceramic material with a suitable wall thickness 50. [0021] The holder is cup-shaped with a base 54 in which is formed a relatively small, centrally positioned hole 56. An end 58 of the holder, opposing the base, is open. This facilitates the manufacturing process for the material which comprises 6610124 1 (GHMa11ers) P91921.AU INT1327/sb 6 AELO78 the time delay element can be packed into the holder through the open end 58 and compacted as required. After the secondary and primary explosives have been placed in the housing 12, the holder 36 with its contents is placed over the primary explosive 32. An ignition component 60, which is notionally shown only, is engaged with the hole 56 and is sealed thereto using an appropriate sealing technique based, for example, on the use of a laser, a chemical or heat treatment process, a mechanical process, an ultrasonic or ultraviolet bonding technique, or the like. The ignition component 60 can include an electrical element which is heated through the application of a control voltage via electrical leads, or it could comprise an appropriate connector which facilitates connection of the device 10 to an end of a shock tube. Other techniques can be employed if necessary. [0022] The detonator, once installed in a blast hole, is used in a substantially conventional manner in that, when required, the ignition component 60 ignites the time delay element 34 by means of thermal energy or a shock wave which is passed through the opening 56. [0023] The utility of the invention lies in the incorporation of the RFID in the housing. The RFID is engaged with the housing in such a manner that it cannot be detached from the housing without compromising the integrity of the initiator. To enable bidirectional communications to take place in a wireless manner, the RFID is surrounded by non-electrically conductive material. The high temperature, time delay element is enclosed in a thermal barrier (the holder 36) so that the housing 12 is protected against melting or burning until such time as the primary and secondary explosives are initiated. [0024] The RFID 44 means that it is possible to determine the identity of a detonator in a wireless and contactless manner. This feature facilitates inventory control. 6610124 1 (GHMa11ers) P91921.AU INT1327/sb 7 AELO78 Unauthorized use or movement of a detonator can also be detected, with relative ease, through the use of suitably positioned RFID interrogating structures. [0025] Figure 2 shows an initiator 1 0A, according to a variation, wherein an RFID 44 is embedded in a secondary explosive 20. Another possibility is to place the RFID in a primary explosive, or to attach the RFID to an inner wall surface 46 of an initiator housing. Normally the RFID would not be attached to an outer surface 48 of the housing for, conceivably, it could then be detached from the housing. [0026] Figure 3 shows another variation which has a number of similarities to the construction shown in Figure 1 and, for this reason, like reference numerals are used in Figure 3 to designate components which are the same as those shown in Figure 1. [0027] In the Figure 1 construction the housing 12 is integrally formed, for example, from a suitable plastics material. Figure 3 shows an initiating device 1OB which has a housing 12A formed from a tubular, open-ended metallic sleeve 70 e.g. of copper or aluminium, and a second portion 72, with a blind end 16, which is made from a non-electrically conductive material, such as a suitable plastics material. An RFID 44 is embedded in a wall of the portion 72. A time delay element 34 is wholly enclosed in the metallic sleeve and is capped by a cover 74. The metallic sleeve is well able to withstand the high temperatures which are produced during burning of the element 34. Thus the thermal insulating holder 50 is not required. The sleeve 70 is fixed to the component 72 at a junction 76 in a manner which effectively prevents detachment of the sleeve from the plastic component which houses the primary explosive 32 and the secondary explosive 20. [0028] The explosive initiator 1 OB thus has a housing with a two-part construction. One part of the housing is designed to withstand the effects of heat arising upon 6610124 1 (GHMa11ers) P91921.AU INT1327/sb 8 AELO78 initiation of a thermal time delay element while a remainder of the housing, which is non-electrically conductive, is adapted to accommodate the RFID. [0029] In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. [0030] It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 6610124 1 (GHMa11ers) P91921.AU