Abstract:
A semiconductor radiation sensor ( 100 ), comprising a substrate ( 102 ), a carrier material ( 104 ) mounted to the substrate ( 102 ), and a semiconductor detector ( 106 ) mounted to the carrier material ( 104 ). A radiation sensitive portion of the semiconductor detector ( 106 ) is oriented towards the carrier material ( 104 ) and generally away from the substrate ( 102 ), and the carrier material is adapted to transmit radiation to the radiation sensitive portion of the semiconductor detector ( 106 ). A dosimeter comprising the radiation sensor ( 100 ) and a method of manufacturing the radiation sensor ( 100 ) are also provided.

Description:
RELATED APPLICATION 
       [0001]    This application is based on and claims the benefit of the filing date of AU application no. 2007903003 filed 4 Jun. 2007, the content of which as filed is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a radiation sensor and dosimeter based thereon, of particularly but by no means exclusive application in measuring radiation dose in photon or electron fields such as for radiation medicine, including radiotherapy and radiation based diagnosis. 
       BACKGROUND OF THE INVENTION 
       [0003]    Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and diode sensors have been used for radiation dosimetry in radiation therapy [1] and other applications such as space and individual dosimetry. A MOSFET operates by trapping a positive charge in the gate oxide proportional to the absorbed dose deposited in a gate. The accumulating charge changes the threshold voltage of the MOSFET sensor measured under constant current [2]. The advantage of MOSFET sensors is in their thin sensitive volume-gate oxide (typically of less than 1 micron); this allows the measurement of dose patterns with high spatial resolution, which is important in Intensity Modulated Radiation Therapy (IMRT) and conformal therapy and brachytherapy. Another advantage of MOSFET detectors is their ability to accumulate dose during irradiation and retain dose information after multiple readouts, which is not possible with Thermo-Luminescent Dosimeter (TLD) detectors. 
         [0004]    Silicon diodes for radiation dosimetry usually operate in a passive mode. The internal electric field produced by the p-n junction of the diode collects charge induced by radiation within the diffusion length in a base of the diode; this provides a short circuit current proportional to the radiation dose rate. Integration of this current provides a measure of the total dose. Usually the sensitive region of the diode is of the order of 50 microns. The simplicity and robustness of these diodes make them attractive for radiotherapy and in vivo real time dosimetry. In many applications diodes are used for entrance and exit dose measurements by placing a diode on a patient during irradiation. Both of these doses are associated with measurements at the water equivalent depth (WED) D max  where charged particle equilibrium is achieved; this depth can be varied from 1.5 to 4 cm by changing the X-ray photon energy of 6 to 18 MV. For miniaturization of the diode sensor&#39;s build-up, different materials are used, such as Al or Cu for 1 to 4 MV photons, brass for 4 to 12 MV photons and Ti for 12 to 25 MV photons [3]. 
         [0005]    Various MOSFET dosimetry systems have been developed over the last 10 years. Important MOSFET parameters for radiation therapy dosimetry include the anisotropy of the response, insensitivity of the response to changes in ambient temperature and the ability to measure skin dose and depth dose. Dual MOSFET sensors have been proposed, to compensate for the temperature dependence of the threshold voltage; the gate voltages of an irradiated MOSFET sensor and a control MOSFET sensor with the same temperature coefficient are compared using a comparative circuit [4]. 
         [0006]    Another dosimetry probe comprises dual MOSFETs produced on a single substrate (so that they are essentially identical). During irradiation, the gates of the paired MOSFETs are biased with different positive voltages leading to different responses; a differential signal in test mode is then proportional to absorbed dose, while compensating for the temperature instability of the MOSFET [5]. 
         [0007]    In many radiation MOSFET probes including dual MOSFET probes, the MOSFET die—which is usually of the order of 1×1 mm and 0.35 to 0.5 mm thick—is placed on one end of a tail of Kapton brand polymer that has embedded copper leads whose ends act as connecting pads to which the substrate, source drain and gate of each MOSFET are wire bonded (with aluminium or gold wires). The other ends of the copper leads are connected to the socket plugged into the data reader. An epoxy envelope is applied to fix the bonding wires and to protect the MOSFET die from environmental conditions. The copper leads and epoxy result in additional response anisotropy especially with lower energy photons such as those used in HDR brachytherapy (where an Ir-192 source is used with an average photon energy of 360 keV) and in diagnostic techniques. This packaging has the additional problem that the shape of the epoxy is not reproducible, which results in a different build-up for each probe and differences in WED of about 0.7 to 1 mm [14]. 
         [0008]    MOSFET sensors for radiation therapy applications, whether passive or active, are presently provided as either disposable (“OneDose” [7]), multiuse (Thompson Nelson MOSFET system) or wirelessly powered and implantable MOSFETs for internal use; all have an epoxy envelope or other cover build up. This is less problematic for internal use when dose measurements are made inside the body (with an implantable wireless MOSFET [8]), where charged particle equilibrium exists and MOSFET response is driven mostly by high energy secondary electrons generated from surrounding tissue rather then from the epoxy. However, the epoxy and the current packaging makes it impossible to realize the full advantages of the micron thickness of the gate oxide of MOSFET dosimeters when used for skin dosimetry or inside anatomical cavities where dosimetry at the tissue-air interface is critical for Dose Planning System (DPS) verification. Comparable problems apply to diode sensors for such applications. 
         [0009]    One technique [9] for improving the anisotropy of MOSFET detector response places the MOSFET die on the surface of a Kapton tail with the rear of the die and wire bonded to the top side (essentially as described above), but with a dummy Si die of the same thickness as the MOSFET die adjacent to the surface of the MOSFET die so as to overlay an active region of the MOSFET surface gate and surrounding area; an epoxy envelope is again used to provide mechanical fixation and protection from the environment. Sandwiching the sensitive dosimetric micron surface layer (i.e. the MOSFET gate or diode p-n junction) between two Si bulk layers makes the radiation path more isotropic. A similar approach [10] has also been suggested for edge-on MOSFET detectors for the measuring the dose deposited by narrow micron synchrotron X-ray micro-beams in Microbeam Radiation Therapy (MRT), to make scattering conditions uniform when scanning the edge-on MOSFET across the microbeam. However, this approach does not solve the problems of skin dosimetry with the MOSFET detector or diode, increasing the WED of dose measurements and leaving the problem of large WED and poor WED reproducibility due to the epoxy. 
         [0010]    Accurate measurement of the skin dose is important in X-ray MV therapy and radiation diagnosis owing to radiation damage of basal layer of epidermis at the depth of 70 to 200 microns. It can be important to determine skin dose due to electron contamination from photon interaction in the air column between the accelerator and patient. The skin dose depends on the incident angle of the beam on the patient, the curvature of the surface of the patient; it increases with increasing beam angle of incidence and beam size, especially with tangential radiation beams in breast cancer treatment. Owing to the absence of a charged particle equilibrium on a surface of the body, the dose gradient is steep which leads to error in skin dosimetry if the WED of the MOSFET measurements is high and a loss of irreproducible within a batch of MOSFETs. Skin dose control in real time is important for avoiding radiation burns, which can lead to serious complications especially during the treatment of breast cancer. 
         [0011]    A MOSFET has been reported [11, 14] having a round epoxy envelope as described above and a WED of 1.8 mm for measurements with a 6 MeV X-ray beam with a field size of 10×10 cm. Manual attempts to partially remove the epoxy led to a spreading of the WED in a range 0.04 to 0.15 mm in the same batch of MOSFETs [12], leading to an unacceptable lack of WED reproducibility for skin dosimetry. 
         [0012]    Improved anisotropy of the response of the MOSFET with packaging of the type described in reference [8] nonetheless does not provide correct skin dosimetry in radiation therapy. 
         [0013]    It has been demonstrated [13] that using a bare (unpackaged) MOSFET allows accurate measurements of dose on the surface on a phantom; this exploits the advantages of the thin dosimetric layer (viz. gate oxide) but it is impractical to leave the gate of the MOSFET unprotected against moisture and mechanical damage. 
         [0014]    Temperature stabilization of the sensor response is commonly achieved in the background art by using dual MOSFET sensors and dual bias supplies for differentially biasing the gates of the sensors during irradiation. This technique is employed in [5]. 
         [0015]    Alternatively, a thermo-stable point on the current-voltage characteristic of the MOSFET may be identified maintained during measurements. However, the readout current corresponding to the thermo-stable point of the current-voltage characteristic is unique for any particular MOSFET and can vary from one batch to another [14]. 
       SUMMARY OF THE INVENTION 
       [0016]    According to a first broad aspect, the present invention provides a semiconductor radiation sensor, comprising:
       a substrate;   a carrier material mounted to the substrate; and   a semiconductor detector mounted to the carrier material;   wherein a radiation sensitive portion of the semiconductor detector is oriented towards the carrier material and generally away from the substrate, and the carrier material is adapted to transmit radiation to the radiation sensitive portion of the semiconductor detector.       
 
         [0021]    Thus, the carrier material covers and protects the radiation sensitive portion of the semiconductor detector. 
         [0022]    The sensor will usually be provided with electrical connectors coupled to the semiconductor detector. 
         [0023]    Sensors according to this aspect can be small in size and hence may be located in, for example, narrow catheters or body cavities for monitoring radiation dose or dose rate in vivo in real time, including during a medical procedure. A physically or wirelessly connected electronic reader may be employed as a data collector to collect the data. 
         [0024]    The carrier material preferably comprises a flexible polymeric material, such as a polyamide. 
         [0025]    The substrate may include an aperture (or a recess) for receiving a rear portion of the semiconductor detector. The rear portion may be covered with an epoxy material. 
         [0026]    The carrier material and semiconductor detector may be coated at least partially with a polyamide or other polymeric material. 
         [0027]    The semiconductor detector may comprise a planar diode or MOSFET die. 
         [0028]    According to this aspect, there is also provided a radiation sensor, comprising:
       a carrier material mountable to a substrate; and   a semiconductor detector mounted to the carrier material;   wherein a radiation sensitive portion of the semiconductor detector is oriented towards the carrier material and generally away from the substrate, and the carrier material is adapted to transmit radiation to the radiation sensitive portion of the semiconductor detector.       
 
         [0032]    According to another aspect of the invention, there is provided a dosimeter comprising a radiation sensor as described above. 
         [0033]    According to still another aspect of the invention, there is provided a method of manufacturing a radiation sensor, comprising:
       mounting a semiconductor detector to a carrier material with a radiation sensitive portion of the semiconductor detector oriented towards the carrier material, said carrier material being adapted to transmit radiation to the radiation sensitive portion of the semiconductor detector; and   mounting the carrier material on a substrate with the radiation sensitive portion of the semiconductor detector directed generally away from the substrate.       
 
         [0036]    According to yet another aspect of the invention, there is provided a method for thermo-stabilizing a semiconductor detector (such as a MOSFET or diode sensor), comprising correcting threshold voltage drift arising owing to temperature changes according to the forward drop voltage of the source-substrate p-n junction. 
         [0037]    In one embodiment, the method comprises biasing a substrate of the semiconductor detector relative to the source to provide a substantially constant current for the p-n junction;
       scaling and sampling the change in the voltage drop across the source-substrate p-n junction (such as by scaling the change in the voltage drop across the source-substrate p-n junction, then sampling the scaled change in the voltage drop); and   subtracting the scaled change in the voltage drop from the threshold voltage during each readout cycle.       
 
         [0040]    The method may further comprise converting the measured change in threshold voltage to dose using a predetermined calibration (such as a calibration curve) or a look-up table. 
         [0041]    In another embodiment, the method comprises:
       selecting a forward current through the forward biased p-n junction such that the change in the voltage drop across the source-substrate p-n junction with changing temperature may be used to control the threshold voltage though the substrate of the semiconductor detector.       
 
         [0043]    As will be understood by those skilled in the art, each of the optional features of the above aspects of the invention may be employed where suitable in any combination with the other optional features and with any other aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0044]    In order that the invention may be more clearly ascertained, embodiments will now be described, by way of example, with reference to the accompanying drawing, in which: 
           [0045]      FIG. 1  is a plan schematic view of a radiation sensor according to an embodiment of the present invention; 
           [0046]      FIG. 2  is an elevational schematic view of the radiation sensor of  FIG. 1 ; 
           [0047]      FIG. 3A  is a plan view of the aluminium connecting strips of the radiation sensor of  FIG. 1 , in use located above the polyamide flexible carrier of the radiation sensor; 
           [0048]      FIG. 3B  is an side view of the aluminium connecting strips of  FIG. 3A ; 
           [0049]      FIG. 4  is a schematic view of the MOSFET die of the radiation sensor of  FIG. 1 , with contact pads corresponding to source, gate, drain and substrate; 
           [0050]      FIG. 5  is a schematic view of a dual-MOSFET MOSFET die adapted for use in a radiation sensor according to another embodiment of the present invention; 
           [0051]      FIG. 6  plots skin dose measured i) with the radiation sensor of  FIG. 1 , ii) with a radiation sensor of the background art, and iii) with an ATTIX plane parallel ionization chamber under the same conditions for different incident angles and 6 MeV radiation from a LINAC; 
           [0052]      FIG. 7  is a plot of the response of a OneDose brand single use MOSFET sensor with epoxy bubble of the background art located (a) on the surface of, and (b) at a depth of 5 mm in, a solid water phantom, superimposed on the response of an ATTIX chamber under the same conditions of normal incidence of the 6 MV X-ray beam; 
           [0053]      FIG. 8  plots measured surface dose as a percentage of depth dose at D max  for different field size obtained i) with the radiation sensor of  FIG. 1 , ii) with a radiation sensor of the background art containing epoxy bubble above sensitive volume of the detector, and iii) with an ATTIX plane parallel ionization chamber under the same conditions of the normal incidence of 6MV X-ray beam; 
           [0054]      FIG. 9  is a plot of the angular response of the radiation sensor of  FIG. 1  in a solid water phantom at a distance of 18 mm from an Ir-192 source; 
           [0055]      FIG. 10  is a diagram of a MOSFET threshold voltage thereto-stabilization circuit according to the embodiment of  FIG. 1 ; and 
           [0056]      FIG. 11  is a plot of the relationship between threshold voltage V th  (mV) and temperature T (° C.) in a p-MOSFET sensor, both with and without thermo-stabilization according to the embodiment of  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0057]      FIGS. 1 and 2  are plan and elevational schematic views respectively of a radiation sensor  100  according to an embodiment of the present invention. The sensor  100  is designed for use in a dosimetry system that is suitable for skin and internal real time in vivo dosimetry in radiation therapy (including HDR brachytherapy), where reproducible skin dose measurements with WED prescribed by ICRU [14] are required, and for temperature independent internal dosimetry with minimal MOSFET probe anisotropy. 
         [0058]    Sensor  100  includes a tissue equivalent (or close to tissue equivalent) substrate  102  (10×2.5 mm and 0.38 mm thick) of—in this embodiment—ceramic material, which provides mechanical rigidity. Sensor  100  also includes a flexible polyamide carrier  104  (of 0.02 mm thickness) located on substrate  102 , and a MOSFET chip or “die”  106  mounted to—but under—carrier  104  (so is shown in ghost in  FIG. 1 ), and protruding into a rectangular aperture  108  in substrate  102 . 
         [0059]    Substrate  102  can, in other embodiments, be of other dimensions as required, including having a length of less than 10 mm where desired. The thickness of substrate  102  is selected to accommodate MOSFET die  106 . A thin layer of epoxy  202  is used to cover the rear of MOSFET die  106  and fix the MOSFET die in aperture  108 . As discussed above, the active area of the MOSFET die (viz. the gate oxide) faces upwards but is covered, and hence protected, by a thin layer of polyamide film in the form of a portion of carrier  104 . 
         [0060]    The carrier  104  is attached to the substrate  102  by ultrasonic welding at three spots  110  adjacent to aperture  108  (to pre-deposited aluminium pads), so that MOSFET die  106  remains securely located relative to the substrate  102 . Aperture  108  is sized to accommodate the rear of the MOSFET die; the die has dimensions of 0.8×0.6 mm and a thickness (i.e. in the vertical direction in the view of  FIG. 2 ) of between 150 and 350 μm. In some other embodiments, however, the die is of greater thickness, in some cases up to 500 μm with substrate  102  having a comparable thickness. 
         [0061]    In another embodiment, a planar diode is employed rather than the MOSFET die  106 . In some embodiments, the substrate  102  may instead be of Kapton brand polymer. 
         [0062]    The sensor  100  includes four thin elongate aluminium connecting strips (shown schematically at  112 ) above carrier  104  that are electrically coupled to the MOSFET die  106 ; carrier  104  is thus between substrate  102  and aluminium strips  112 . These aluminium strips  112  provide the required electrical connection to the MOSFET die  106 , as is described in greater detail below. Carrier  104  thus acts both as a protective tissue equivalent layer of fixed and reproducible thickness above the MOSFET gate, and as a carrier of the aluminium strips  112 . 
         [0063]    If a dual MOSFET is employed, sensor  100  will include additional aluminium connecting strips (cf.  FIG. 5 ). 
         [0064]      FIGS. 3A and 3B  are plan and elevational views of aluminium strips  112 , identified separately at  302 ,  304 ,  306 ,  308 . These strips have a thickness of 0.02 mm. Shown in ghost at  310  is the front of MOSFET die  106  (located, in sensor  100 , below the aluminium strips  302 ,  304 ,  306 ,  308  and separated therefrom by carrier  104 ), including its four aluminium contact pads.  FIG. 4  is an enlarged view of the front  400  of MOSFET die  106  including these contact pads  402 , including substrate, source, gate and drain pads. In embodiments that include a planar diode rather than a MOSFET die, electrical contact is made with the cathode and anode of the diode die. 
         [0065]    Carrier  104  includes four apertures (not shown) that coincide with the positions of the contact pads  402  so that electrical contact can be made between the aluminium strips and the contact pads  402  of MOSFET die  106 . 
         [0066]    Aluminium strips  302 ,  304 ,  306 ,  308  may be attached to the contact pads  402  of the MOSFET die  106  by ultrasound welding. The aluminium strips  302 ,  304 ,  306 ,  308 , however, do not cover the sensitive area of the MOSFET gate, which avoids dose enhancement effects from electron scattered from the aluminium. 
         [0067]    The ends  312  of the aluminium strips remote from the MOSFET die  106  are provided with soldering balls  312  located in a mounting zone  314  of each strip, and are connected (by ultrasound welding) to connecting copper wires or strips  114  deposited on—or alternatively embedded in—substrate  102 ; copper strips  114  are connected either to a plug (in the case of a disposable MOSFET) or a ribbon cable (in the case of multi-use MOSFET). This embodiment has the latter: sensor  100  includes a ribbon cable  116 , which is connected to copper strips  114 . 
         [0068]    This arrangement provides a flat reproducible mount and protects the sensor  100  from environmental hazards. The thickness of the mount can be adjusted to reproduce a required WED by laminating the sensor with an additional polyamide of desired thickness; such lamination also provides a water- or moisture-proof envelope. The result is a sensor that has the form of a planar strip with a width of 1.8 to 2.5 mm, a thickness of 0.4 mm (though in some variants of this embodiment up to 0.5 mm) and a length of 10 mm (though this can be varied as desired, and it is envisaged that the sensor will commonly be shorter). 
         [0069]    In another embodiment, the sensor includes a MOSFET die with two MOSFETs, each with a different gate oxide thickness and hence sensitivity.  FIG. 5  illustrates such a MOSFET die schematically at  500 . MOSFET die  500  includes a substrate  502 , a first transistor  504   a  with a relatively thin oxide layer and a second transistor  504   b  with a relatively thick oxide layer. The die  500  includes aluminium substrate, source, gate and drain connection pads  506   a ,  508   a ,  510   a ,  512   a  for first transistor  504   a  and aluminium substrate, source, gate and drain connection pads  506   b ,  508   b ,  510   b ,  512   b  for second transistor  504   b.    
         [0070]    The die has a size of 0.55×1.7 mm and a thickness comparable to that of MOSFET die  106  of  FIGS. 1 and 2  (which is essentially that half of die  500  with second transistor  504   b ). 
       EXAMPLES 
       [0071]      FIG. 6  is a plot showing the angular dependence of the measured surface dose as a percentage of depth dose at D max  with two MOSFET sensors, one according to the embodiment of  FIGS. 1 to 4  (termed “MOSKIN” (trade mark)) and a “RADFET” MOSFET sensor of the background art (with a covering epoxy envelope), with a radiation field of 20×20 cm 2  from a 6 MV LINAC and SSD=100 cm. The plot compares these data with results from an ATTIX plane parallel ionization chamber (the “gold standard” for such determinations), which measures surface dose. It is apparent that the MOSKIN response is close to the ATTIX ionizing chamber over a wide angular range of the incident beam on the surface of the solid water phantom, bearing in mind that the WED in the ATTIX measurements was less than that of the MOSKIN measurements. The WED of the measurements with this particular MOSKIN sensor was 0.12 mm and for the RADFET sensor with epoxy bubble about 1 mm. 
         [0072]      FIG. 7  is a plot of the response of a OneDose brand single use MOSFET sensor with epoxy bubble placed on the surface and at a depth of 5 cm in a solid water phantom, superimposed on the response of an ATTIX chamber under the same conditions (i.e. 10×10 cm 2  radiation field, 6 MV LINAC and SSD=100 cm). The OneDose brand sensor—when placed on the surface—measures dose at a depth of 1.2 mm (data point (a)), that is, 10 times deeper than depth of clinical interest; the same 1.2 mm depth offset is observed at a depth of 5 cm (data point (b)). 
         [0073]      FIG. 8  is a plot of skin dose measured by MOSKIN, RADFET with epoxy bubble and ATTIX ionizing chamber for field sizes from 5 to 40 cm 2  at normal beam incidence. It is apparent that the MOSKIN sensor provides reliable skin dose measurements at a depth of 0.12 mm for all radiation fields used in radiation therapy, while the epoxy covered RADFET sensor overestimates the skin dose by as much as a factor of two or more. 
         [0074]    The MOSKIN sensor also has more uniform angular response, which is especially important for lower energy photons as employed in HDR brachytherapy (the average energy of an Ir-192 source being about 360 keV):  FIG. 9  is a plot of the angular response of the MOSKIN sensor in a solid water phantom at a distance of 18 mm from an Ir-192 source. The MOSKIN sensor exhibits an anisotropy of generally within ±2%. 
       Temperature Stabilization of Sensor Response 
       [0075]    According to the present invention, thermo-stabilization is achieved using the source-substrate p-n junction of the same MOSFET sensor during readout mode of the threshold voltage on the MOSFET sensor. 
         [0076]      FIG. 10  is a diagram of a MOSFET threshold voltage thermo-stabilization circuit  1000  according to this embodiment. The circuit  1000  operates by using the forward drop voltage of the source-substrate p-n junction, which is a function of the temperature inside the MOSFET die  106 , to correct the V th  drift arising from temperature changes. To do so, the substrate  1002  of the MOSFET die  106  of the sensor  100  is biased through a resistor R with a potential of −V st  relative to the source  1004 , which is under virtually zero bias, to provide constant current for the p-n junction. The change in the voltage drop across the source-substrate p-n junction is scaled, sampled and subtracted from the threshold voltage V th  during each readout cycle in a microprocessor controlled reader  1006 , which also converts the measured V th  to dose and wirelessly transmits the dose to a data collection computer  1008 . 
         [0077]    Alternatively, if the proper forward current through the forward biased p-n junction is selected, the change in the voltage drop across the source-substrate p-n junction with changing temperature may be used to control V th  though the substrate  1002 . In this case a feedback loop is created inside the MOSFET die  106  so the Scaler  1010  and the Sample &amp; Hold module  1012  of the circuit of  FIG. 10  are not required. 
         [0078]      FIG. 11  is a plot of the relationship between threshold voltage V th  (mV) and temperature T (° C.) in a p-MOSFET sensor, for a read-out current of 150 μA, without thermo-stabilization (crosses) and with thermo-stabilization provided with the circuit of  FIG. 10  (squares). Threshold voltage V th  changes negligibly between 30° C. and 50° C. 
         [0079]    The advantage of this approach is its simplicity, due to the direct probing of the temperature in the MOSFET die  106  close to the gate  1014 . This makes it applicable to essentially any MOSFET sensor. 
       CONCLUSION 
       [0080]    Thus, the sensor  100  has the following key features and advantages: 
         [0081]    i) Sensor  100  employs a “drop in” packaging with electrical connections and mechanical fixation of MOSFET die  106  provided from the top side of the die; 
         [0082]    ii) Aluminium strips  302 ,  304 ,  306 ,  308  connect the aluminium substrate, source, gate and drain pads of MOSFET die  106  through holes in flexible polyamide carrier  104 ; 
         [0083]    iii) Polyamide carrier  104  protects the top side of the MOSFET die (and hence the sensitive element, the gate, of the MOSFET) and simultaneously provides a reproducible mount thickness with a WED of 0.12 mm (corresponding to the skin dose where basal layer of epidermis is situated); 
         [0084]    iv) Substrate  102  (of Kapton or ceramic) has an aperture into which the MOSFET die is “dropped in” and connecting copper strips for connection to the end of the aluminium strips; 
         [0085]    v) A laminate of polyamide or other TE material is used to provide a final adjustment of the WED and to render the sensor waterproof or resistant; 
         [0086]    vi) The source-substrate p-n junction of the MOSFET die  106  is used for thermo-stabilization of the threshold voltage V th  to avoid error in dosimetry associated with temperature changes; and 
         [0087]    vi) A wireless reader  1006  converts the measured V th  to the dose. 
       REFERENCES 
       [0000]    
       
         [1]. A. B. Rosenfeld (invited paper), “MOSFET dosimetry in modern radiation oncology modalities”, Rad. Prot. Dosim, 101(1) (2002) 393-398. 
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         [7]. R. A. Kinhikar et al., “Dosimetric evaluation of a new OneDose MOSFET for Ir-192 energy”, Phys. Med. Biol., 51 (2006) 1261-1268. 
         [8]. U.S. Patent Application Publication No. 2005/0010110 (Black et al.), published 13 Jan. 2005. 
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         [12]. Scalchi et al., “Characterization of a new MOSFET detector configuration for in vivo skin dosimetry” Med. Phys., 32(6) (2005) 1571-1578. 
         [13]. M. Butson, A. Rozenfeld, J. Mathur, M. Carolan, T. Wong, P. Metcalfe, “A new radiotherapy surface dose detector: the MOSFET”, Med. Phys. 23(5) (1996) 655-658. 
         [14]. M. G. Buehler, B. R. Blaes, G. A. Soli, G. R. Tradio, “On-chip p-MOSFET dosimetry”, IEEE Trans. On Nucl. Sci., 40(6) (1993) 14442-14449. 
       
     
         [0102]    Modifications within the scope of the invention may be readily effected by those skilled in the art. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove. 
         [0103]    In the claims that follow and in the preceding description of the invention, except where the context requires otherwise owing to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, that is, 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. 
         [0104]    Further, any reference herein to prior art is not intended to imply that such prior art forms or formed a part of the common general knowledge in Australia or any other country.