Abstract:
A radiation detector can perform both rate and dose measurements for personal safety and also to provide measurements that are sufficiently sensitive for security applications. In one embodiment, a radiation detector has a first measurement channel and a second measurement channel, where the second measurement channel can measure radiation at levels that would saturate the first measurement channel. The detector can automatically switch between the high and low sensitivity channels while continuously integrating the measured rates to determine a radiation dose. The detector can also conserve power by automatically shutting off the unused measurement channel and still give a personal safety warning when the radiation rate or the dose reaches respective alarm thresholds.

Description:
BACKGROUND 
   Radiation detectors are needed today for both safety and security applications. For safety applications, a personal radiation detector may be needed to monitor both the rate of radiation exposure and a total dose. For example, a portable rate meter can monitor a subject&#39;s exposure to radiation and provide a warning if the rate of radiation exposure reaches an unacceptable level, and a portable dosimeter can measure the accumulated radiation exposure that a subject receives during a period of time. Traditionally, implementations of dose rate meters and dosimeters have differed significantly. For example, one class of rate meters employs Geiger-Müller tubes to detect or count the rate at which ionizing radiation passes into the Geiger-Müller tube. In contrast, dosimeters can be implemented simply be carrying film in a light-tight package and developing the film to measure the amount of radiation that has penetrated the packaging and exposed the film. 
   Radiation measurements for security purposes confront a different set of demands and concerns. For example, detecting a source of radiation, which may pose a security threat, generally requires the ability to sense relatively low levels of radiation because the device containing radioactive material may be hidden, shielded, or separated from the detector. As a result, the rate of radiation reaching a detector from such security threats may be low when compared to the rates consider unacceptable for personal safety. Radiation detectors for security applications thus may need to be orders of magnitude more sensitive than a personal radiation rate meters suitable for safety applications 
   The differences in requirements for personal radiation rate meters and portable radiation detectors for security have led to adoption of separate standards for such detectors. For example, recent actions to improve homeland security have led to development of ANSI standard N42.32 for alarming personal radiation detectors and ANSI standard N42.33 for portable radiation detectors for security applications. However, in many situations, security personnel or others will want to have a personal radiation dosimeter and a safety alarm in addition to a portable radiation detector for detection of security threats. The need to purchase and carry two types of radiation detectors can be costly and cumbersome. 
   SUMMARY 
   In accordance with an aspect of the invention, a radiation detector can employ multiple measurement channels and perform both rate and dose measurements for personal safety and also to provide radiation rate measurements that are sufficiently sensitive for security applications. In one embodiment, a dual-channel radiation detector has a first channel including a scintillator and a photodiode. The scintillator has a sampling volume that provides photon events at a rate that is high enough for the radiation measurement accuracy needed in security application. The second channel provides a wide measurement range using a sensor that may consist of a photodiode without a scintillator to provide a lower count rate. Accordingly, the second channel can thus measure radiation at levels that would saturate counting if the scintillator were used. The detector can continuously integrate measured radiation rates to determine an accumulated dose while automatically switching between the high sensitivity channel and the wide rate channel as radiation rates change. The detector can then give a personal safety warning when the radiation rate or the dose rises above respective thresholds. Further, the detector can selectively operate only one of the measurement channels at a time to conserve power in a portable or hand held detector. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2  show block diagrams of multi-channel radiation detectors in accordance with embodiments of the invention capable of measuring radiation at levels associated with personal safety alarms and at levels associated with detection of security threats. 
       FIG. 3  shows an expanded view of a portable radiation detector in accordance with an embodiment of the invention. 
   

   Use of the same reference symbols in different figures indicates similar or identical items. 
   DETAILED DESCRIPTION 
   In accordance with an aspect of the invention, a radiation detector can include multiple channels that permit implementation of a dosimeter with alarms for personal safety and a rate meter with alarms and the accuracy needed for security or safety applications. 
     FIG. 1  illustrates a radiation detector  100  in accordance with an exemplary embodiment of the invention including two radiation measurement channels  110  and  120 . Measurement channel  110  is for measurement of radiation at relatively low rates, e.g., about 1 to 4000 μR/h. In contrast, measurement channel  120  is designed to measure radiation at much higher rates, e.g., up to about 8 or 10 R/h. In one specific embodiment of the invention, channel  110  measures radiation rates in the range required for detection and interdiction of radioactive security treats as set forth in IEEE standard N42.32, and channel  120  is for measurements over a wide dynamic range for hazard assessment as set forth in IEEE standard N42.33. Accordingly, one embodiment of detector  100  complies with both IEEE standards N42.32 and N42.33. 
   In the illustrated embodiment, measurement channel  110  includes a high sensitivity sensor  112 , a preamplifier or charge amplifier  114 , a discriminator  116 , and a counter  118 . High sensitivity sensor  112  can be implemented using conventional sensing systems including but not limited to Geiger-Mueller tubes, solid state detectors, or scintillation detectors. Sensor  112  produces an electric charge or signal when radiation penetrates and interacts with sensor  112 . Amplifier  114  then amplifies or converts the charge or signal from sensor  112  to produce an electrical signal having a voltage suitable for operation of discriminator  116  and counter  118 . Discriminator  116  filters the signal from amplifier  114  to reduce or eliminate electronic noise and produces a signal containing pulses that occur when the input signal to discriminator  116  is above a threshold voltage. Counter  118  counts the number of pulses in the signal from discriminator  116 . 
   Measurement channel  110  can be calibrated by exposing sensor  112  to radiation from a radioactive source that is situated to provide a known radiation rate and then determining a conversion factor from the ratio of the known radiation rate to the measured count rate. The product of the count of pulses in counter  118  during a specific time and the conversion factor then indicates a radiation rate without energy correction. Alternatively, an energy corrected radiation rate measurement might be obtained by taking into account the magnitude of each pulse in the output signal from discriminator  116 . 
   Measurement channel  120  can be implemented using measurement technology that is the same as or different from that used in measurement channel  110 , but measurement channel  120  is generally capable of measuring higher radiation rates and a wider measurement range than can be accurately measured with measurement channel  110 . In the embodiment of  FIG. 1 , measurement channel  120  includes of a wide range sensor  122 , a preamplifier or charge amplifier  124 , a discriminator  126 , and a counter  128 . Wide range sensor  122  produces an electrical charge or signal when radiation interacts with sensor  122 , but to provide a wider measurement range, sensor  122  is selected to provide a lower rate of pulses than does high sensitivity sensor  112  when exposed to the same radiation levels. This may be achieved, for example, by using a smaller or less efficient sensing system in sensor  122  than in sensor  112  or using a different type of sensor, e.g., a Geiger-Müller tube in sensor  122  and a scintillator in sensor  112 . 
   Pre-amplifier or charge amplifier  124 , discriminator  126 , and counter  128  in measurement channel  120  operate in substantially the same manner as described above for amplifier  114 , discriminator  116 , and counter  118  in measurement channel  110 , and measurement channel  120  can be calibrated in the same manner as measurement channel  110 . However, since sensor  122  is less sensitive to radiation than is sensor  112 , the count rate of counter  128  will be lower than the count rate for counter  118  for the same radiation rate, and therefore the conversion factor for the count rate from counter  128  will be less than the conversion factor for the count rate from counter  118 . 
   Circuitry in detector  100  can process the count signals from measurement channels  110  and  120  in a user controlled manner to provide radiation rate and dose measurements and safety alarms. In the embodiment of  FIG. 1 , detector  100  includes a control unit  130  that is connected to measurement channels  110  and  120  and to user interface hardware  140 . Control unit  130  can include dedicated circuitry, a general purpose microprocessor or microcontroller, or a combination of the two that implement the desired functions of detector  100 . In particular, control unit  130  can perform functions including: selecting one of measurement channels  110  and  120  for a radiation rate measurement; determining a current radiation rate from the count rate in the selected channel  110  or  120 ; integrating the measured radiation rates over time to determine an accumulated dose; operating user interface hardware  140  to provide measurement results and/or alarms when the radiation rate or dose reach alarm threshold levels, and monitoring user interface hardware  140  to receive user initiated commands. 
   A rate unit  132 , which may be implemented in hardware, software, or a combination of the two, determines radiation rates from the measured count rates from counters  118  and  128  and can select which measurement channel  110  or  120  to use. Channel selection can be based on the counts in one or both of counters  118  and/or  128 . In particular, for low level background radiation, rate unit  132  can periodically read the count from high sensitivity channel  110 , for example, by reading and resetting counter  118  once a second. The ratio of the change in the count since the previous reading and the time since the previous reading indicates a count rate, which count unit  132  can convert to a radiation rate, e.g., through multiplication by the conversion factor found for channel  110  during calibration. For high levels of radiation, counting in measurement channel  110  becomes “saturated” when pulses in the signal to counter  118  consistently overlap each other. A typical minimum pulse width achieved for conventional photodiodes and associated amplification circuits is about 10 μs (FWHM), so that count rates at or above about 100 kcps may be unreliable. When the count rate from counter  118  is above a threshold level, rate unit  132  can use the count from measurement channel  120 , which provides a lower count rate due to the efficiency of sensor  122 . Similarly, detector  100  can automatically switch from measurement channel  120  back to measurement channel  110  when the count rate from measurement channel  120  is less than a threshold level. The automatic change from one measurement channel  110  or  120  to the other  120  or  110  can provide accurate measurements over a wide range and be transparent to a user that sees the rate measurement displayed through user interface hardware  140 . 
   In accordance with a further aspect of the invention, control unit  130  may automatically deactivate the unused measurement unit  110  or  120 . In particular, control unit  130  can turn off power to one measurement unit  110  or  120  to preserve battery power in a portable detector when the other measurement channel  120  or  110  provides the more accurate rate measurement. 
   Control unit  130  in  FIG. 1  also includes a dose unit  134 . Dose unit  134  can determine a radiation dose by integrating the rate measurements from rate unit  132 . Integration can be simply performed by accumulating weighted count rates or radiation rates. For example, if the radiation rate measurements are performed at a constant frequency, the accumulated dose depends on the sum of the measured rates. Alternatively, each measured count rate can be multiplied by a factor that depends on the source measurement channel  110  or  120  and on the time since the last count rate measurement. The accumulated dose value may be stored in non-volatile memory to preserve the accumulated dose value when detector  100  is turned off or the power source for detector  100  is removed. Control unit  130  can direct user interface hardware  140  to produce an audio, visual, or tactile alarm to draw a user&#39;s attention to a high accumulated dose. 
     FIG. 1  as noted above includes two measurement channels  110  and  120  and shared circuitry such as control unit  130  and interface hardware  140 . In other embodiments of the invention, multiple measurement channels can share more or less circuitry. For example, some or all of amplifier  114 , discriminator  116 , and counter  118  may be shared by multiple measurement channels if appropriate multiplexing or selection circuitry is provided. 
     FIG. 2  shows a detector  200  having measurement channels  210  and  220  that contain exemplary radiation sensors. More specifically, the radiation sensor in high sensitivity measurement channel  210  includes a scintillator  211  and a light sensor  212 . Scintillator  211  can made of any material that emits light or electromagnetic radiation of a detectable wavelength in response to ionizing radiation such as alpha rays, beta rays, gamma rays, neutrons, or other emissions from radioactive materials. In one specific embodiment, scintillator  211  is a crystal of cesium iodide (CsI) that is about 3 cm 3 . Light sensor  212 , which can be implemented, for example, using photodiode or a photomultiplier tube, is positioned to sense photon events corresponding to emission of light that ionizing radiation causes when entering scintillator  211 . As a result, sensor  212  produces charge movements that charge amplifier  114  amplifies to produce an electrical signal have a voltage suitable for discriminator  116  and counter  118 . 
   Measurement channel  220  achieves a wider measurement range using a sensor  222  having a much lower rate of capture and detection of radiation. For example, sensor  222  may consist of a photodiode without a scintillator. A photodiode will then produce charge movement each time incident radiation induces the photoelectric effect in the photodiode. A photodiode generally has a much smaller volume/area and lower efficiency for absorbing radiation in comparison to a high-Q scintillator  211 , so that sensor  222 , when implemented using only a photodiode, generates pluses less frequently than scintillator  211  for a fixed radiation level. In an alternative embodiment, measurement channel  220  may employ a scintillator (not shown) that is smaller than scintillator  211 . The remainder of measurement channel  220  operates with control unit  130  and interface hardware  140  as described above with reference to  FIG. 1 . 
   Detectors  100  and  200  are preferably constructed as portable devices that a user can wear or carry.  FIG. 3  illustrates an exploded view of a personal radiation detector  300  in accordance with one embodiment of the invention. Detector  300  has overall dimensions of about 125 mm by 68 mm by 35 mm and includes an external housing, which can be made of durable light-weight material such as plastic. In the illustrated embodiment, the housing of detector  300  includes a removable back  310 , a housing base  312 , a battery cover  314 , and an anti-shock device  340 . A clip  316  on back  310  allows detector  300  to clipped or worn on a user&#39;s apparel. 
   Electrical components of detector  300  include a battery  320 , a main circuit board  322 , a network board  324 , a high sensitivity radiation sensor  326 , a wide range radiation sensor  328 , and a display  330 . Main circuit board  322  can include circuitry such as control unit  130  described above. Display  330 , which is part of the interface hardware, may be an LCD or other display capable of displaying measurement results, and network board  324  can implement a wireless communication protocol such as Bluetooth that permits detector  300  to communicate with a network of other devices. 
   Radiation sensors  326  and  328  of  FIG. 3  can correspond to measurement channels  110  and  120  of  FIG. 1  or measurement channels  210  and  220  of  FIG. 2 . In particular, radiation sensor  326  has high sensitivity for low range measurements and may include a scintillator, a photodiode, and a preamplifier for measuring lower radiation rates. Radiation sensor  328  has lower sensitivity but a wider measurement ranges for sensing higher levels of radiation and may include a photodiode without a scintillator. 
   Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. For example, although the above-described embodiments employ two measurement channels to extend the range for rate and dose measurement, three or more measurement channels could similarly be used to if necessary or desired to improve accuracy and further extend the applicable measurement ranges. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.