WO2007070541A2

WO2007070541A2 - Radiation detection with change of resonant frequency

Info

Publication number: WO2007070541A2
Application number: PCT/US2006/047452
Authority: WO
Inventors: Babak Ziaie; Chulwoo Son
Original assignee: Regents Of The University Of Minnesota
Priority date: 2005-12-14
Filing date: 2006-12-13
Publication date: 2007-06-21
Also published as: WO2007070541A3

Abstract

An apparatus (100, 106, 144) includes a passive resonant circuit (102) having a resonant frequency that changes as ionizing radiation (104) is received. A method includes charging one plate (114) of a capacitor (110) in a resonant circuit (102) having a resonant frequency, receiving ionizing radiation (104) at the capacitor (110) and detecting a change in the resonant frequency.

Description

APPARATUS AND METHOD FOR RADIATION DETECTION

[0001] The U.S. Government has a paid-up license in the disclosure and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Grant No. ECS-0400637 awarded by NSF.

Field [0002] This disclosure relates to detection of ionizing radiation.

Background [0003] Active radiation detectors, which include active devices such as transistors and diodes, are used to detect and measure ionizing radiation. Exemplary systems in which active radiation detectors are used include medical systems, such as cancer treatments systems, and high-energy experimental physics systems, such as particle accelerators. Unfortunately, radiation detectors that utilize active devices may be damaged when used in these environments. The ionizing radiation being detected can destroy the active devices. As active devices become smaller, which is the trend in the industry, this problem becomes more acute. Further, active radiation detectors are expensive and difficult to manufacture.

Brief Description of the Drawings [0004] Figure IA is an illustration of an apparatus including a passive resonant circuit in accordance with some embodiments receiving radiation from an ionizing radiation source. [0005] Figure IB illustrates an apparatus including an embodiment of the passive resonant circuit, shown in Figure IA, including an inductor and a capacitor, including an electret, an ionization chamber, and a plate in accordance with some embodiments. [0006] Figure 1C illustrates a fabrication process suitable for use in connection with the fabrication of the electret, shown in Figure IB, in accordance with some embodiments. [0007] Figure ID illustrates a fabrication process suitable for use in connection with fabrication of the plate, shown in Figure IB, in accordance with some embodiments. [0008] Figure IE illustrates forming the capacitor, shown in Figure IB, by coupling the electret, as shown and fabricated in Figure 1C, and the plate, as shown and fabricated in Figure ID, through spacers in accordance with some embodiments.

[0009] Figure IF is an illustration of an apparatus including an embodiment of the passive resonant circuit, shown in Figure IA, including a foam dielectric in accordance with some embodiments. [0010] Figure IG is an illustration of the results of a simulation of one embodiment of the plate, as shown in Figure IB.

[0011] Figure IH is an illustration of a graph of surface charge density versus deflection for a membrane, shown in Figure IG, in accordance with some embodiments. [0012] Figure 2 is a flow diagram of a method for detecting ionizing radiation in accordance with some embodiments.

[0013] Figure 3 is a flow diagram of a method for determining the amount of radiation received by a tumor in accordance with some embodiments. [0014] Figure 4 is a flow diagram of a method for determining the amount of radiation delivered by a beam of ionizing radiation in accordance with some embodiments.

[0015] Figure 5 is a flow diagram of a method of determining the amount of radiation received from a source of ionizing radiation in accordance with some embodiments. [0016] Figure 6A is an illustration of an apparatus including the passive resonant circuit, shown in Figure IA, and an interrogating radiation source in accordance with some embodiments.

[0017] Figure 6B is an illustration of an apparatus including the passive resonant circuit, shown in Figure IA, implanted in a human body and an interrogating radiation source in accordance with some embodiments. [0018] Figure 6C is an illustration of an apparatus including the passive resonant circuit, shown in Figure IA, inserted in a particle accelerator in accordance with some embodiments.

Description

[0019] Figure IA is an illustration of an apparatus 100 including a passive resonant circuit 102 in accordance with some embodiments receiving radiation from an ionizing radiation source 104. The passive resonant circuit 102 includes passive electronic components, such as resistors, capacitors, and inductors. The passive resonant circuit 102 does not include active electronic components, such as transistors or diodes. The passive resonant circuit has a resonant frequency that changes in response to ionizing radiation. The resonant frequency of the passive resonant circuit 102 is the natural frequency of oscillation of the passive resonant circuit 102. A resonant circuit, such as the passive resonant circuit 102, absorbs more energy at the resonant frequency than at other frequencies. Ionizing radiation is high-energy radiation capable of producing ionization in substances, such as gases and foams, through which it passes. Exemplary types of ionizing radiation suitable for use in connection with the passive resonant circuit 102 include nonparticulate radiation, such as x-rays, and radiation produced by energetic charged particles, such as alpha and beta rays, and by neutrons, as from a nuclear reaction. In operation, the passive resonant circuit 102 receives ionizing radiation from a source of ionizing radiation, such as the ionizing radiation source 104. Exemplary sources of ionizing radiation include accelerators, such as particle accelerators used in high energy physics, and medical systems, such as cancer treatment systems. In response to the ionizing radiation received from the ionizing radiation source 104, the resonant frequency of the passive resonant circuit 102 changes. The change in resonant frequency of the passive resonant circuit 102 is used to determine the amount of ionizing radiation received at the passive resonant circuit 102. Further description of the response of the passive resonant circuit 102 to ionizing radiation is provided below.

[0020] Figure IB is an illustration of an apparatus 106 showing one embodiment of the passive resonant circuit 102, shown in Figure IA, including an inductor 108 coupled to a capacitor 110 having an electret 112 and a plate 114 in accordance with some embodiments. The electret 112 is a dielectric that exhibits persistent dielectric polarization. In some embodiments, the electret 112 has a dielectric constant of about 2.1 when measured at one megahertz. Exemplary materials suitable for use in the fabrication of the electret 112 for use in connection with the apparatus 106 include fluorocarbon- based polymers and inorganic oxides, such as silicon oxide and silicon nitride.

PolyTetraFuoroEthylene is one example of a fϊuorocarbon-based polymer suitable for use in fabrication of the electret 112. Teflon^® is one brand of polymer suitable for use in connection with the fabrication of the apparatus 106. The plate 114 is a deflectable structure. In some embodiments, the plate 114 is a membrane, such as a thin layer of silicon coated with a conductor. An air gap or ionization chamber 116 separates the electret 112 from the plate 114. Ionizing radiation entering the air gap or ionization chamber 116 ionizes gases present in the air gap or ionization chamber 116. Increasing the air pressure in the air gap or ionization chamber 116 increases the sensitivity of the apparatus 106. In some embodiments, the apparatus 106 is operated with the air pressure in the air gap or ionization chamber 116 set to about two atmospheres. The air gap or ionization chamber 116 is not limited to being filled with air. Other gases and foams or other materials having voids capable of holding gases are also suitable for filling the air gap or ionization chamber 116. In operation, the electret 112 is charged, for example through a corona discharge. A corona discharge is produced by one of two energized electrodes in a gas having a shape causing the electric field at its surface to be significantly greater than the electric field between the electrodes. Ionizing radiation entering the air gap or ionization chamber 116 after charging of the electret 112 causes ionization of gases in the air gap or ionization chamber 116. Ionization of gases in the air gap or ionization chamber 116 changes the charge on the electret 112. As a result of the change in the charge on the electret 112, the plate 114 deflects, the capacitance of the capacitor 110 changes, and the resonant frequency of the apparatus 106 changes. The apparatus 106 in interrogated, for example by a phase-dip method technique, to detect a change in the resonant frequency of the apparatus 106. In the phase-dip method, an interrogation coil scans a range of frequencies, phase as a function of frequency is observed, and a dip in phase occurs at the resonant frequency of the apparatus 106. The change in resonant frequency is related to the amount of ionizing radiation received by the apparatus 106. [0021] Figure 1C illustrates a fabrication process 116 suitable for use in connection with fabrication of the electret 112, shown in Figure IB, in accordance with some embodiments. As shown in Figure lC-(a), a titanium layer 118 is formed on a substrate 120, such as a glass substrate. An exemplary thickness for the titanium layer 118 is about 200A. A gold layer 122 is formed on the titanium layer 118. An exemplary thickness for the gold layer 122 is about 3000A. E-beam evaporation is one method suitable for use in connection with forming the titanium layer 118 and the gold layer 122 . The titanium layer 118 and the gold layer 122 are patterned. Wet etching is one method suitable for use in connection with patterning the titanium layer 118 and the gold layer 122. The resulting patterned titanium /gold layer 124 is shown in Figure lC-(b). The patterned titanium/gold layer 124 is coated by a fluorocarbon-based polymer 126. Spin-coating is one method suitable for use in connection with forming the fluorocarbon-based polymer 126 on the patterned titanium/gold layer 124. The resulting spincoated titanium/gold layer 128 is shown in Figure lC-(c). To solidify the fluorocarbon-based polymer 126, the substrate is baked at about 330 degrees Centigrade for about fifteen minutes. [0022] Figure ID illustrates a fabrication process 130 suitable for use in connection with fabrication of the plate 114 shown in Figure IB in accordance with some embodiments. [0023] As shown in Figure lD-(a), on a substrate 132, such as a silicon substrate, layers of low-stress silicon nitride 134 and 136 each having a thickness of about lμm are formed. Low-pressure chemical vapor deposition is one method suitable for use in connection with forming the low-stress silicon nitride layers 134 and 136.

[0024] As shown in Figure lD-(b), on the low-stress silicon nitride layer 134, a Ti/Au layer 138 of about 200A/3000A is formed. E-beam evaporation is one method suitable for 5 use in connection with forming the Ti/Au layer 138. The Ti/Au layer 138 is patterned. Wet etching is one method suitable for use in connection with patterning the Ti/Au layer 138.

[0025] As shown in Figure lD-(c), a meander shaped support 140 is formed. Spin- coating and patterning a photoresist is one method suitable for use in connection with

10 forming the meander shaped support 140.

[0026] As shown in Figure lD-(d), the Ti/Au layer 138 is etched. Wet etching is one method suitable for use in connection with etching the Ti/Au layer 138. The low-stress silicon nitride layer 134 is etched. CHF3/O2 plasma etching is suitable for use in connection with etching the low-stress silicon nitride layer 134.

15 [0027] As shown in Figure lD-(e), the substrate 132 is etched to about 20μm deep. This etching defines membrane thickness of the plate 114. Deep reactive ion etching is one method suitable for use in connection with etching the substrate 132. [0028] As shown in Figure lD-(f), the low-stress nitride layer 136 is patterned and etched. Dry etching is one method suitable for use in connection with etching the low-stress

^'20 nitride layer 136.

[0029] As shown in Figure lD-(g), the substrate 132 is etched until 50μm silicon is left. Wet etching is one method suitable for use in connection with etching the substrate 132. [0030] Finally, as shown in Figure lD-(h), the substrate 132 is etched. Dry etching is one method suitable for use in connection with etching the substrate 132. The membrane and

25 meander shaped supports 140 are released. Deep reactive ion etching is one method of etching suitable for use in connection with releasing the membrane and meander support 140.

[0031] Figure IE illustrates forming a capacitor 110, shown in Figure IB, by coupling the electret 112, as shown and fabricated in Figure 1C, and the plate 114, as shown and

30 fabricated in Figure ID₅ through spacers 142 in accordance with some embodiments. Prior to coupling the electret 112 and the plate 114 through the spacers 142, the electret 112 is charged using a corona discharge source and the surface charge density of the electret 112 is measured. The spacers 142 are not limited to being fabricated using a particular material or to having a particular length. Aluminum is one material suitable for use in connection with fabrication of the spacers 142. In some embodiments, the spacers 142 have a length of about 50μm. To avoid discharging the electret 112 during fabrication of the capacitor 110, the spacers are coupled to the electret 112 and the plate 114 using a process that operates at a low-temperature, such as a temperature of less than about 150 degrees Centigrade. Adhesives, such as adhesive tapes, and glues, are exemplary materials, suitable for use in connection with the fabrication of the capacitor 110 in a low- temperature coupling process. In operation, the electret 112 is discharged through ionization of gas in the ionization chamber 116. [0032] Figure IF is an illustration of an apparatus 144 including an embodiment of the passive resonant circuit 102, shown in Figure IA, including a foam dielectric 146 in accordance with some embodiments. The apparatus 144 includes the inductor 108 coupled to the capacitor 110 having the foam dielectric 146. The foam dielectric 146 includes voids or gaps 148 in the foam dielectric 146. The voids or gaps 148 are physical locations in the dielectric that trap gases, maintain surface charge, and charge and discharge. An exemplary foam dielectric suitable for use in connection with the apparatus 144 includes porous polypropylene. In operation, the voids or gaps 148 are charged and discharged. An exemplary method of charging the voids or gaps 148 includes charging through a corona discharge. Exposure to a corona discharge creates dipoles in the voids or gaps 148 of the foam dielectric 146 and changes the dielectric constant of the capacitor

110. Exposure to ionizing radiation discharges the surface charge in the voids or gaps 148 of the foam dielectric 146 and changes the dielectric constant of the capacitor 110 and the resonant frequency of the apparatus 144. [0033] Figure IG is an illustration of the results of a simulation of one embodiment of the plate 114, as shown in Figure IB. The plate 110 is modeled as a membrane (2x2mm² and 20μm thick) with a 50 μm width meander shaped spring support as shown in Figure 1 G- (a). This support configuration enhances the sensitivity and linearizes the membrane movement. Figure 1 G-(b) shows a plot of the membrane deflection. The plot shows a linear relationship between the deflection and applied force/area (linear fitting error 0.0004%). Therefore, the membrane restoring force can be formulated by a linear model F=kx. The spring constant k is calculated with the linear fitting of the simulated data points shown in Figure IG. In this case, k is calculated as 2.1 N/m. [0034] Figure IH is an illustration of a graph of surface charge density versus deflection for a membrane, shown in Figure IG, in accordance with some embodiments. The data shown in Figure IH is for the case of 50μm air gap between an electret surface and the membrane. As can be seen, a surface charge density of about 800uC/m² results in a membrane deflection of about 16μm. Above this value, the membrane is pulled-down due to an increase in the electric field and resulting elastic instability (pull-in charge density). Upon exposure to the ionizing radiation, surface charge (i.e., electrical force) is reduced. This results in a decrease in the membrane deflection and the capacitance between the membrane and the electret (hence an increase in the resonance frequency). [0035] Figure 2 is a flow diagram of a method 200 for detecting ionizing radiation in accordance with some embodiments. The method 200 includes charging one plate of a capacitor in a resonant circuit having a resonant frequency (block 202), receiving ionizing radiation at the capacitor (block 204), and detecting a change in the resonant frequency in accordance with some embodiments (block 206). In some embodiments, charging one plate of the capacitor in the resonant circuit having the resonant frequency includes exposing the capacitor to a corona discharge. In some embodiments, receiving ionizing radiation at the capacitor includes receiving gamma radiation at the capacitor. In some embodiments, receiving ionizing radiation at the capacitor includes receiving x-ray radiation at the capacitor. In some embodiments, detecting the change in resonant frequency includes measuring the change in resonant frequency. In some embodiments, detecting the change in the resonant frequency includes monitoring the resonance frequency change with a phase-dip technique. In some embodiments, the method 200 further includes determining the amount of ionizing radiation received by the resonant circuit from the change in the resonant frequency. [0036] Figure 3 is a flow diagram of a method 300 determining the amount of radiation received by a tumor in accordance with some embodiments. The method 300 includes positioning a passive resonant circuit near a tumor (block 302), irradiating the tumor (block 304), and interrogating the passive resonant circuit to determine the amount of radiation received by the tumor (block 306). In some embodiments, interrogating the passive resonant circuit to determine the amount of the radiation received by the tumor includes measuring a change in resonant frequency of the passive resonant circuit. [0037] Figure 4 is a flow diagram of a method 400 for determining the amount of radiation delivered by a beam of ionizing radiation in accordance with some embodiments. The method 400 includes positioning a passive resonant circuit in a path of a beam of ionizing radiation (block 402), and interrogating the passive resonant circuit to determine the amount of radiation received delivered by the beam (block 404). In some embodiments, interrogating the passive resonant circuit to determine the amount of radiation delivered by the beam includes measuring a change in resonant frequency of the passive resonant circuit.

[0038] Figure 5 is a flow diagram of a method 500 of determining the amount of radiation received from a source of ionizing radiation. The method 500 includes positioning a passive resonant circuit near a source of ionizing radiation (block 502), and interrogating the passive resonant circuit to determine the amount of radiation received from the source of ionizing radiation (block 504). In some embodiments, interrogating the passive resonant circuit to determine the amount of radiation received from the source of ionizing radiation includes measuring a change in resonant frequency of the passive resonant circuit. [0039] Figure 6 A is an illustration of an apparatus 600 including the passive resonant circuit 102, shown in Figure IA, and an interrogating radiation source 602 in accordance with some embodiments. An exemplary radiation source suitable for use in connection with the apparatus 600 includes a radio-frequency radiation source. In operation, the radiation source 602 interrogates the passive resonant circuit 102 to determine the amount of ionizing radiation received at the passive resonant circuit 102. The phase-dip technique is one method of interrogation suitable for use in connection determining the change in the resonant frequency of the passive resonant circuit 102 and thereby the amount of ionizing radiation received at the passive resonant circuit 102. [0040] Figure 6B is an illustration of an apparatus 604 including the passive resonant circuit 102, shown in Figure IA, implanted in a human body 606 and the interrogating radiation source 602 in accordance with some embodiments. In operation, after ionizing radiation treatment of the tumor, the interrogating radiation source 602 provides radiation 608 to interrogate the passive resonant circuit 102 to determine the amount of ionizing radiation received at the tumor 608 located in the human body 606. Locating the passive resonant circuit 102 in close proximity to the tumor 608 permits accurate measurement of ionizing radiation applied to the tumor 608. [0041] Figure 6C is an illustration of an apparatus 610 including the passive resonant circuit 102, shown in Figure IA, inserted in an accelerator 612 in accordance with some embodiments. In operation, after inserting the passive resonant circuit 102 in the accelerator 612 particle path, which provides a source of high-energy particles, the interrogating radiation source 602 provides radiation 608 (radio frequency radiation) to interrogate the passive resonant circuit 102 to determine the amount of ionizing radiation received from the accelerator 612. The passive resonant circuit 102 provides a robust detector for detecting high-energy particles provided by the accelerator 612. [0042] In the foregoing specification the subject matter of the disclosure has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the disclosure as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense.

Claims

What is claimed is:

1. An apparatus comprising: a passive resonant circuit (102) having a resonant frequency that changes as ionizing radiation (104) is received.

2. The apparatus of claim 1, wherein the passive resonant circuit (102) includes a capacitor (110) having an electret (1 12) and a plate (114) that deflects in response to changes in charge on the electret.

3. The apparatus of claim 2, wherein the plate (114) includes a spring support.

4. The apparatus of claim 2, wherein the capacitor (110) includes a dielectric (146) to receive the ionizing radiation (104).

5. The apparatus of claim 1 , wherein the passive resonant circuit ( 102) includes an ionization chamber.

6. The apparatus of claim 5, wherein the ionization chamber (116) includes an electret (112) formed thereon.

7. The apparatus of claim 6, wherein the electret (112) comprises a foam.

8. The apparatus of claim 1, further comprising an interrogating radiation source (602) to interrogate the passive resonant circuit (102).

9. The apparatus of claim 1 , further comprising an accelerator (612) to provide a source of ionizing radiation (104) to the passive resonant circuit (102).

10. A method comprising : charging one plate (114) of a capacitor (110) in a resonant circuit (102) having a resonant frequency; receiving ionizing radiation (104) at the capacitor (110); and detecting a change in the resonant frequency.

11. The method of claim 10₃ wherein charging one plate ( 114) of the capacitor ( 110) in the resonant circuit (102) having the resonant frequency includes exposing the capacitor to a corona discharge.

12. The method of claim 10, wherein receiving ionizing radiation (104) at the capacitor (1 10) includes receiving gamma radiation at the capacitor.

13. The method of claim 10, wherein receiving ionizing radiation (104) at the capacitor (110) includes receiving x-ray radiation at the capacitor.

14. The method of claim 10, wherein detecting the change in resonant frequency includes measuring the change in resonant frequency.

15. The method of claim 10, wherein detecting the change in the resonant frequency includes monitoring the resonance frequency change with a phase-dip technique.

16. The method of claim 10, further comprising determining the amount of ionizing radiation (104) received by the resonant circuit (102) from the change in the resonant frequency.

17. A method comprising: positioning a passive resonant circuit (102) near a tumor; irradiating the tumor; and interrogating the passive resonant circuit (102) to determine the amount of radiation received by the tumor.

18. The method of claim 17, wherein interrogating the passive resonant circuit to determine the amount of radiation received by the tumor includes measuring a change in resonant frequency of the passive resonant circuit (102).

19. A method comprising: positioning a passive resonant circuit (102) in a path of a beam of ionizing radiation (104); interrogating the passive resonant circuit (102) to determine the amount of radiation delivered by the beam.

20. The method of claim 19, wherein interrogating the passive resonant circuit (102) to determine the amount of radiation delivered by the beam includes measuring a change in resonant frequency of the passive resonant circuit.

21. A method comprising: positioning a passive resonant circuit (102) near a source of ionizing radiation (104); interrogating the passive resonant circuit (102) to determine the amount of radiation received from the source of ionizing radiation (104).

22. The method of claim 21 , wherein interrogating the passive resonant circuit (102) to determine the amount of radiation received from the source of ionizing radiation (104) includes measuring a change in resonant frequency of the passive resonant circuit.