Neutron detector using Gd-based scintillator and wide-bandgap semiconductor photovoltaic

Abstract

An enhanced neutron sensing device, that couples a gadolinium based scintillator with at least two wide bandgap photodiodes to achieve a high sensitivity, low power, and portable neutron detector with high gamma discrimination. Once coupled with electrical signal processing and read-out electrons, the device will output the incident neutron flux in the environment and can be used in locations with known sources of neutrons or for identifying clandestine nuclear materials.

Claims

1. A neutron detector, comprising: a 157-Gd isotopically enriched dopant-activated single-crystal or nano-crystalline Gd-based scintillator coupled with two or more wide bandgap epitaxially grown photodiodes, wherein: the scintillator has a photo yield exceeding 6000 photons/neutron and an emission lifetime of 50 ns or less; each photodiode has a shunt resistance that is 3-5 orders of magnitude larger than GaAs and 5-7 orders of magnitude larger than Si; each photodiode has a dark current, at zero bias, of 1 nA/cm.sup.2 or less; each photodiode has an output voltage greater than 0.75 V under excitation by a 1 nA/cm.sup.2 pulse and greater than 0.5 V under excitation by a 0.1 nA/cm.sup.2 pulse; each photodiode absorbs greater than 98% of incident light with a wavelength of 650 nm or less in less than 3 microns; each photodiode has a base doping gradient that generates a built in field that drives minority carriers to respective sides of the photodiode; and each photodiode has a cross section thickness of less than or equal to 3 microns; and signal processing electronics configured to perform pulse-shape, coincidence signal analysis that assesses timing, shape, and amplitude of incident signals generated by the two or more photodiodes to achieve neutron sensing and gamma ray discrimination.

2. The neutron detector of claim 1, wherein the photodiodes comprise InGaP.sub.2.

3. The neutron detector of claim 1, wherein the Gd-based scintillator comprises GdF.sub.3, Gd.sub.2O.sub.3, GdI.sub.3, GdBr.sub.3, GdCl.sub.3, or Gd.sub.(x)Li.sub.(1-x)F.sub.3 and a scintillator dopant.

4. The neutron detector of claim 1, wherein a scintillator dopant comprises Eu, Ce, Tb, Er, Tm, or any combination thereof.

5. The neutron detector of claim 1, wherein the detector comprises a single-crystal Gd-based scintillator.

6. The neutron detector of claim 1, wherein the photodiodes operate in photovoltaic mode.

7. The neutron detector of claim 1, wherein the photodiodes operate in photosensing mode.

8. The neutron detector of claim 1, additionally comprising a non-Gd based scintillator configured to serve as a background reference to help gamma discrimination and to determine a source of radiation.

9. The neutron detector of claim 1, wherein the detector comprises a nano-crystalline Gd-based scintillator.

10. The neutron detector of claim 1, wherein the detector has one or more of the following features: (1) the scintillator is rare-earth activated; (2) the scintillator has a thickness greater than 1 cm; (3) the scintillator has an emission wavelength near 610 nm; (4) each photodiode has a time constant having a fast component of about 74 ps and a long-lived component of 3-4 ns; and (5) each photodiode has an intrinsic region forming a p-type-intrinsic-n-type (PIN) structure.

11. The neutron detector of claim 10, wherein the detector has all of the features (1)-(5).

12. A method for detecting neutrons, the method comprising: generating incident signals using a neutron detector comprising a 157-Gd isotopically enriched dopant-activated single-crystal or nano-crystalline Gd-based scintillator coupled with two or more wide bandgap epitaxially grown photodiodes, wherein: the scintillator has a photo yield exceeding 6000 photons/neutron and an emission lifetime of 50 ns or less; each photodiode has a shunt resistance that is 3-5 orders of magnitude larger than GaAs and 5-7 orders of magnitude larger than Si; each photodiode has a dark current, at zero bias, of 1 nA/cm.sup.2 or less; each photodiode has an output voltage greater than 0.75 V under excitation by a 1 nA/cm.sup.2 pulse and greater than 0.5 V under excitation by a 0.1 nA/cm.sup.2 pulse; each photodiode absorbs greater than 98% of incident light with a wavelength of 650 nm or less in less than 3 microns; each photodiode has a base doping gradient that generates a built in field that drives minority carriers to respective sides of the photodiode; and each photodiode has a cross section thickness of less than or equal to 3 microns; and processing the incident signals from the photodiodes using signal processing electronics configured to perform pulse-shape, coincidence signal analysis that assesses timing, shape, and amplitude of the incident signals to achieve neutron sensing and gamma ray discrimination.

13. The method of claim 12, wherein the photodiodes comprise InGaP.sub.2.

14. The method of claim 12, wherein the Gd-based scintillator comprises GdF.sub.3, Gd.sub.2O.sub.3, GdI.sub.3, GdBr.sub.3, GdCl.sub.3, or Gd.sub.(x)Li.sub.(1-x)F.sub.3 and a scintillator dopant.

15. The method of claim 12, wherein a scintillator dopant comprises Eu, Ce, Tb, Er, Tm, or any combination thereof.

16. The method of claim 12, wherein the detector comprises a single-crystal Gd-based scintillator.

17. The method of claim 12, wherein the detector comprises a polycrystalline Gd-based scintillator.

18. The method of claim 12, wherein the photodiodes operate in photovoltaic mode.

19. The method of claim 12, wherein the photodiodes operate in photosensing mode.

20. The method of claim 12, wherein the detector further comprises a non-Gd based scintillator configured to serve as a background reference to help gamma discrimination and to determine a source of radiation.

21. The method of claim 12, wherein the detector has one or more of the following features: (1) the scintillator is rare-earth activated; (2) the scintillator has a thickness greater than 1 cm; (3) the scintillator has an emission wavelength near 610 nm; (4) each photodiode has a time constant having a fast component of about 74 ps and a long-lived component of 3-4 ns; and (5) each photodiode has an intrinsic region forming a p-type-intrinsic-n-type (PIN) structure.

22. The method of claim 21, wherein the detector has all of the features (1)-(5).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(a) is a schematic of the neutron detector structure and operating mechanisms. FIG. 1(b) is an overlay of the InGaP.sub.2 spectral responsivity (line with dots) and the characteristic emission spectrum of Eu.sup.3+ doped phosphors.

(2) FIG. 2 is a comparison of limitations on the neutron absorption cross section for (a) the present invention and (b) traditional solid-state approaches.

(3) FIG. 3(a) shows measured dark current density vs. voltage characteristics for Si, GaAs, and InGaP.sub.2 photovoltaic device. FIG. 3(b) shows simulated radioisotope battery power output of the three devices under an excitation source that yields a generated current of 10 A/cm.sup.2Si, 1 A/cm.sup.2GaAs and InGaP.sub.2. A larger generated current is assumed in the Si cell because it has a greater generation volume, although 10 more current is a very generous assumption.

(4) FIG. 4 shows photoluminescence transient of InGaP.sub.2 epitaxially layer (squares) overlaid with the instrumental response and a parametric fit to the data.

DETAILED DESCRIPTION OF THE INVENTION

(5) The present invention is aimed at the development of battery operated, high efficiency, low-power solid-state neutron detectors with high gamma discrimination. In the ideal instantiation of the device, a 157-Gd enriched rare-earth activated single-crystal or transparent nano-crystalline Gd-based scintillator is paired with high-efficiency low-noise InGaP.sub.2 solid-state detectors with signal processing circuitry. Single-crystal or nano-crystalline Gd-based scintillators and back-end signal processing electronics are used to maximize neutron sensitivity and gamma ray discrimination by reducing internal scattering of the generated photons.

(6) The neutron detection technology of the present invention combines the favorable aspects of direct-conversion solid-state detection technologies (low power, scalability, low cost, high gamma discrimination, etc.) with the largest-know thermal neutron cross section of 157-Gd. FIG. 1(a) provides a schematic representation of the detector and operating mechanism. First, neutrons are absorbed by Gd in the Gd-based scintillator, which results in the emission of the nuclear reaction products. As shown on the right inset, the reaction products of 157-Gd include a 79 keV electron and gamma ray or, with much lower probability, an alpha particle or proton can be emitted. Regardless of the reaction, all of the reaction products (electrons, alphas, or protons) of 157-Gd (or other Gd isotopes) have sufficient energy to ionize the surrounding scintillator, which subsequently converts the energy to photons. The emitted photons propagate through the scintillator to one of two adjacent InGaP.sub.2 photovoltaic devices that convert the optical signal into a proportional electrical signal. Europium is currently specified as the scintillator dopant (or activator) because of the ideal match to the peak spectral responsivity of the InGaP.sub.2 converter. However, the Gd-based scintillators may include GdF.sub.3, Gd.sub.2O.sub.3, GdI.sub.3, GdBr.sub.3, or GdCl.sub.3, and other rare-earth dopants (Ce, Tb, Er, Tm) as well as co-doping may also be considered for the purposes of increased density, faster (or slower) luminescence lifetimes, ease of crystal production, etc. The light output

(7) $({}_p=\frac{S\left(E\right)}{E_g}{}_{\mathrm{RL}},\mathrm{where}S\left(E\right)={}^R\frac{E}{x}\left(E\right)x$
dx is the total ionizing energy deposited over the range, R, of the particle with initial energy E, E.sub.g is the bandgap energy of the scintillator, and .sub.RL is the radioluminescent efficiency, of the scintillator) can exceed 6000 photons/n in Gd-based scintillators with Ce doping. This would result in a photon power intensity of P=.sub.phv/t=0.19 nW/neutron, where t=10 s is included as a conservative estimate of the pulse width. This amount of incident optical power is well above the minimum sensitivity for InGaP.sub.2 photovoltaic converters, meaning this technology can be used for real-time single neutron counting. FIG. 1(b) is an overlay of the InGaP.sub.2 spectral responsivity (line with dots) and the characteristic emission spectrum of Eu.sup.3+ doped phosphors. (Cress et al., Alpha-particle-induced luminescence of rare-earth-doped Y.sub.2O.sub.3 nanophosphors, Journal of Solid State Chemistry, 181(8), 2041-45 (2008)).

(8) Following amplification, the output current pulse from the InGaP.sub.2 devices could potentially serve as a direct signal output of the detector. However, the scintillator crystal and InGaP.sub.2 converters may also have an unwanted sensitivity to background gamma radiation in the environment. Signal processing techniques assessing the timing, shape, and amplitude of incident signals, from both InGaP.sub.2 converters, will be applied to discriminate neutrons from the background gamma events with high efficacy. Depending on the intended application, both full spectrum energy resolved data may be provided using multiple channel pulse high signal processing or miniaturized versions that provide single channel resolution (single energy) but with reduced mass and energy overhead for using in unattended sensing applications.

(9) FIG. 2 provides a comparison of the limitation on neutron sensitive layer thickness for the present invention and alternative solid state approaches. In the present invention, the limiting thickness of the Gd-based scintillator is defined by the range of the emitted photons from the crystal, which can be greater than 1 cm for high quality crystalline material. In contrast, solid state detectors based on direct-ionization are limited by the range of the nuclear reaction products, which for low energy electrons and alpha-particles is typically 15 m or less. For a given neutron flux, the maximum sensitivity is dictated by the total cross section of neutron absorbing material (i.e., the Gd-based crystal). Therefore, the size of the Gd-based scintillator crystal employed can be used to tune the sensitivity of the detector to the desired level for the given application. For hand-held operation, a device that has adjustable crystal sizes for real-time sensitivity tuning is conceivable, and would be ideal for environments where large variations in neutron fluxes are possible.

(10) Detector

(11) The detector envisioned for this invention is based on InGaP.sub.2, which has many favorable qualities as compared to more traditional solid-state photodiodes. Some advantages of InGaP.sub.2 photodiodes include an extremely low dark current, high absorption coefficient, short lifetime, and thin active volume. These attributes yield photodiodes that can be operated in photovoltaic mode and achieve high-speed and high sensitivity while requiring very low power. For enhanced sensitivity and speed, the devices may also be operated in photosensing mode. In all modes, the thin active volume allows for high gamma discrimination.

(12) InGaP.sub.2, a wide bandgap semiconductor, is useful over more traditional photodiode materials, including Si and GaAs, as shown in FIG. 3. FIG. 3(a) depicts the measured current densityvoltage characteristics for the three devices under dark conditions. Under operation, the dark current flows in opposition to the scintillation-generated current, thus it is an unwanted source of leakage within the device. For high sensitivity, an important factor is the shunt resistance, i.e., the ability to oppose current flow through the device under low (near zero) biasing conditions when the photodiode is nominally considered to be in the off state. Since resistivity depends on the carrier density of a material, the high bandgap and concomitantly low intrinsic carrier concentration of InGaP.sub.2 yield a shunt resistance that is 3-5 orders of magnitude larger than GaAs and 5-7 orders of magnitude larger than Si. The much lower (off the scale in FIG. 1(a)) zero-bias current density for the InGaP.sub.2 device is evidence of this attribute. Furthermore, the lower intrinsic carrier concentration also reduces the saturation current, which effectively downward shifts the magnitude of the dark current.

(13) The impact of the lower dark current on the power output of a RPC is illustrated in FIG. 3(b) by simulating a large optical pulse emitted from the scintillator, resulting in a current that flows in the device opposite to the dark current (the current axis has been inverted to plot the result in the first quadrant). This method is called the linear superposition method and is often performed to simulate the power output of photovoltaic devices under optical excitation. A pulse 10 larger was assumed for the Si cell to emphasize the point, resulting in generated currents of 10 A/cm.sup.2 (Si) and 1 A/cm.sup.2 for both the GaAs and InGaP.sub.2 devices. Under these conditions, the Si device is not capable of generating a photo-voltage since the dark current is greater than the generated current. For the GaAs and InGaP.sub.2 devices a short circuit current density of 1 A/cm.sup.2 is achieved since at zero bias the dark current in both devices is 1 nA/cm.sup.2 or less. The larger shunt resistance in these devices is the main contributor in the stark contrast between the performance of Si and the other two devices. This results in an output voltage of 0.52 V and 1.05 V for the GaAs and InGaP.sub.2 photodiodes, respectfully. Following a similar analysis, the output voltage of the InGaP.sub.2 photodiode would exceed 0.75 V under excitation by a 1 nA/cm.sup.2 pulse and is over 0.5 V under excitation by a 0.1 nA/cm.sup.2 pulse. The latter could be generated by a single neutron capture reaction of 157-Gd as a part of a GdI.sub.3:Ce scintillator crystal which has a photon yield of over 6000 photons.

(14) The speed of the InGaP.sub.2 photodiodes is an important consideration and necessary to apply pulse-shape analysis techniques. FIG. 4 depicts the time-resolved photoluminescence measurements conducted on a high quality epitaxially grown InGaP.sub.2 layer. The experimental data (squares) show a fast initial decay followed by a long-lived lifetime component. The time constant of the fast component is 74 ps, while the long-lived is 3-4 ns. In comparison, the lifetime of Si typically exceeds 1 s because it is an indirect semiconductor. This information supports the notion that extremely fast photodiodes can be developed from InGaP.sub.2.

(15) The design and fabrication of the InGaP.sub.2 photodiodes also draws important distinctions with other materials. As a direct bandgap III-V semiconductor, InGaP.sub.2 has an extremely high absorption coefficient allowing it to absorb >98% of incident light (with a wavelength of 650 nm or less) in less than 3 m. When operated in photovoltaic mode (no external bias), the carriers have an extremely short path to transit for collection enabling high speed operation in photovoltaic mode. By introducing a doping gradient in the base of the structure, a built in field can be generated that drives the minority carriers to their respective sides of the diode further speeding the device. For operation in photo-sensing mode (an externally supplied reverse bias applied), even greater speed and sensitivity may be achieved because carriers are driven out of the device. To further suppress dark current and increase shunt resistance, a thin intrinsic region may be grown within the structure forming a p-type-intrinsic-n-type (PIN) structure. For comparison, Si photodiodes and avalanche photodiodes rely on absorption by Si which is an indirect semiconductor with a much lower absorption coefficient. Therefore, to achieve full absorption, the photodiode structures use the entire thickness of the Si wafer (100 m to 300 m thick). Therefore, the transit time for generated carriers is 30 to 100 times longer for these devices necessitating a tradeoff between device speed and sensitivity since thinner faster devices have less absorption. Furthermore, the large absorption cross section in Si-based photodiodes increases their susceptibility to false counts from incident background radiation such as gamma rays and x-rays. This effect is even worse in Si avalanche photodiodes (APDs), which have an internal gain mechanism. The >3 m cross section of InGaP.sub.2 greatly suppresses this effect making it more applicable for applications that necessitate high gamma discrimination.

(16) Scintillator

(17) The present invention is not restricted to the halides, crystals formed from rare-earth doped Gd.sub.2O.sub.3; and those that incorporate Li, such as Gd.sub.(x)Li.sub.(1-x)F.sub.3 would also be applicable. Essentially any Gd-based scintillator with a high photon yield under neutron exposure is applicable. An ideal scintillator would be one comprising isotopically enriched 157-Gd to maximize the thermal neutron cross section, has a photon yield exceeding 6000 photons/neutron (ideally 12,000 photons/neutron), and emission wavelength near 610 nm, and has an emission lifetime of 50 ns or less.

(18) For the present invention, a non-Gd scintillator may be used in addition to a Gd-based scintillator. The non-Gd scintillator can serve as a background reference to help gamma discrimination. Also, the non-Gd scintillator may be used in combination with a Gd-based scintillator to determine the source of the radiation, meaning that the actual isotope emitting the neutrons and gammas can be identified.

(19) Assembled Device

(20) The compilation of fast InGaP.sub.2 diodes, neutron sensitive Gd-based scintillators, and pulse-shape signal analysis techniques yields a truly unique neutron-sensing device. Some advantages of the device are its low-power, small size, and potentially reduced cost as compared to state-of-the-art detectors based on 3-He ionization counters or photomultiplier tube coupled scintillator approaches. The device uses a Gd-based scintillator bi-facially coupled with two (or more) InGaP.sub.2 photodiodes. This multi-diode coupling enables coincidence signal processing to be performed on output signals from the photodiodes allowing for improved suppression of background signals resulting from gamma rays incident with the scintillator or one of the photodiodes. Finally, the use of InGaP.sub.2 photodiodes enables the device to operate in photovoltaic mode while still achieving high sensitive and speed. This operation is important for achieving low power operation in a standalone device.

(21) The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles a, an, the, or said, is not to be construed as limiting the element to the singular.