Alpha indirect conversion radioisotope power source

Alpha indirect conversion radioisotope power source

ARTICLE IN PRESS Applied Radiation and Isotopes 66 (2008) 173–177 www.elsevier.com/locate/apradiso Alpha indirect conversion radioisotope power sour...

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ARTICLE IN PRESS

Applied Radiation and Isotopes 66 (2008) 173–177 www.elsevier.com/locate/apradiso

Alpha indirect conversion radioisotope power source Maxim Sychov, Alexandr Kavetsky, Galina Yakubova, Gabriel Walter, Shahid Yousaf, Qian Lin, Doris Chan, Heather Socarras, Kenneth Bower TRACE Photonics Inc., 1680 West Polk, Charleston, IL 61920, USA Received 1 October 2006; received in revised form 5 August 2007; accepted 3 September 2007

Abstract Advantages of radioisotope-powered electric generators include long service life, wide temperature range operation and high-energy density. We report development of a long-life generator based on indirect conversion of alpha decay energy. Prototyping used 300 mCi Pu-238 alpha emitter and AlGaAs photovoltaic cells designed for low light intensity conditions. The alpha emitter, phosphor screens, and voltaic arrays were assembled into a power source with the following characteristics: Isc ¼ 14 mA; Uoc ¼ 2.3 V; power output 21 mW. Using this prototype we have powered an eight-digit electronic calculator and wrist watch. r 2007 Elsevier Ltd. All rights reserved. Keywords: Microbatteries; Plutonium; Phosphor; Indirect conversion; Power generation

1. Introduction A new generation of ultralow power devices needs miniature long-lasting electric sources. One option is the utilization of radioisotope-powered microgenerators as shown by Olsen (1992) and Sims et al. (1995). Advantages of radioisotope-powered energy sources are long service life (over 10 years depending on isotope), low weight, small size, wide operating temperature range and high reliability (Bower et al., 2002; Sychov et al., 2002). Among the many radioisotope decay energy conversion techniques, we here describe alpha indirect conversion. The kinetic energy of alpha particles is converted to light in a phosphor screen. The matched photon wavelengths are then guided to the photovoltaic. The phosphor screen is optically coupled to the photovoltaic device, as shown in Fig. 1. Indirect conversion using an alpha source has several advantages to both direct conversion of alphas and utilization of beta emitters:



The high energy of alpha particles provides higher

Corresponding author. Tel.: +1 217 348 6703; fax: +1 217 348 6394/ 6713. E-mail address: [email protected] (M. Sychov).

0969-8043/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2007.09.004

 

specific power (power per unit surface) compared to beta particles. Therefore, less photovoltaic surface area is needed, making the device smaller and cheaper. For the same reason, alpha radioluminescence (RL) is brighter compared to beta-excitation and photovoltaic conversion is more efficient at brighter light levels. Compared to direct conversion of alphas, initial overall efficiency is lower, but the stability is much higher since phosphors are more stable than voltaics. Over the long term, the indirect power output surpasses that of direct alphavoltaics.

In this paper we describe fabrication and testing of power sources using Pu-238. 2. Phosphor screen fabrication Phosphor was deposited on glass substrates using the following procedure: glass slides 2.5  2.5 cm2 were washed with alkaline solution and then with a solution of potassium dichromate in sulfuric acid. Phosphor powder was mixed with a 5% solution of phosphoric acid in acetone and that mixture was poured into a vial in which was placed the weighed glass slides. After several hours of sedimentation, the slides were taken out of the vial, baked at 250 1C and weighed again. The difference between

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For comparison, data using Sr-90 excitation is also shown in Fig. 2. Sr-90 and its Y-90 daughter, emit highenergy electrons. Its penetration in phosphor layers is much deeper compared to alpha particles. Therefore, thicker phosphor layers are needed to capture all the kinetic energy, as is clear in Fig. 2. However, the thicker layers self-absorb and scatter the produced light. Therefore, most of photonic energy is lost, showing the advantage of alpha emitting isotopes having high energy and short range of particles in phosphor screen.

Fig. 1. Alpha indirect conversion setup.

3. Battery prototype

Fig. 2. Radioluminescent intensity vs ZnS phosphor layer thickness.

starting and final glass weight divided by surface area is the surface density of the phosphor layer, H (mg/cm2). Layers of different surface density were fabricated from ZnS phosphor and tested against Pu-238 and Sr-90 sources. Radioluminescent intensity was measured with a photomultiplier tube and results are shown in Fig. 2. Curves do not cross the origin due to binder weight. With alpha excitation, the optimum thickness of ZnS phosphor is found to be 7–10 mg/cm2, which corresponds to 17–24 mm. The optimum layer thickness should be approximately equal to the alpha range in the phosphor since thicker layers absorb and scatter the light. The alpha range may be calculated using the empirical formula: 104 Rx ¼

qffiffiffiffiffiffiffiffiffiffiffi Ax E 30 rx

,

For alpha indirect conversion, we used a 10 mW plutonium-238 alpha source with external flux of 2.15 mW and active area 2.5 cm  6 cm ¼ 15 cm2, so energy flux was about 0.143 mW/cm2. For the light-to-electricity conversion, we used AlxGa1xAs/GaAs photovoltaics fabricated in the laboratory of Prof. V. Andreev (Ioffe Institute). Details may be found in the paper of Andreev et al. (2001). Photovoltaics were used for the fabrication of assemblies of approximately 2.5  6 cm2 size each to match the active area of available Pu-238. Voltaics were exposed to the radioluminescent light of phosphor-coated slides coupled with the plutonium source, as shown in Fig. 1, and their I–V curves were measured. To increase the efficiency and power output of indirect conversion models, we found thin reflective metal layers between the alpha source and phosphor layer to be useful. The idea was to reflect back light emitted from the phosphor screen toward source toward the voltaic, and thus increase light flux on the photovoltaics. Approximately, 1 mm thick aluminum and gold foils were used. Performance of the indirect conversion cell composed of one plutonium source with and without foils is presented in Fig. 3. An aluminum reflector increased power output 60%, namely from 6.3 to 10 mW. Since light emission is isotropic in the phosphor screen, the intensity of light emitted toward the source is the same as toward the voltaic. Then

(1)

where Rx is the range (cm), rx is the density (g/cm3), and E0 is the initial energy (MeV). For the ZnS Ax ¼

AZn þ AS , 2

(2)

where AZn and AS are atomic weights of zinc and sulfur, respectively. If E0 ¼ 5.5 MeV, and rx ¼ 4.1 g/cm3, then R0.0022 cm ¼ 22 mm, or 9 mg/cm2 in surface density units. That value corresponds nicely to the experimentally determined optimum.

Fig. 3. Indirect conversion cell with and without reflectors.

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some 90% of that additional light is reflected by the aluminum mirror. According to our measurements, only 75% of the reflected light is diffusively transmitted by the phosphor layer. That should give about a 68% improvement of light flux toward the voltaics. However, the reflector absorbs 5% of the alpha energy, so the total light flux was reduced to the same degree and we obtain 64% improvement as observed experimentally. Gold foil has lower reflectivity, while its density, and hence absorbed alpha energy, is higher. Therefore, gold foil gives only 20% improvement. To show the operational capabilities of the indirect conversion prototype, we powered an eight-digit electronic calculator with the indirect conversion cell. Photos given in Fig. 4 show the sequence of cell assembly. Photo #6 of Fig. 4 shows the same assembly as photo #5 except the cell is coated with black cloth to exclude the effect of ambient light on the generator. The cell functions fine in both situations. An electronic watch was also powered in the same manner. It should be noted that radioluminescence brightness of 15–20 cd/m2 achieved in our experiments is adequate for backlighting of liquid crystal displays. When five plutonium cells were connected in one power source, we achieved short circuit current of 14 mA, open circuit voltage of 2.3 V and power output of 21 mW.

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4. Phosphor stability We conducted stability investigations of various phosphors including some experimental samples. All stability measurements were done in vacuum to prevent corrosion of the alpha source. 2.4 mW Pu-238 (300 mCi) was used for the irradiation of phosphors. Radioluminescence intensity was measured as the current of a photomultiplier tube. Various phosphors were tested including ZnS samples, oxides, oxysulfides, yttrium aluminum garnet as well as SiAlON:Eu and tiogallates. The most stable samples among tested ones were identified and subjected to longer-term tests. A description of these samples is given in Table 1. Fig. 5 shows radioluminescent emission spectra under alpha excitation (measured using KSVU-23/MDR23 spectrofluorimeter). These phosphors emit light mostly in the yellow-red region of the spectrum, where these photovoltaics have the highest efficiency. Alpha radiation stability was characterized by the normalized radioluminescent intensity as well as by a radiation stability coefficient, K. This coefficient reflects the percentage of radioluminescent intensity change per kGy of absorbed dose and is calculated according to the formula

Fig. 4. Calculator is working when powered by the indirect conversion cell.

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Table 1 Properties of phosphors tested under alpha excitation Phosphor

Chemical composition

K for 26,300 kGy (% per kGy)

B-3g Mih-Y RST-612 R K-78 R K-78 B-W

(Zn,Cd)S:Ag,Cl (Zn,Mg)F2:Mn Gd2O3:Eu Y2O3:Eu Y2O2S:Tb,Dy

3.80E03 7.36E04 1.93E03 2.65E03 2.68E03

Fig. 5. RL spectra of tested phosphors.

taken from Mikhalchenko (1988) K¼

I0  I  100%, I 0D

Fig. 6. Irradiation dose effect on RL intensity (a) and K (b) of tested phosphors.

(3)

where I0 is initial RL intensity, I is the RL intensity at certain absorbed dose D in kGy. To calculate absorbed dose in Gy (J/kg), we used D¼

Pa t , H

(4)

where Pa is the energy flux from the alpha source in W/cm2, t the is exposure time in seconds, and H is the surface density of phosphor layer absorbing all alpha energy, kg/ cm2. Results of radiation stability tests are presented in Fig. 6. Some of the curves in Fig. 6a, show periodic increases in RL intensity. This was observed when stability experiments were interrupted for a weekend. Therefore, these phosphors show self-repair of radiation damage at room temperature. The curves of Fig. 6b show change in the radiation stability coefficient, K, with increased dose. Initial degradation (high K values) level out for oxide and oxysulfide phosphors. For the sulfide-type phosphor, K stabilizes at a higher value of absorbed doses. The fluoride-type phosphor show initial improvement of RL (negative values of K), and then K levels at the lowest value among tested phosphors. Values of the radiation stability coefficient for 26.3 MGy dose are presented in the last column of Table 1.

Electric generators based on plutonium emitters, phosphor screens and voltaic arrays were successfully fabricated and tested. Optimized generators produced short circuit current of 14 mA, open circuit voltage of 2.3 V and maximum power output of 21 mW at 0.11% overall efficiency. This battery powered electronic calculators and an electronic watch clearly showing the ability of radioisotope-based power sources to produce enough energy for modern circuits. Stability tests allowed us to identify the most stable phosphor under the Pu-238 exposure: (Zn,Mg)F2:Mn lost only 19% of initial RL intensity after the 26.3  107 J/kg absorbed dose which corresponds to 411 h of continuous irradiation with high-activity Pu-238. Acknowledgments Authors gratefully acknowledge project support provided by DARPA and US Army, Picatinny Arsenal, under Contract W15QKN-04C-1123. Low light arrays provided by Prof. V.M. Andreev were greatly appreciated. References Andreev, V.M., Kavetsky, A.G., Kalinovsky, V.S., Khvostikov, V.P., Ustinov, V.A., Khvostikova, O.A., Shvarts, M.Z., 2001. Betavoltaic cells and arrays based on AlGaAs/GaAs heterostructures. In: Proceedings of the 17th EPVSEC, Munich, p. VA1/38.

ARTICLE IN PRESS M. Sychov et al. / Applied Radiation and Isotopes 66 (2008) 173–177 Bower, K., Shreter, Y., Barbanel, Y., Bohnert, G. (Eds.), 2002. Polymers, Phosphors and Voltaics for Radioisotope Microgenerators. CRC Press, Boca Raton, FL, p. 441. Mikhalchenko, G.A., 1988. Radioluminescent Radiators. Energoatomizdat, Moscow, p. 152. Olsen, L.C., 1992. Review of betavoltaic energy conversion. In: Proceedings of the XII Space Photovoltaic Research and Technology Conference, p. 256.

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Sims, P.E., Dinetta, L.C., Goetz, M.A., 1995. Gallium phosphide energy converters. In: Proceedings of the 14th Space Photovoltaic Research and Technology Conference, p. 33. Sychov, M.M., Bower, K.E., Kavetsky, A.G., Andreev, V.M., 2002. Radioluminescent glass based light and power source. In: Guo, R. (Ed.), Optoelectronics—Materials and Technology in the Information Age, vol. 126. ACerS, Ohio Ceramic Transactions.