Alpha indirect conversion radioisotope power source

Elsevier Editorial System(tm) for Applied Radiation and Isotopes Manuscript Draft Manuscript Number: Title: ALPHA INDIRECT CONVERSION RADIOISOTOPE POWER SOURCE Article Type: Full Length Article Section/Category: Radiation Sources and Applications Keywords: Microbatteries; Plutonium; Phosphor; Indirect conversion; Power generation Corresponding Author: Maxim Sychov, PhD Corresponding Author's Institution: First Author: Maxim Sychov, PhD Order of Authors: Maxim Sychov, PhD; Alexandr Kavetsky, Ph.D.; Galina Yakubova; Gabriel Walter, PH.D.; Shahid Yousaf, Ph.D.; Kenneth Bower, Ph.D. Manuscript Region of Origin: UNITED STATES Abstract: Advantages of radioisotope-powered electric generators include long service life, wide operation temperature range and high energy density. We report development of a long-life generator based on indirect conversion of alpha decay energy. Prototyping used 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=14uA; Uoc=2.3V; power output - 21uW. Using this prototype we have powered 8-digit electronic calculator.

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ALPHA INDIRECT CONVERSION RADIOISOTOPE POWER SOURCE

Maxim Sychov∗ , Alexandr Kavetsky, Galina Yakubova, Gabriel Walter, Shahid Yousaf, Qian

Lin, Doris Chan, Heather Socarras, and Kenneth Bower

TRACE Photonics Inc., 1680 West Polk, Charleston IL, 61920 USA 217-348-6394, 217-348-

6713 (fax), [email protected]

Abstract

Advantages of radioisotope-powered electric generators include long service life, wide

operation temperature range and high energy density. We report development of a long-life

generator based on indirect conversion of alpha decay energy. Prototyping used 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=14uA; Uoc=2.3V; power output – 21uW. Using this prototype

we have powered 8-digit electronic calculator.

Keywords: microbatteries, Plutonium, phosphor, indirect conversion, power generation

1. Introduction

A new generation of ultralow power devices needs miniature long-lasting power sources.

One option is the utilization of radioisotope-powered microgenerators as shown by Olsen

(1992) and by Sims, Dinetta and Goetz (1995). Advantages of radioisotope-powered energy

∗ Corresponding author. Tel. +1-217-348-6703; fax: +1-217-348-6713. E-mail address: [email protected]

Manuscript

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sources are long service life (over 10 years depending on isotope), low weight, small size,

wide operating temperature range and high reliability, see for example Bower et al. (2002)

and Sychov et al. (2002).

Among the several types of radioisotope decay energy conversion, we here demonstrate

alpha indirect conversion. The device uses a source of alpha particles coupled to a phosphor

screen. The phosphor converts the kinetic energy of alpha particles into light of suitable

wavelengths for the photovoltaic. The phosphor screen is then optically coupled to the

photovoltaic device, see Fig. 1.

Indirect conversion using alpha-source has several advantages compared to both direct

conversion of alphas or utilization of beta emitters:

• The high energy of alpha particles compared to beta ones provides higher specific

power (power per unit surface). Therefore, less photovoltaic surface area is

needed, device may be 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 better since phosphors are generally more stable than

voltaics. Over the long term, the power output is expected to surpass that of direct

conversion cells.

In this paper we describe fabrication and testing of power sources using Pu-238 alpha-

emitters.

2. Phosphor screen fabrication

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Phosphor layers on glass substrates were fabricated according to the following

procedure: glass substrates were washed with alkaline solution and a solution of dichromate

potassium 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 a horizontal

2.5x2.5cm2 glass plate of known weight. After several hours of sedimentation, the glass

substrate was taken out of the vial, baked at 250°С and weighed again. The difference

between starting and final glass weight divided by surface area is the thickness of phosphor

layer, H (mg/cm2).

Layers of different thickness 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 the Fig. 2.

Curves do not cross the origin due to binder weight. From results of Fig.2, for alpha

excitation the optimum thickness of the ZnS phosphor is 7-10 mg/cm2. The optimum layer

thickness should be approximately equal to the alpha range in the phosphor, as for the thicker

layers generated light is absorbed and scattered. Alpha range may be calculated using

empirical formula below:

x

xx

EAR

ρ

30

410 ⋅⋅=

−

, (1)

where Rx is range (cm); ρx is density (g/cm3); and Е0 is initial energy (MeV). For the ZnS:

2SZn

xAA

A+

= , (2)

where AZn and AS are atomic weights of zinc and sulfur respectively. If Е0=5.5 MeV, and ρx =

4.1 g/cm3, then R ~ 0,0022 cm = 22 um, or 9 mg/cm2 in surface density units. That value

corresponds nicely to the experimental optimum.

For comparison, data for Sr-90 excitation are also shown in Fig. 2. Isotope Sr-90 has high

energy of particles, but range of electrons in phosphor layer is much larger comparing to

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alpha particles. Therefore much thicker phosphor layers are needed to capture all the kinetic

energy, as it is clear from the Fig. 2. But in the thick layers most of light is absorbed and

scattered so we can’t use most of useful energy; that shows advantage of alpha emitting

isotopes having high energy of particles and short range of particles in phosphor screen.

3. Battery Prototype

For alpha indirect conversion we used a 10mW plutonium-238 alpha source with external

flux of 2.15mW and active area 2.5x6=15 cm2, so energy flux was about 0.143 mW/cm2. For

the light-to-electricity conversion we used AlxGa1-xAs/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

2.5x6cm2 size each to match active area of available Pu-238 sources. Voltaics were exposed

to the radioluminescent light of phosphor-coated slides coupled with the plutonium source, as

shown in Fig. 1, and their IV curves were measured.

To increase the efficiency and power output of indirect conversion prototypes we used

reflective layers between the alpha source and phosphor layer. The idea was to reflect back

light emitted from the phosphor screen toward source, and thus increase light flux on the

photovoltaics. Thin metal foils were placed between the source and phosphor screen.

Approximately 1u-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 10uW. 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 some 90% of that additional light is reflected

by the aluminum mirror. According to our measurements, only 75% of the reflected light is

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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 almost as it

is observed experimentally. Gold foil has lower reflectivity, while its density, and hence

absorbed alpha energy, is higher. Therefore, gold foil gives less improvement of power

output, only 20%.

To show the operational capabilities of the indirect conversion prototype, we powered an

8-digit electronic calculator with the indirect conversion cell. Photos given in the Fig. 4 show

the sequence of cell assembly. Photo #6 of the 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 under 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 also adequate for backlighting of liquid crystal displays.

When five plutonium based cells like described above were connected in one power

source we have achieved short circuit current of 14uA, open circuit voltage of 2.3V and

maximum power output of 21uW.

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. 2.4mW 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

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given in Table 1. Fig. 5 shows their radioluminescence emission spectra under alpha

excitation (measured using KSVU-23/MDR-23 spectrofluorimeter). Phosphors mostly emit

light in the yellow-red region of spectrum in which available 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 taken from the book of Mikhalchenko (1988):

%100DI

IIKo

o −= (3)

where Io is initial RL intensity; I – RL intensity at certain absorbed dose D in kGy. To

calculate absorbed dose in Gy (J/kg) we used following equation:

HtPD α= (4)

where Pαααα is energy flux from the alpha source in W/cm2; t 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 on

Fig. 6a, show periodic increase 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 the 26.3 MGy dose are presented in the last column of Table 1.

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Thus power source based on plutonium emitters, phosphor screens and voltaic arrays was

successfully fabricated and tested. Optimized battery produced short circuit current of 14uA,

open circuit voltage of 2.3V and maximum power output of 21uW. Developed battery was

used for the powering of electronic calculator and electronic watch clearly showing ability of

radioisotope-based power sources to produce enough energy for electronic circuitry. Stability

tests allowed us to identify the most stable phosphor under the Pu-238 exposure:

(Zn,Mg)F2:Mn lost of only 19% of initial RL intensity after the 26.3*107 J/kg absorbed dose.

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.

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References

Andreev V.M., Kavetsky A.G., Kalinovsky V.S., Khvostikov V.P., Ustinov V.A.,

Khvostikova O.А., Shvarts M.Z., 2001. Betavoltaic cells and arrays based on

AlGaAs/GaAs heterostructures. Proc. of 17th EPVSEC, Munich, p. VA1/38

Bower K., Shreter Y., Barbanel Y., and Bohnert G., Ed. Polymers, 2002. Phosphors and

Voltaics for Radioisotope Microgenerators, CRC Press., Boca Raton, p. 441

Mikhalchenko G.A. Radioluminescent radiators. 1988, Moscow, Energoatomizdat, p. 152 (In

Russian)

Olsen L.C., Review of betavoltaic energy conversion. 1992, Proc. of XII Space Photovoltaic

Research and Technology Conf., p.256

Sims P.E., Dinetta L.C., Goetz M.A.. Gallium phosphide energy converters. 1995,

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: Optoelectronics - Materials & Technology in the

Information Age. Ed. R.Guo. Ceramic Transactions, Volume 126. ACerS, Ohio.

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Figure and table captions

Figure 1. Alpha indirect conversion setup.

Figure 2. Radioluminescent intensity vs ZnS phosphor layer thickness.

Figure 3. Indirect conversion cell with and without reflectors.

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

Figure 5. RL spectra of tested phosphors.

Figure 6. Irradiation dose effect on RL intensity (left) and K (right) of tested phosphors.

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Table 1.

Properties of phosphors tested under alpha excitation

Phosphor Chemical composition K for 26,300 kGy, %/kGy

B-3g (Zn,Cd)S:Ag,Cl 3.80E-03

Mih-Y (Zn,Mg)F2:Mn 7.36E-04

RST-612 R Gd2O3:Eu 1.93E-03

K-78 R Y2O3:Eu 2.65E-03

K-78 B-W Y2O2S:Tb,Dy 2.68E-03

Figure(s) 1Click here to download high resolution image

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Figure(s) 6aClick here to download high resolution image

Figure(s) 6bClick here to download high resolution image

Dear Sirs, To your kind attention we present the paper which is devoted to the important problem: development of isotope-powered long-life energy sources for low-power microelectronics applications. Sincerely, Maxim Sychov Consultant TRACE Photonics Inc. 1680 West Polk Avenue Charleston, IL 61920 USA 217-348-6703, 217-348-6713 (fax) [email protected]

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