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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 12, Iss. 12 — Jun. 14, 2004
  • pp: 2662–2669
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Characterization of a bit-wise volumetric storage medium for a space environment

Y. Zhang, J. Butz, J. Curtis, N. Beaudry, W. L. Bletscher, K. J. Erwin, D. Knight, T. D. Milster, and E. Walker  »View Author Affiliations


Optics Express, Vol. 12, Issue 12, pp. 2662-2669 (2004)
http://dx.doi.org/10.1364/OPEX.12.002662


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Abstract

We report playback performance results of volumetric optical data storage disks that are made from a class of light-absorbing (photo-chromic) compounds. The disks are exposed to a simulated space environment with respect to temperature and radiation. To test for temperature sensitivity, a vacuum oven bakes the disks for certain amount of time at a designated temperature. Radiation exposure includes heavy ions and high energy protons. Disks fail in high temperature and large proton-dose conditions. Heavy ions do not cause significant disk failure. The prevention of disk failure due to harsh space environments is also discussed.

© 2004 Optical Society of America

1. Introduction

Bit-wise volumetric data storage techniques are attractive candidates to increase data capacity in optical storage devices [1

1. H. Zhang, E. P. Walker, W. Feng, Y. Zhang, A. S. Dvornikov, S. Esener, and P. M. Rentzepis, “Multi-layer optical data storage based on two-photon recordable fluorescent disk media,” Eighteenth IEEE Symposium on Mass Storage System, 225–236 (2000).

,2

2. S. Kawata, “Three-dimensional Digital Optical Data-storage with Photorefractive Crystals,” Proc. SPIE 3470, 56–63, (1998). [CrossRef]

,3

3. M. Hisaka, H. Ishitobi, and S. Kawata, “Three dimensional optical recording with the ferroelectric domain reversal in a Ce-doped SBN:75 crystal: experiment and calculation,” Proc. SPIE 3740, 109–112 (1999).

,4

4. A. Toriumi and S. Kawata, “Reflection confocal microscope readout system for three-dimensional photochromic optical data storage,” Opt. Lett.23, (1998). [CrossRef]

,5

5. T. D. Milster, Y. Zhang, J. Butz, T. Miller, and E. P. Walker, “Volumetric Bit-Wise Memories, ” NASA earth science technology conference (2002).

,6

6. T. D. Milster, Y. Zhang, C. D. Pinto, and E. P. Walker, “A Volumetric Memory Device based on Photo-Chromatic Compounds,” NASA earth science technology conference (2001).

]. Configuration of the optical system for this type of recording is similar to conventional disk systems, in which a spinning disk is illuminated by a laser beam that is focused into the disk by an objective lens. The laser beam is modulated by the data-mark pattern interacting with the laser focus. The modulated light is then collected by a detector. Inter-layer crosstalk is controlled by placing a pinhole in front of the detector. This type of data storage has the potential to achieve 1 Tb/in-2 with 1000 data layers and 64 Mb/s data rate. Because of the massive storage capability of this technology, it is very attractive to build an on-board volumetric optical storage device in a space vehicle. Note that the vacuum environment in a low-orbit space vehicle prohibits the use of the magnetic hard drives, and optical memory is more power efficient than solid state disks. Hence, testing the robustness of the storage medium in a harsh space environment is crucial.

In this work, a dynamic test stand is constructed to write and read multiple layers of data marks inside disks that are made from a photo-chromic material developed at Call/Recall Inc [1

1. H. Zhang, E. P. Walker, W. Feng, Y. Zhang, A. S. Dvornikov, S. Esener, and P. M. Rentzepis, “Multi-layer optical data storage based on two-photon recordable fluorescent disk media,” Eighteenth IEEE Symposium on Mass Storage System, 225–236 (2000).

]. The written marks are microns in size. With an appropriate spherical aberration compensator, multiple layers of data marks are written inside the disks and then retrieved from the disks. By varying the distance between data layers, the inter-layer crosstalk is controlled to under -30 dB.

Section 2 provides background on the photochromic process. A detailed description of the dynamic test stand is presented in Section 3. Section 4 describes media characterization experiments and results of the temperature and radiation sensitivity studies. A short summary is presented with conclusions in Section 5.

2. Background on the photochromic process

With a tightly focused laser beam, the photochromic process is initiated and controlled within micrometer-size spaces. A data mark is written within the volume only at points of sufficiently high irradiance (20 GW/cm2) [7

7. S. Hunter, “Potentials of two-photon 3-D optical memories for high performance computing,” Appl. Opt. 29, 2058–2066 (1990). [CrossRef] [PubMed]

,8

8. D. C. Hutchings, “Kramers-Kronig relations in nonlinear optics,” Opt. Quantum Electron. 24.1–30 (1992). [CrossRef]

,9

9. E. W. Van Stryland, “Characterization of nonlinear optical absorption and refraction,” Prog. Crys. Grow. And Chac. 27, 279–311 (1993). [CrossRef]

,10

10. H. Zhang, “Single-beam two-photon-recorded monolithic multi-layer optical disks,” Proc. SPIE 4090, 174–178 (2000). [CrossRef]

]. At these points, two-photon absorption occurs, resulting in a bond dissociation. Thus, the molecular structure is changed into a new ‘written’ molecule with a different absorption and emission spectrum, as shown in Fig. 1. To read the information written within the volume, the approach exploits the fact that the written form absorbs at longer wavelengths than the unwritten form. As shown on the right side of Fig. 1, nondestructive excitation of written molecules is followed by fluorescence at ~660 nm, which returns the molecule to its ground state. The presence or absence of this fluorescence is detected and classified as a physical ‘1’ or ‘0’ for the stored data mark. Since the decay lifetime is ~5 nanoseconds and the concentration of molecules is high, it is possible to excite the written molecules many times in a single read cycle and increase the total light collected at the detector.

The advantage of a 2-photon absorption process is based upon its ability to selectively excite molecules inside a volume without populating molecules on the surface of the device.

Fig. 1. A physical description of two-photon absorption. (a) Energy-level diagram and molecular structure of unwritten and written forms, showing fluorescence (b) absorption spectra and fluorescence spectrum of the unwritten and written forms of the material.

This selection may be achieved because the laser photons have less energy than the energy gap between the ground state and first allowed electronic level. Therefore, photons propagate through the medium without being absorbed by a one-photon process. However, in the vicinity of the laser beam focus, the intensity is high enough so that two photons can combine to excite carriers across the energy gap. The transition probability of a 2-photon absorption process partly depends upon the writing beam irradiance and carrier diffusion, so lasers producing high irradiance in short pulses, i.e., picosecond and sub-picosecond pulses, must be used.

The recording material is dispersed in a polymer host polymethylmethacrylate (PMMA), which is a thermoplastic with good optical characteristics, and is shaped to produce disks. PMMA disks used in this experiment are 25mm in diameter and 3mm thick. They are homogeneously dispersed with storage material.

3. Dynamic test stand

A dynamic test stand, which is called Arizona Readout Test Stand (ARTS), is built as the read-out system for this medium. A system schematic is shown in Fig 2. A diode laser with wavelength 638 nm is reflected 90° by a dichroic beamplate. This laser beam is then delivered through a relay telescope to a biaspheric molded glass lens mounted on a voice-coil actuator. The beam is focused into a disk sample. The disk is mounted on a miniature tip-tilt-x-y stage that aligns written data mark planes inside the disk in the focus of the Geltech lens. The data marks inside the disk absorb energy and emit the fluorescent light, the spectrum of which ranges from 650nm to 700nm. The dichroic beam plate combined with a high pass filter (650 nm cutoff wavelength) is used to separate the reflected laser beam and the fluorescent beam. A flip mirror is used to direct the fluorescent beam into two different paths. In the first path, the fluorescent beam is focused onto a CCD camera. In the second path, a lens is used to focus the fluorescence onto a knife-edge prism, which splits the beam into two parts, A and B. Two PMTs are used to collect beams A and B. The carrier-to-noise (CNR) is measured from the A+B signal and is measured with a spectrum analyzer over a 500 kHz bandwidth. The main purpose of splitting this fluorescent beam is as a tracking servo [11

11. T. D. Milster, “Semi-kinematic rails for construction of optical test stands”. SPIE Annual Meeting, San Diego, Aug. 2 (2001).

]. Careful alignment produces a tracking error signal when the laser illumination is not directly centered over the data bits. The error signal is used in a feedback control loop to correctly reposition the actuator. Because of the extreme sensitivity of the PMT, a 50 µm diameter pinhole is used to let only the fluorescence from the bits inside the disk pass through. The size of the pinhole is determined by the size of the image of a data mark at the pinhole plane. The PMT/pinhole combination is mounted on a xyz stage, which enables precise adjustment of the position of the PMT and achieves the best signal quality. Figure 3(a) shows a read back signal from the disk displayed on an oscilloscope from a 175 kHz data pattern. The carrier-to-noise (CNR) of this signal is 37 dB and associated spectrum analyzer output is shown in Fig. 3(b). The CNR is measured by measuring the difference between signal levels at A and B in Fig. 3(b). The resolution bandwidth of the spectrum analyzer is set to 3 KHz. The second harmonic peak is also observed in Fig. 3(b). Although this CNR limits the single-channel data rate to around 1 MHz, parallel channels can be used to improve readout performance [1

1. H. Zhang, E. P. Walker, W. Feng, Y. Zhang, A. S. Dvornikov, S. Esener, and P. M. Rentzepis, “Multi-layer optical data storage based on two-photon recordable fluorescent disk media,” Eighteenth IEEE Symposium on Mass Storage System, 225–236 (2000).

]. The jitter of this signal, which is defined as the standard deviation of the lengths of all pulses in the data signal, is 1.667 µsec. Since the modulated fluorescence is used as the read out signal, the primary noise component is shot noise.

Fig. 2. The schematic description of the Arizona Readout Test Stand (ARTS) dynamic test stand. The optical components are mounted on a semi-kinematic rail structure. A knife-edge prism is used to split the fluorescence and the difference between the two signals is used as a track error signal (TES).
Fig. 3. (a) The read out signal from ARTS displayed on a oscilloscope; (b) the read out signal from ARTS displayed on a spectrum analyzer. The resolution bandwidth is 3 kHz, the video bandwidth is 300 Hz and the CNR is 37 dB.

4. Media characterization experiments

In this section, the playback performance of the disks made from photo-chromic material is reported after they are exposed to a simulated space environment, which includes temperature and radiation cycles. Although the particular media studied in these experiments requires a high laser power to write data, similar media is now developed that use commercial laser diode to write data. Since readout performance is of primary concern for data integrity, writing performance is not addressed in this study. The temperature in a well-conditioned low orbit space vehicle is maintained between -20° C and 50° C. Two major radiation sources in the low orbit are heavy ion and proton radiation. The energy level for heavy ions range from 100 Mev to 350 Mev with flux up to 2×105 p/cm2-s; while the energy level for heavy ions range from 30 Mev to 60 Mev with flux up to 109 p/cm2-s.[14

13. D. Malacara, “Optical Shop Testing,” Wiley series in pure and applied optics, 2nd edition.

,15

14. J. Barth, “Ionizing Radiation Environment Concerns,” conference of single event effect criticality analysis, http://radhome.gsfc.nasa.gov/radhome/papers/seeca3.htm.

,20

19. S. Guertin, “Single-Event Upset Test Results for the Xilnx XQ1701L PROM,” IEEE Radiation Effects Data Workshop, pp. 14–21 (1999) http://radnet.jpl.nasa.gov/reports/1/ReportFiles/Xilinx_R1701L.pdf.

] A failure event of the disk is defined as a factor of two increase in jitter compared to the jitter of the same disk before exposure to the space environment.

4.1 Temperature testing

The temperature range in this study is from 50° C to 90° C. Firstly, pre-written disk substrates are baked in a vacuum chamber for certain amount of time at a specific temperature. After being removed from the oven and allowed to cool, readout signals are evaluated on ARTS. The failure rate is defined as

λ=SumoffailuresΣ(Quantity×Timetofailure)1hours.
(1)

The test results are shown in Table 1. The temperature dependence of the failure rate is described by the Arrhenius model [12

11. E. P. Walker, “Servo error signal generation for two-photon-recorded monolithic multilayer optical data storage,”. Proc. SPIE 4090179–184, (2000). [CrossRef]

]

λ(T2)=λ(T1)×exp[(EAK)×(1T11T2)].
(2)

where T1 and T2 are two temperatures that disks are exposed to in the vacuum environment and k is Boltzmann’s constant. EA is the activation energy found by fitting experimental failure rates λ (T 1) and λ (T 2) in Eq.(2). A critical temperature envelope of 1.5 hours at 70C and 1.0 hour at 80C exhibited 40% and 70% disk failures, respectively. EA determined from these results is 0.4677 eV. The long term failure rate at 50° C and a 3 year mission is expected to be unacceptably high at 21%.

Table 1. The percentage of disks failed versus temperature and heating period

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Interestingly, disk failure is not due to intrinsic photochromic damage. For example, Fig. 4(a) shows a CCD image of data marks inside the disk before heating, and Fig. 4(b) shows a typical image of marks inside the disk after heating. There is no visible degradation of fluorescence between Figs. 4(a) and 4(b). Figure 5(a) shows the time-domain readout signal before heating, which exhibits a straight baseline and large signal amplitude. Figure 5(b) shows the time-domain readout signal after heating, which exhibits a strongly curved baseline without a significant reduction in signal amplitude. Disk readout failure occurs when the servo circuit becomes unstable and unable to lock track when the track error signal has a curved baseline, as shown in Fig 5(b). ARTS is not able to follow the data track of the exposed disk sufficiently well. CCD images in Fig. 4 and signal amplitudes in Fig. 5 indicate that the intrinsic photochromic properties of the medium are not significantly affected in this temperature range.

Fig. 4. (a) CCD image of fluorescent bits before heating. (b) CCD image of fluorescent bits after heating. No apparent change is observed compared to (a).
Fig. 5. (a) Time domain readout signal before heating exhibits a straight baseline and large signal amplitude. (b) Time domain readout signal after heating exhibits a strongly curved baseline, without a significant reduction in signal amplitude.

In order to identify cause of the servo error, the front surface of the disk is examined with a Ronchi test [13

12. For the detailed information of Arrhenius model and the failure rate, http://www.vishay.com/docs/rect_reliability.pdf.

]. Figure 6(a) shows the front surface of the disk before heating. Ronchi lines are patterns of dark fringes covering the surface. If the surface is perfectly flat, Ronchi lines are straight and equally spaced. Magnitude of surface departure is proportional to displacement of fringes from the straight pattern. In Fig. 6(a), fringes are mostly straight, except near edges and near the center mounting hole. In Fig 6(b), which shows the disk surface after heating, a significant distortion of the surface is apparent. The magnitude of the surface departure is estimated to be tens of microns.

We conclude that disk failure is due to a servo error caused by deformation of the molded PMMA disk substrate. The fluorescent molecules inside the disk are not damaged within the tested temperature range.

Fig. 6. (a) Ronchi test interferogram of the disk front surface before heating. Straight lines indicate a relatively flat surface. (b) Ronchi test interferogram of the disk surface after heating. Complex line patterns indicate a deformed surface.

4.2 Heavy ion testing

Table 2. Test results of the photo-chromic disks after they are exposed to heavy ion radiation.

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4.3 Proton radiation testing

Table 3. Test results of the photo-chromic disks after they are exposed to proton radiation.

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Table 4. Minimal thickness for shielding metal when the proton energy is 60 Mev

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5. Conclusion

The play-back performance of photon-chromic disks are tested after they are exposed to temperature cycles, heavy-ions and proton radiation. We conclude that the current two-photon disk is not an ideal candidate for data storage on a low-orbit satellite because of the unacceptably high long-term failure rate at 50°C. An improved servo system and temperature-tolerant substrates are required to improve performance at high temperatures. Heavy ion radiation has no effect on the disk performance, due to poor penetration range. Protons with high flux (109 p/cm2-s) erase the bits inside a disk in a short amount of time, while protons with relatively low flux (106 p/cm2-s) have no effect on the disk.

Acknowledgments

This work is supported by NASA contract NAS2-00117.

References and links

1.

H. Zhang, E. P. Walker, W. Feng, Y. Zhang, A. S. Dvornikov, S. Esener, and P. M. Rentzepis, “Multi-layer optical data storage based on two-photon recordable fluorescent disk media,” Eighteenth IEEE Symposium on Mass Storage System, 225–236 (2000).

2.

S. Kawata, “Three-dimensional Digital Optical Data-storage with Photorefractive Crystals,” Proc. SPIE 3470, 56–63, (1998). [CrossRef]

3.

M. Hisaka, H. Ishitobi, and S. Kawata, “Three dimensional optical recording with the ferroelectric domain reversal in a Ce-doped SBN:75 crystal: experiment and calculation,” Proc. SPIE 3740, 109–112 (1999).

4.

A. Toriumi and S. Kawata, “Reflection confocal microscope readout system for three-dimensional photochromic optical data storage,” Opt. Lett.23, (1998). [CrossRef]

5.

T. D. Milster, Y. Zhang, J. Butz, T. Miller, and E. P. Walker, “Volumetric Bit-Wise Memories, ” NASA earth science technology conference (2002).

6.

T. D. Milster, Y. Zhang, C. D. Pinto, and E. P. Walker, “A Volumetric Memory Device based on Photo-Chromatic Compounds,” NASA earth science technology conference (2001).

7.

S. Hunter, “Potentials of two-photon 3-D optical memories for high performance computing,” Appl. Opt. 29, 2058–2066 (1990). [CrossRef] [PubMed]

8.

D. C. Hutchings, “Kramers-Kronig relations in nonlinear optics,” Opt. Quantum Electron. 24.1–30 (1992). [CrossRef]

9.

E. W. Van Stryland, “Characterization of nonlinear optical absorption and refraction,” Prog. Crys. Grow. And Chac. 27, 279–311 (1993). [CrossRef]

10.

H. Zhang, “Single-beam two-photon-recorded monolithic multi-layer optical disks,” Proc. SPIE 4090, 174–178 (2000). [CrossRef]

11.

T. D. Milster, “Semi-kinematic rails for construction of optical test stands”. SPIE Annual Meeting, San Diego, Aug. 2 (2001).

11.

E. P. Walker, “Servo error signal generation for two-photon-recorded monolithic multilayer optical data storage,”. Proc. SPIE 4090179–184, (2000). [CrossRef]

12.

For the detailed information of Arrhenius model and the failure rate, http://www.vishay.com/docs/rect_reliability.pdf.

13.

D. Malacara, “Optical Shop Testing,” Wiley series in pure and applied optics, 2nd edition.

14.

J. Barth, “Ionizing Radiation Environment Concerns,” conference of single event effect criticality analysis, http://radhome.gsfc.nasa.gov/radhome/papers/seeca3.htm.

15.

T. Miyahira “Initial SEE Tests of SanDisk Flash Memory,” http://radnet.jpl.nasa.gov/reports/1/ReportFiles/078-SanDisk2.PDF.

16.

J. Coss, “Device SEE Susceptibility Update: 1996–1998,” http://radnet.jpl.nasa.gov/reports/1/ReportFiles/Update.PDF.

17.

T. Miyahira, “Summary of SEE test results from BNL heavy ion test,” http://radnet.jpl.nasa.gov/reports/1/ReportFiles/9902bnl.PDF.

18.

For more information of calculating penetration depth, http://tvdg10.phy.bnl.gov/LETCalc.html.

19.

S. Guertin, “Single-Event Upset Test Results for the Xilnx XQ1701L PROM,” IEEE Radiation Effects Data Workshop, pp. 14–21 (1999) http://radnet.jpl.nasa.gov/reports/1/ReportFiles/Xilinx_R1701L.pdf.

20.

L. Scheick, “SEE measurement at Brookhaven National Laboratory for the SRAMs: WMS128K8 128*8, MT5C2568 32K*8, MT5C2564 64K*4,” http://radnet.jpl.nasa.gov/reports/1/ReportFiles/SRAMS.PDF.

21.

G. Swift “In-Flight Observations of Multiple-Bit Upset in DRAMs” http://radnet.jpl.nasa.gov/reports/1/ReportFiles/CassDRAM.pdf.

OCIS Codes
(160.4670) Materials : Optical materials
(210.0210) Optical data storage : Optical data storage
(210.2860) Optical data storage : Holographic and volume memories

ToC Category:
Research Papers

History
Original Manuscript: May 10, 2004
Revised Manuscript: May 24, 2004
Published: June 14, 2004

Citation
Y. Zhang, J. Butz, J. Curtis, N. Beaudry, W. Bletscher, K. Erwin, D. Knight, T. Milster, and E. Walker, "Characterization of a bit-wise volumetric storage medium for a space environment," Opt. Express 12, 2662-2669 (2004)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-12-12-2662


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References

  1. H. Zhang, E. P. Walker, W. Feng, Y. Zhang, A. S. Dvornikov, S. Esener, P. M. Rentzepis, �??Multi-layer optical data storage based on two-photon recordable fluorescent disk media,�?? Eighteenth IEEE Symposium on Mass Storage System, 225-236 (2000)
  2. S. Kawata, �??Three-dimensional Digital Optical Data-storage with Photorefractive Crystals,�?? Proc. SPIE 3470, 56-63, (1998) [CrossRef]
  3. M. Hisaka, H. Ishitobi, S. Kawata, �??Three dimensional optical recording with the ferroelectric domain reversal in a Ce-doped SBN:75 crystal: experiment and calculation,�?? Proc. SPIE 3740, 109-112 (1999)
  4. A. Toriumi, S. Kawata, �??Reflection confocal microscope readout system for three-dimensional photochromic optical data storage,�?? Opt. Lett. 23, (1998). [CrossRef]
  5. T. D. Milster, Y. Zhang, J. Butz, T. Miller, E. P. Walker, �??Volumetric Bit-Wise Memories, �?? NASA earth science technology conference (2002).
  6. T. D. Milster, Y. Zhang, C. D. Pinto, E. P. Walker, �??A Volumetric Memory Device based on Photo-Chromatic Compounds,�?? NASA earth science technology conference (2001)
  7. S. Hunter, �??Potentials of two-photon 3-D optical memories for high performance computing,�?? Appl. Opt. 29, 2058-2066 (1990) [CrossRef] [PubMed]
  8. D. C. Hutchings, �??Kramers-Kronig relations in nonlinear optics,�?? Opt. Quantum Electron. 24. 1-30 (1992) [CrossRef]
  9. E. W. Van Stryland, �??Characterization of nonlinear optical absorption and refraction,�?? Prog. Crys. Grow. And Chac. 27, 279-311 (1993) [CrossRef]
  10. H. Zhang, �??Single-beam two-photon-recorded monolithic multi-layer optical disks,�?? Proc. SPIE 4090, 174-178 (2000) [CrossRef]
  11. T. D. Milster, �?? Semi-kinematic rails for construction of optical test stands�??, SPIE Annual Meeting, San Diego, Aug. 2 (2001)
  12. For the detailed information of Arrhenius model and the failure rate, <a href="http://www.vishay.com/docs/rect_reliability.pdf">http://www.vishay.com/docs/rect_reliability.pdf</a> [CrossRef]
  13. D. Malacara, �??Optical Shop Testing,�?? Wiley series in pure and applied optics, 2nd edition
  14. J. Barth, �??Ionizing Radiation Environment Concerns,�?? conference of single event effect criticality analysis, <a href="http://radhome.gsfc.nasa.gov/radhome/papers/seeca3.htm">http://radhome.gsfc.nasa.gov/radhome/papers/seeca3.htm.</a>
  15. T. Miyahira �??Initial SEE Tests of SanDisk Flash Memory,�?? <a href="http://radnet.jpl.nasa.gov/reports/1/ReportFiles/078-SanDisk2.PDF">http://radnet.jpl.nasa.gov/reports/1/ReportFiles/078-SanDisk2.PDF.</a>
  16. J. Coss, �??Device SEE Susceptibility Update: 1996-1998,�?? <a href="http://radnet.jpl.nasa.gov/reports/1/ReportFiles/Update.PDF">http://radnet.jpl.nasa.gov/reports/1/ReportFiles/Update.PDF.</a>
  17. T. Miyahira, �??Summary of SEE test results from BNL heavy ion test,�?? <a href="http://radnet.jpl.nasa.gov/reports/1/ReportFiles/9902bnl.PDF">http://radnet.jpl.nasa.gov/reports/1/ReportFiles/9902bnl.PDF.</a>
  18. For more information of calculating penetration depth, <a href="http://tvdg10.phy.bnl.gov/LETCalc.html"http://tvdg10.phy.bnl.gov/LETCalc.html</a>.
  19. S. Guertin, �??Single-Event Upset Test Results for the Xilnx XQ1701L PROM,�?? IEEE Radiation Effects Data Workshop, pp. 14-21 (1999) <a href="http://radnet.jpl.nasa.gov/reports/1/ReportFiles/Xilinx_R1701L.pdf">href="http://radnet.jpl.nasa.gov/reports/1/ReportFiles/Xilinx_R1701L.pdf.</a>
  20. L. Scheick, �??SEE measurement at Brookhaven National Laboratory for the SRAMs: WMS128K8 128*8, MT5C2568 32K*8, MT5C2564 64K*4,�?? <a href="http://radnet.jpl.nasa.gov/reports/1/ReportFiles/SRAMS.PDF">http://radnet.jpl.nasa.gov/reports/1/ReportFiles/SRAMS.PDF.</a>
  21. G. Swift �??In-Flight Observations of Multiple-Bit Upset in DRAMs�?? <a href="http://radnet.jpl.nasa.gov/reports/1/ReportFiles/CassDRAM.pdf">http://radnet.jpl.nasa.gov/reports/1/ReportFiles/CassDRAM.pdf.</a>

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