A new effect, gas amplification of electron energy is reported here; namely
when a cylindrical pyroelectric crystal such as (LiNbO3) is contained in a
concentric cylindrical chamber and is heated and then allowed to cool in a
dilute gas the maximum energy of the resultant focused electron beam more than
doubles as the pressure increases from 0.05 to 4 mTorr for seven different
gases.Comment: 5 pages, 5 figures, pd

Brownridge, James D.

Shafroth, Stephen M.

English

arXiv

A new effect, gas amplification of electron energy is reported here; namely when a cylindrical pyroelectric crystal such as (LiNbO3) is contained in a concentric cylindrical chamber and is heated and then allowed to cool in a dilute gas the maximum energy of the resultant focused electron beam more than doubles as the pressure increases from 0.05 to 4 mTorr for seven different gases

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  1 Pressure Dependence of Energetic (≤160 keV) Focused Electron Beams Arising From Heated or Cooled (LiNbO3) Pyroelectric Crystals  James D. Brownridgea)  Department of Physics, Applied Physics and Astronomy, State University of New York at Binghamton, P.O. Box 6000, Binghamton, New York 13902-6000   Stephen M. Shafrothb)  Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3255   Abstract. A new effect, “gas amplification of electron energy” is reported here; namely when a cylindrical pyroelectric crystal such as (LiNbO3) is contained in a concentric cylindrical chamber and is heated and then allowed to cool in a dilute gas the maximum energy of the resultant focused electron beam more than doubles as the pressure increases from 0.05 to 4 mTorr for seven different gases.   Even though pyroelectric crystals have been known since ancient Greek times1, surprising new effects and practical applications are constantly being discovered1-12, for example; electrons accelerated by heating and cooling such crystals in dilute gases have produced energetic brehmsstralung and characteristic K x-rays up to Au.8 There is a voluminous literature on electron emission by pyroelectric crystals.  Reference 10, a comprehensive review article, is representative but does not suggest that such crystals can produce focused, energetic electron beams.  Much of the current industrial use of these crystals is in infrared detection, sensitive temperature-change detectors and photonics, e.g. the emission of THz radiation in LiNbO3 crystals11.   For these reasons few studies of the behavior of these crystals in dilute gases have previously been done so no one has yet discovered the phenomena reported here, namely that if a LiNbO3 crystal is heated      and then allowed to cool in a dilute gas, the electron beam energy and intensity  depends strongly on the gas pressure and less on gas type.   Fig. 1.  Experimental arrangement. (a) Thermocouple and heater wires. (b) Axial motion of crystal. (c) 62 ohm resistor. (d) Cylindrical LiNbO3 crystal. (e) 0.1 mm slit and surface barrier detector. (f) Motor, which moves the slit-detector system vertically. (g) Valve. (h) Gas container (i) Pump.                                      There are interesting practical applications of pyroelectric crystals in dilute gases for laboratory teaching4 of X-ray and electron physics.  The various parameters such as pressure, temperature etc. can be controlled by student-written LabView programs5. A further    ab c d efgh i  2 application of this work is to use a pyroelectric crystal to produce bremsstrahlung X-rays, which can fluoresce high-Z targets and produce very background-free characteristic K X-ray spectra9 of elements at least up to Pb. The experimental arrangement used for this work is diagrammed schematically in Fig. 1.  It was designed expressly to study the behavior of the focused electron beams, Vs pressure and gas type. Since results are geometry dependent, we used a cylindrical LiNbO3 crystal, 10 mm x 4 mm dia. whose + z base was epoxied to a 62 ohm resistor.  This assembly, which could be moved along the axis, was mounted axially in a glass cylinder 45 cm x 7.5 cm dia.  The chamber was capped by metal end plates.  The crystal is heated to ~180 oC by passing a current (~120 mA) through the resistor.  The crystal then cools naturally.  The electrons are detected by a fully depleted 100 μm surface barrier detector located behind a 0.1 mm slit. Varying the distance between the slit and the crystal does not significantly effect  Fig.  2.  Electron spectra at different temperatures and pressures as the crystal cooled.  The pressure reduction began between (g) and (h) and continued through (i).  the electron energy until the distance is ≤5 mm.  The slit-detector system can be moved vertically across the beam  (See ref. 7).  These data were taken with the slit located at the focal point of the beam, i.e. ~22 mm.  Typical electron  spectra taken at 2 mTorr N2 and different temperatures as the crystal cooled are shown in Fig. 2(a-g) The lowest energy peak at energy E is the true energy of the electrons.  The peaks at 2E and 3E are due to pile up.  (See ref. 6 for additional spectra).  Figure 2(h and i) show the result of reducing the pressure from 2 mtorr to 0.2 mtorr.  The electron energy     Fig.  3.  Crystal temperature as indicated by thermocouple, solid circles, electron counting rate, solid squares, (log scale) and pressure, open squares.  Inset:  Log of electron count rate and gas pressure when the gas inlet valve was closed.   drops by almost a factor of two.  Figure 3 shows the electron counting rate (on a log scale) Vs. time.  It also shows the cooling curve for the crystal and the pressure in the chamber.  It can be seen from the inset that at 2470 sec the pressure is reduce over a period of ~ 30 sec by closing the leak valve to the gas supply which causes the electron count rate to increase dramatically and then fall to below its former value while simultaneously the beam is also defocused.  This change in electron count rate and electron energy as the pressure is reduced is also shown in fig. 2(g-i).  0 100 200 300 4000K2K4K6K8K (a)  2 mTorr71 keV at 90 oC0 100 200 300 40002000400060008000  (b)  2 mTorr88 keV at 80 oC0 100 200 300 40002000400060008000  (c)  2 mTorr117 keV at 60 oC0 100 200 300 4000K2K4K6K8K  (d)  2 mTorr133 keV at 45 oC0 100 200 300 40002000400060008000  (e) 2 mTorr134 keV at 35 oC0 100 200 300 40002000400060008000  (f)  2 mTorr131 keV at 32 oC0 100 200 300 4000K2K4K6K8KElectron Energy (keV)Counts  (g)  2 mTorr130 keV at 30 oC0 100 200 300 40002000400060008000  (h) 0.5 mTorr71 keV at 29.5 oC0 100 200 300 40002000400060008000  (i)  0.2 mTorr68 keV at 28 oC0 400 800 1200 1600 2000 2400050100150200250300 ~0.1 mTorrPressure ~1 mTorrTime (sec)Temperature (oC)0.111010010002425 2450 2475 2500 25250100200300 ~ 24 oC Time (sec)0.11101001000~ 24 oCRelative count rate (c/sec)  3        Fig.  4.  Electron energy Vs crystal temperature at 3 different pressures as the crystal cools.   Figure 4 shows the electron energy as a function of temperature, which is directly related to cooling time (see fig 3) at three different pressures.  At the lowest pressure (8x10-7 Torr) the electron energy slowly increases as the crystal cools.  In this case the electron energy is more closely proportional to the exposed surface polarization charge3 density.  If the pressure is increased to 1.67 mTorr the electron energy slowly increases from ~50 keV to ~ 100 keV as the crystal cools from 900 C to 250 C.  This is the gas amplification effect.  We believe it is related to the formation of plasma at the crystal surface.  Finally at 2.3 mTorr the electron energy increases further until the crystal cools to about 30 oC and then starts to drop with further cooling.  Again this is the result of the net charge on the crystal surface (polarization and adsorbed ions) as well as the plasma at the surface.              Figure 5 summarizes the main effect,” gas amplification of electron energy”, reported in this paper, which as far as we know has not previously been reported in   Fig.  5.  Maximum electron energy Vs pressure for 7 gases. Each data point is the maximum electron energy observed as the crystal cooled from ~180 oC at the pressure indicated.   the literature. Data points were collected after adjusting the pressure and then cycling the crystal once completely over a thermal cycle.   It shows that the maximum electron energy more than doubles in the range from 0.05 to 4 mTorr for a variety of gases.  The maximum electron energy then levels off and decreases with further pressure increase.  Also the rate of increase in energy with pressure starts out the same for most gases (~50 keV/mTorr) whereas in the case of He it is ~ 25 keV/mTorr starting at 1.5 mtorr, and in the case of Ar it is 57 keV/mTorr.   We are most grateful to our colleagues, Sol Raboy, Tom Clegg and Eugen Merzbacher for insightful discussions, and continued encouragement.  a) e-mail jdbjdb@binghamton.edu  b) e-mail shafroth@physics.unc.edu    10 20 30 40 50 60 70 80 90 100406080100120140160Energy (keV)Temperature (oC) 8x10-7 Torr 1.67 mTorr 2.30 mTorr0.0 2.0 4.0 6.0 8.0 10.0 12.06080100120140160180O2ArN2HeEnergy (keV)Pressure (mTorr) H2 He O2 N2 Ar CO2 Air  4  References  1.   S. B. Lang, Sourcebook of Pyroelectricity, Gordon and Breach, New York 1974  2.  J. D. Brownridge, Nature (London) 358, 278 (1992)  3.  James Brownridge, and Sol Raboy, J. Appl. Phys. 86, 640  (1999)  4.  S. M. Shafroth and J. D. Brownridge, CP 475 American Institute of Physics, Applications of Accelerators in Research and Industry, Edited by J. L. Duggan and I.L. Morgan, 1100 (1999)  5.  S. Bedair, S.M. Shafroth and J.D. Brownridge, Bull Am Phys Soc 47, 65 (2002)  6.  J. D. Brownridge, S. M. Shafroth, D. Trott, B. Stoner, W. Hooke, Appl. Phys. Lett, 78, 1158 (2001)  7. J. D. Brownridge and S. M. Shafroth, Appl. Phys. Lett. 79, 3364 (2001) 65 (2002)  8.   J. D. Brownridge and S. M. Shafroth, International Conference on High Power Electron Beam Technology, Hilton Head, Oct 27-29 (2002) and           http://www.arxiv.org/abs/physics?0209079  9.   J. D. Brownridge and S.M.        Shafroth, private communication (2003)  10. G. Rosenman,  D. Shur,  Ya. E. Krasik and A. Dunaevsky,           J. Appl.  Phys. 88, 6109 (2000).  11 Y.Sasaki and A. Yuri, App. Phys.           Letters, 81, 3323 (2002)   12     J. D. Brownridge, Video clip, Dynamical Behavior of Beam as Pressure Increase, http://www.binghamton.edu/          physics/brownridge.html        

Pressure Dependence of Energetic (160 keV) Focused Electron Beams Arising From Heated or Cooled (LiNbO3) Pyroelectric Crystals

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