A simple non-interrupting method of measuring pulsed electron beam current in low energy electron linac

NUCLEAR INSTRUMENTS

AND M E T H O D S 33 (1965) 3 4 3 - 3 4 4 ;

A SIMPLE NON-INTERRUPTING METHOD

© NORTH-HOLLAND

PUBLISHING

CO.

OF M E A S U R I N G P U L S E D E L E C T R O N BEAM

C U R R E N T IN L O W E N E R G Y E L E C T R O N LINAC P. SUBRAHMANYAM and P. AMMIRAJU Tata Institute of Fundamental Research, Bombay, India

Received 20 January 1964 Absolute current measurement. An absolute measurement of electron beam current is carried out with a Faraday cage and a d.c. micro-volt-ammeter (Hewlett Packard Model 425-A). Fig. 1 shows the arrangement. The Faraday cup consists of 6" x 4" x 4" graphite block with a hole of 1½" dia. and 2" deep in the front face to minimise the loss of electrons due to back scattering. D

GRAPHITE ABSORBER

m LEADSHIELDING FIG. NOTTO SCALE

~

ELECTRON BEAM

TO

R~N RECORDER

/ ~~'~'__~~'PORT

HOLES

Fig. I. Graphite absorber inside the chamber. The diameter of the hole is fixed from the measured beam profile which is determined by measuring with a photo diode the optical coloration of 2 mm thick transparent glass plates exposed to the electron beam. The dimensions of the Faraday cup are so chosen 1) as to completely stop the 3.5 MeV electrons. Graphite is chosen for its low scattering properties due to low atomic number and good electrical conductivity. It is placed in an evacuated scattering chamber (pressure < 10 -6 m m Hg) from which it is insulated by teflon, the chamber itself being isolated from the diffusion p u m p and the body of the accelerator by means of perspex rings. The sides of the cup are covered by lead blocks ~ thick to absorb any gamma-rays and subsequent electrons produced by the fast electrons stopped in graphite. The accelerator duty cycle is 2 x I0 -4. The electron gun is pulsed by a 50 kV, 2/~s pulse. Since only a fraction of all the elcctrons emitted from the gun are bunched, there will be a sizable bunch of 50 keV electrons present in the 3.5 MeV beam. Since these are not part of the beam current of interest, they are effectively stopped by a 6 mg/cm 2 AI foil placed in front of the Faraday cup. The variation of the measured cup current I as a

1it

function of gun filament current (varied from 5.5-7.3 A) after making the necessary corrections to be described further on, is shown in fig. 3. As one would expect the cup current closely follows the variation of thermionic emission behaviour. The cup current I as measured above should be corrected for the loss of electrons through back scattering and bremsstrahlung production. For the geometry of the cup the inner back scattering angle 20 in the graphite is approximately 44 ° so that assuming 2) an angular distribution per unit solid angle, which is proportional to cos0, then the part of the back scattered radiation for 2 0 is 13~. The back scattering coefficient of AI is 5% for 1.75 MeV electronsZ). Since the back scattering coefficient decreases with decreasing Z and increasing electron energy, we can assume a back scattering coefficient for graphite at this energy a value approximately 2%. Using this the correction factor comes out to be 1.0026. To estimate the correction due to bremsstrahlung, a second graphite cup is placed next to the main cup and the residual current i in this is measured. The correction factor is simply ( I + i)/1 which is found to be < 1% for all filament currents. The correction for residual gas ionisation is also very small as the residual gas pressure is < 10 -6 mm Hg. Further the effect of any stray radiation to falsify the current measurement is eliminated as the Faraday cup is electrically isolated from the outside environment. Current monitor. A ~ " thick and 6" × 6" brass sheet bent in a sector form is placed in the forward beam direction inside the chamber and is electrically isolated. Fig. 2 shows the arrangement. The current from the [ ] BRASS MONITOR FIG NOT TO SCALE

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BEAM

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E

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Fig. 2. Brass monitor. 343

P. SUBRAHMANYAMAND P. AMMIRAJU

344

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Fig. 3. Electron beam current as a function of electron gun filament current. brass monitor is shown in fig. 3. The current closely follows the cup current. In any experiment such as electron scattering or bremsstrahlung measurements using thin targets where measurements are not usually

done in the forward direction, this brass monitor with holes at the appropriate port holes in the scattering chamber except in the forward direction, will serve as a simple and effective non-interrupting beam current monitor. We have also measured the current collected by the external scattering chamber without any graphite absorber or the monitor inside, to find the feasibility of using it as a continuous monitor. The current recorded by this is found to be very small. In fact, for a filament current of 7 A, the beam current in this case is less than 10% of the monitor current. Also the external chamber is more vulnerable to the influence of external radiation like electromagnetic pick-up in the accelerator room. The leakage current between the brass monitor and the chamber is found to be of the order of 0.003 pA/V, where as it is of the order of 2 p A / V between the external chamber and ground, which further demonstrates the effectiveness of the brass monitor.

References l) j. Fleeman, N.B.S. Circ. 527 (1954) 91. 2) H. Frank, Z. Naturf. 14a (1959) 247.