?/vl - noo 
 
 UNCLASSIFIED 
 
 UNCLASSIFIED 
 
 ORNL-1700 
 
 Subject Category: PHYSICS 
 
 UNITED STATES ATOMIC ENERGY COMMISSION 
 
 EFFECT OF RADIATION ON THE 
 DIELECTRIC CONSTANT AND ATTENUATION 
 OF TWO COAXIAL CABLES 
 
 By 
 
 R. A. Weeks 
 
 D. Binder 
 
 March 19, 1954 
 
 Oak Ridge National Laboratory 
 Oak Ridge, Tennessee 
 
 Technical Information Service, Oak Ridge, Tennessee 
 
Date Declassified: February 23, 1955. 
 
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 GPD B2Z900 - 1 
 
OML-1700 
 
 Contract No. W-7405-eng-26 
 
 SOLID STATE DIVISION 
 
 EFFECT OF RADIATION ON THE DIELECTRIC CONSTANT AND 
 ATTENUATION OF TWO COAXIAL CABLES 
 
 R. A. Weeks and D. Binder 
 
 ABSTRACT 
 
 Measurements have been made on radiation induced changes in the phase 
 constant and attenuation of two coaxial cables while being irradiated. The 
 measurements were made in the region of four megacycles. At this frequency 
 the change in dielectric constant was (1.4 + 0.4) f> for both dielectrics 
 after roughly 2 x 10^° nvt. The change in attenuation was (9 + 2) "jo for 
 polyethylene and within the range of error for teflon. The phase constant 
 and attenuation were found by measuring the input impedance of an open-ended 
 length of cable in the neighborhood of its quarter -wave frequency. Assuming 
 a uniform cable dielectric and no other variables the input impedance has a 
 minimum at this frequency. From the minimum the attenuation and phase con- 
 stant are found. 
 
 March 19, 1954 
 
 Oak Ridge National Laboratory 
 operated by 
 Carbide and Carbon Chemicals Company 
 A Division of Union Carbide and Carbon Corporation 
 Post Office Box P 
 Oak Ridge, Tennessee 
 
 xix 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation 
 
 http://archive.org/details/effectofradiatio3043oakr 
 
EFFECT OF RADIATION ON THE DIELECTRIC CONSTANT 
 AND ATTENUATION OF WO COAXIAL CABLES 
 
 R. A. Weeks and D. Binder 
 Oak Ridge National Laboratory, Oak Ridge, Tennessee 
 
 Introduction 
 
 Plastic dielectrics in the form of coaxial cables are often used in and 
 around radiation sources. Many of the physical properties of such plastics have 
 been studied*' ' as a function of irradiation in neutron and gamma ray sources. 
 It is of some interest to extend these studies to the effect of radiation on 
 certain electrical properties, dielectric constant and attenuation, of importance 
 in high frequency applications. 
 Experimental Method 
 
 An accurate technique of measuring relative changes in the dielectric con- 
 stant and attenuation in coaxial cables during irradiation has been devised. 
 With this technique and the theory developed below, it was possible to determine 
 changes in the relative value of the attenuation and specific dielectric constant, 
 e', with an error less than 2% and 0.1$ respectively. Two types of coaxial 
 cable have been inserted in the ORNL graphite reactor and measurements of 
 attenuation and e 1 have been made during irradiation. One of the cables was 
 type RG-ll/U with a polyethylene dielectric. The other cable was a high 
 voltage cable with a teflon dielectric. Its characteristic impedance, Z , was 
 approximately 50 ohms. The latter cable was irradiated for approximately 700 
 hours (Fig. II) and the RG-ll/U for approximately 900 hours in a flux of 9.2 x 
 10 11 thermal neutrons (Fig. III). The observed e' increased about 1% for both 
 
 (1) 0. Sisman, C. D. Bopp, "Physical Properties of Irradiated Plastics", ORNL-928 
 Refer to Bibliography for other reports. 
 
 - 1 - 
 
- 2 - 
 
 cables. The observed attenuation of the teflon cable showed little change 
 during the period of irradiation. The observed change in attenuation of the 
 SG-ll/U cable was approximately 5>%* The measurements were made in the region 
 of four megacycles. 
 Theory of Measurement 
 
 The equations pertinent to the technique are developed below. The input 
 impedance of a transmission line of length X is 
 
 Z( /) coshY,/ k z o sinh Y./ 
 (1) Z^ « Z z(/ ) sinhYi f Z cosh yj 
 
 where 
 
 Zj^ = input impedance 
 
 Z(/) = impedance of load 
 
 Z * characteristic impedance 
 
 V = propagation constant 
 
 For low loss lines 
 
 Y« C*+ i/3 
 
 <*■ - attenuation per unit length 
 
 /3 = cu f/7c 
 
 Ld « frequency in radians per second 
 P ■ permability 
 
 ^ ■ 1 (for dielectrics of interest here) 
 ^- - dielectric constant 
 
 (2) George C. Southworth, "Principles and Applications of Waveguide Transmission". 
 p. 39-72, 1950, D. Van No strand Co. 
 
 (3) G. G. Montgomery, et al, "Principles of Microwave Circuits", p. 67, 19U3 
 McGraw Hill. 
 
- 3 - 
 
 For an open ended line, Z{$ )» Z , equation (1) becomes 
 
 (2) Z^ * Z cothy/ 
 
 Then if 
 
 ^ s JL^L , n . x, 2 , 3, ... 
 
 equation (2) becomes 
 
 (3) Zin ■ Z tanh */ 
 At the quarter-wave frequency, n » 1 
 
 where c is the velocity of light, Vh is the angular quarter-wave frequency, 
 f is the length of the line, e is the dielectric constant of free space, and 
 e' is the specific dielectric constant. 
 Equation (2) can be written 
 
 (6) p& , I fe e ~ 2 *' ( cos 2 ^ - 1 s^ 2/8/ ) 
 ^° 1 - e- 2 (cos 2/5/ -i sin 2/3/ ) 
 
 When 
 
 (7) 2/S/s^ A 9, 4© « 7/ 
 
 and retaining only first order terms equation (6) becomes 
 
 (8) ha z ac/i L&1 
 
 The absolute magnitude is 
 
 and when A © e 
 
 (10) |f * *' 
 
- h - 
 
 At the quarter wave frequency the magnitude of t? — goes through a minimum. 
 By plotting \'^ u ^\ versus frequency, one can find both the attenuation and the 
 quarter-wave frequency. A curve drawn from equation (9) is compared with ob- 
 served values of hf 111 / in Figure IV. The agreement between observed and 
 
 calculated values of z i J1 is quite good. The value of &■!( used in the equation 
 
 z o 
 was that found from the minimum in the experimental datajC-f ■ 0.016°. 
 
 Apparatus 
 
 A block diagram of the circuit used to measure lir" 1 / is shown in Figure I-A. 
 The procedure of the experiment was set up so that h^— / could be found from 
 two voltages that were easily measured. In Fig. I-B the signal generator is 
 treated as a Thevenin voltage source^'. With the terminals open the potential 
 across them is 
 
 (11) V ■ V X 
 
 where V, = Thevenin voltage. With the cable attached to the generator terminals 
 the Thevenin voltage is increased until the potential across the terminals is 
 the same as that in equation (11). Then 
 
 (12) V * i Z^ 
 
 where i ■ current, and the new Thevenin voltage is 
 
 (13) V 2 = i (Z g f Zin) 
 
 where Zg is the generator impedance. Substituting i = ^r ^ equation (13), 
 we have for the input impedance 
 
 v l Z g 
 C 11 *) Z in ■ v 2 -\ 
 
 At the quarter i^ave frequency (lit) becomes 
 
 (15) Z tanhot/ a y My 
 
 (U) George C. Southworth, "Principles and Applications of Waveguide Transmission 
 p. 26, 19!?0, D. Van Nostrand Co. 
 
-5 - 
 
 If the generator impedance is matched to the characteristic impedance of the 
 cable, Z - Z , tanned « <*/ , then 
 
 (16) «/ 
 
 v. 
 
 1 
 
 v 2 -Vx 
 
 All that is necessary is to measure Vi and V2 in the region of %/),• In- the 
 signal generator used in these measurements, a General Radio 60^-B, a calibrated 
 attenuator preceding the output terminal was used to measure the relative magni- 
 tudes of V]_ and V?. ^ ie signal generator impedance was matched to the cable by 
 a series resistor of appropriate value. The incidental shunting capacitance was 
 negligible at the frequencies used. The value of the series impedance matching 
 resistor was found by measuring the impedance of the generator and the character- 
 istic impedance of the cable. Since the cable impedance was larger than the 
 generator impedance for both cables, the matching resistance was in series with 
 the generator impedance. The cathode follower, receiver, and VTVM shown in Fig. 
 1A were used to set the voltage V at a constant value. This was done by modulat- 
 ing the output of the signal generator; a small fraction of the r-f signal being 
 passed by the cathode follower to the receiver, a National HRO-60, the modula- 
 tion component then going to the VTVM. A cathode follower was used because of 
 its high input impedance, approximately 100 megohms. The deflection of the 
 VTVM was kept constant for the conditions of cable connected and disconnected 
 by varying the calibrated attenuator of the signal generator. The frequency 
 was measured by the receiver tuning condenser of the HRO-60 which had been 
 calibrated. 
 
 The advantages of this method of measuring relative changes in e* and 
 attenuation are: a) with one measurement the relative magnitudes of e' and 
 -attenuation can be determined with an accuracy not achievable by other methods^ 
 b) instruments and personnel can be shielded from the radiation source dur?^" 
 
- 6 - 
 
 irradiation of cable, c) measurements can be made during irradiation, and d) 
 standard instruments can be used. The major disadvantages are: a) the cable is 
 not uniformly irradiated, and b) there is a temperature gradient along the length 
 of the cable. However, the conditions of irradiation are similar to those that 
 would exist if the cables were used in and around a radiation source. 
 Cable Measurements 
 
 Using the above method changes in e' andof/in two coaxial cables have 
 been measured during irradiation in the ORNL graphite reactor. The cables were 
 inserted in Hole B of this reactor and the measurements made over a period of 
 approximately 700 hours (approx. 1.6 x lCr-° nvt) for one cable and 900 hours 
 (approx. 2,1 x 10-'-" nvt) for the other. The one irradiated for the 700 
 hour period had teflon as the dielectric, the other had polyethylene. The 
 results for the two cables are shown in Figs. II and III. It can be seen from 
 these figures that the effect of radiation on the "measured e' " is small, the 
 total observed change for both cables is approximately 1% of the initial value. 
 The change in e 1 is proportional to twice the change in Hj/i i.e., 
 
 (17) -A*' - £-J& 
 
 vu 
 
 The "measured pCf " for the teflon cable exhibited changes that were not outside 
 the range of error for theperiod or irradiation. The polyethylene cable had 
 a total change in the "measuredo(/ " of approximately 9%, 
 
 In order to determine whether there were transient effects resulting from 
 variations in the flux around the cable measurements were made during the weekly 
 shutdowns of the reactor. These were then compared to measurements made just 
 prior to shutdown and just after start-up. In the teflon cable no significant 
 change in e' and e*/ could be detected between the reactor-on and reactor-off 
 
-■? - 
 
 conditions. In the HG-ll/O cable there were observable differences in the value 
 of e' but none in &H between the two conditions . These differences were much 
 smaller than the total change in e' for the period of irradiation. The change 
 in e' for the two conditions seems to be correlated with the change in the 
 temperature of the irradiated section of the cable. This temperature effect 
 can be seen by comparing adjacent points on the f ^ /, graph, Fig. Ill, for 
 which there was a change in the average temperature of the irradiated section 
 of the cable. The changes in e' are of the same sign as the changes in average 
 temperature . 
 
 The average temperature is the average of the temperature of the pile inlet 
 air and outlet air. The air coolant flowed through the channel in which the 
 cables lay, the inlet air being at one end of the cable and the outlet air at the 
 other. Hence the average of the inlet and outlet air temperatures is an 
 approximate measure of the average temperature of the cable. At full pile power 
 the difference between inlet and outlet air temperature was approximately 70°C. 
 
 The reason for speaking of "measured e' " and"measured ©</ " arises from 
 the conditions of the experiment. The entire length of the cables was not 
 irradiated. Approximately 60$ of the cable length was in the active lattice of 
 the reactor, the remaining k-0% passed through the reflector and shield of the 
 reactor and to the instruments. In addition the flux distribution across the 
 active lattice is not uniform. It has a maximum flux of 9.2 x lCr- 1 thermal 
 neutrons at the center, falling off to zero at the reflector. As a result of 
 these conditions the cables should be considered as having, to a first approxi- 
 mation, two dielectrics after the irradiation began. Hence the "measured e' " 
 and "measured 0(-r " are not those of the irradiated section but of the combination 
 of the unirradiated and the irradiated section. The "measured <*( " is simply 
 the arthmetic sum of the oL$ 's of the two sections. The "measured e' " is a 
 
somewhat involved combination of the e's of the two sections. In view of the 
 small change in the "measured e' " it seems reasonable to assume that the ob- 
 served change is approximately that of the irradiated section. Since 6($ of the 
 cable was irradiated the "measured d-S " is approximately 60Jo of the frQ of the 
 irradiated section. Hence the change of 9% in the "measured ofj " of the poly- 
 ethylene cable would be approximately 1%% change ino<* for the irradiated 
 section. These changes in e' and c<i! observed in the region of four megacycles 
 should not be extended to other frequencies. Some preliminary measurements on 
 teflon and polyethylene in the microwave region before and after irradiation in- 
 dicate that changes in e' may be of the same order of magnitude as at four mega- 
 cycles. -The increases in attenuation are much greater for shorter periods of 
 
 irradiation at microwave frequencies than those observed at the lower frequency. 
 
 (6) 
 
 Teflon exhibits quite radical changes in its mechanical properties for 
 
 an integrated flux of lCP-° nvt whereas the effects of radiation damage begin to 
 show up in polyethylene^ ' ' between l(r-' and lCr-°nvt. In the above measurements 
 of e' and W/ the cables were inserted in a horizontal hole in the pile and were 
 not moved until the end of the tests. Thus they were subjected to no mechanical 
 strains. In view of the small changes observed in e' and <AX (in the region of 
 four megacycles) in these two materials as a result of irradiation the criteria 
 of their usefulness in a radiation field would be the effect of this radiation 
 on their mechanical properties. 
 Acknowledgments 
 
 The method used in these experiments was suggested by Mr. B. R. Gossick. 
 We gratefully acknowledge his suggestion. Discussions with R. L. Sproull were 
 very helpful. We wish to thank him for his interest. 
 
 (6) 0. Sisman and C. D. Bopp, "Physical Properties of Irradiated Plastics", 
 ORNL-928, p. 88-92. 
 
 (7) op. cit., p. 78-82. 
 

 
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 CABLE INPUT IMPEDANCE vs. FREQUENCY ABOUT f 
 
 % 
 
 4.II8C 
 
-13- 
 
 BIBLIOGRAPHY* 
 
 (1) 0. Sisman, C. D. Bopp, Physical Properties of Irradiated Plastics, ORNL-928 
 
 (2) J. G. Burr, W. M. Garrison, The Effect of Radiation on Physical Properties 
 of Plastic (19U3) AECD-2078, (decl. 19U8) 
 
 (3) A. 0. Allen, D. M. Richardson, Effect of Clinton Reactor Radiation on 
 Plastics, CNL-16 (19U8) 
 
 (U) M. 3urton, Effects of High-Energy Radiation on Organic Compounds", J. of 
 Phys. Col. Chem., $1, 786 (19k7) 
 
 (5) J. W. Ryan, Effects of Radiation on Organic Materials - II, CEL-57 (1952) 
 J t W. Ryan, Effect of Gamma Radiation on Certain Rubbers and Plastics, 
 Nucleonics, 71, No, 8, 13-15 (1953) 
 
 (6) Memo: S. Gandon, et. al, "Gamma Irradiation of Insulating Materials", 
 
 ANL-OCS-75 (19U'7) 
 
 (7) J. S. Horsman, Summary of NRX Pile Irradiations, NP-1922, 1950 
 
 (8) F. E. Faris, A Compendium of Radiation Effects on Solids, Vol. II, NAA-SR-2hl, 
 1953 
 
 *This bibliography is not, by any means, complete. It is given only to indicate 
 some* of the reports available. 
 
 GPD 822900 - 2 
 
UNIVERSITY OF FLORIDA 
 
 3 1262 08906 3043