180
NOTES
VOl. GG
used in this work. Thc frcquencies of thc measured rotational transitions and the rotational constants are given in Table I together with the calculated frequencies using these constants. The rotational constants B and C were fitted t o Om -t 101, l o l -t 202, llo 211 and 111+ 212 transitions. The rotational constant A was obtained from b-type transition~.~ The Stark measurements are listed in Table 11. The dipole moment components were calculated from the measurements by the method of Golden and Wilson6 using the line strength tables of Schwendeman and Laurie.6 The dipole moment of the molecule is 0.46 D. The author gives sincere thanks to Dr. R. F. Curl, Jr., for the help received during the course of this work.
peak current in the pulse is -0.1 amp. for electron encrgies from 8 to 15 Mev. and the energy spectrum is constant within 0.5 Mev. of the peak energy. By lowering the current through the filament in the accelerator the dose rate can be reduced at any electron energy but with a reduction by a factor >50 the electron spectrum cannot be determined accurately. Calorimetry.-The calorimetric measurements have been carried out under geometrical conditions identical with those for the purely chemical measurcments,4 and were designed so that measurements of energy input and chemical change could be made successively on the same irradiation cell without changing the geometry. The ferrous sulfato solution was contained in a cylindrical Pyrex sample cdl (3.5 cm. diam., 3.5 cm. long containing about 25 ml. of solution) which is an analog of the irradiated part of the syringe used in the chemical studies.‘ This cell was suspended horizontally by Nylon threads inside an aluminum heating jacket, wound with a copper heating element, wluch in ita turn was mounted inside a container of polystyrene foam to provide a thermal insulation barrier. Copperconstantan thermocouples were attached to various parts of (4) The b-type asaignment waa made after learning from Mr. Ira the irradiation cell and located in grooves cut in thc aluLevino that it is possible to observe b-type transitions. minum heating jacket. ( 5 ) S. Golden and E. B. Wilson, Jr., J. Chem. rhus., 16, 008 (1948). The calorimeter was used adiabatically with prior 01.”:(6) R. H. Schwendeman and V. W. Laurie, “Line Strengths for trical calibration over the range of energy input anticiRotational Transitions,” Porgamon Press, London, 1958. pated in the radiation measurements. Adiabatic control waa maintained by placing a thermocouple from the sample in opposition to a jacket thermocouple across a sensitive A CALORIMETRIC DETERMINATION OF potentiometer and the electrical energy supplied to thc T H E OXIDATION YIELD OF TIIE FRICKE pclcet adjusted continuoucrly to maintain a temperature difference within the limits f l pvolt (fO.025O with copperDOSIMETER AT HIGH DOSE RATES OF constantan thcrmocouplea). None of the thermocouples wm calibrated absolutely aa the method of comparative ELECTRONS’“ measurements rcquires only reproducibility of thermocouples during calibration and subsequent measurementa. BY A. R. ANDERSON’) The temperature rise of -100 pvolta in each case was reChemistry Dzczsion, Argonne National Laboratory, Argonne, Ill. corded continuously by feeding the output from each of four Recewed Nouember 17. 1961 thermocouples through a d.c. amplifier (Honeywell, model 2 HLA-7) to a multipoint Honeywell recorder. One of the Accurate chemical dosimetry of ionizing radiation thermocouples mcasured the temperature rise of. the jaclcet, has not been established firmly a t the very high in- which should be equal to that of the sample ~f adiabatic stantaneous dose rates produccd by pulsed elec- conditions are maintained. I n the electrical calibration, energy was supplied continuously through a tightly wound tron accelerators. Measurements previous1 re- copper spiral immersed in water in the sample cell.whlle the ported by Keene,3a by Rotblat and Sutton21and radiation energy was supplied in short bursts with a freby Glazunov and Pikayev3 with aerated ferrous quency ranging from 0.2 to 20 pulses per second depending sulfate solutions in 0.8 N H 8 0 4 have shown a on the instantaneous dose rate. With good adiabatic conit was assumed that the two methods of heating would marked variation in the yield a t comparable dose trol give comparable thermal responses for the same energy rates, As part of the continuing investigations of input. radical diffusion kinetics at this laboratory, the radiFor measurements a t the accclerator the electron beam ation chemistry of simple aqueous systems has been first ivas degraded by passage through a polystyrene disc, 9.5 mm. thick,* and entered the sample cell uia holes cut i n studied a t high dose rates of electrons4 requiring the front of tho outer container and in the heating jacket. an accurate and consistent method of chemical A second hole cut in the aluminum jacket dircctly opposite dosimetry. Consequently we have made a calori- the first, together with a detachable rear section of the outer metric determination of G(E’c3+) for the Frickc container, permitted the visual positioning of the calorimeter by viewing the discoloration spot produccd in a watch dosimeter (0.001 Ai! FeS04, 0.001 NaCl, 0.8 N glass by intense electron bombardment. Once the calorim1&SO4, aerated), not as a comprehensive study of eter had been centered with respect to the beam it was dosimetry problems a t very high dose rates but fixed rigidly and arcew to the sample cell for pipetting specifically to provide dosimetry for the irradia- solutions wus provided by a detachable cover on the outer and by a tightly fitting removable lid on the xlntion conditions applicable to our chemical studies. container minum heating jwket. In this way a series of consecutive Experimental calorimetric and ferrous sulfate oxidation mrasuremcnts All irratiiaLions were carried out with the Argonnc Linc~:ir was made without changing the fixcd geometrical condiElectron Accelerator (Applied Physics Corporation) which tions. generstcs electrons in pulses from 1.4 pscc. to 5.5 psec. duration. Our measurcmcnts have been made with -15 Analysis.-Ferric ion concgntrations were detcrMcv. electrons at a pulse length of -1.4 ~ e c which . h:ts a mined spectrophotometrically at a wave length Gf rise time of 0.2 psec. and a decay time of 0.4 psec. The area of the beam emerging from the exit window is 1 em.*, the 3040 A., where the molar extinction coefficient is
fir
(1) (a) Work psrformed under the auspices of the U.S. Atomic Energy Commission: (b) Chemistry Division, A.E.R.E. Harwell, Nr. Didoot, Berks., Ellgland. (2) (a) J. P. Keene. Radialdon Reeearcfi, 6. 424 (1957); (h) J . Rotblat and €1. C. Sutton, Proc. Rov. Soe. (London), Aa66, 490 (1900). (3) P. Ye. Glazunov and A. Ii. Pikayev, Doklady A W . Nauk, 130, (5) 1051 (1960). (4) A. R. Anderson and Edwin J. IIart, J. Phus. Chsm., 66, 70 (1962).
2225 a t 25” and the temperature coefEicient 0.7% per degree. Definition of Dose Rate.-We have used the energy averaged mean intensity as given by Rotblat and Sutton2b to define our mean instantaneous dose rate. The contours of the irradiated volumc were determined from the color imparted to a block of Plexiglas (dcnsity 1.2 g. ml.-l) by the electron
NOTES
Jan., 1962
181
samc for measurrment,s made on the sample cell and on the heating jucltct. Thcse ohmvations show that, adiabatic conditions were being maintained with both continuous and pulsed energy Optiasl Donsity input. The results of all calorimetric measurc-----------% (cm.)------0 732 1 524 2 30 3.14 ments a t the accelerator are given in Table 11. 1 . ~ 6 0 1.117 1 007 0.6%~ A t each mean instantaneous dosc rate the rate 1.148 1.105 1.000 -677 of temperature rise was measured a t different pulse 1.046 1.068 0 975 .65,5 rates. With constant machine conditions and re1.013 1.000 ,928 .609 producible adiabatic control the relationship be0.813 0.889 ~ ~ 4 0.537 tween rate of energy absorption and pulse rate a t .503 ,663 ,710 .46o each instantaneous dose ratc should be constant .250 .398 .555 .377 as is shown in Table 11, column 4. As the ferrous .I28 .225 .395 .297 sulfate oxidation measurements required only .221 about 1 to 2% of the number of pulses needed to .078 .I29 GO TABLE I1 CALORIMETRY MEASUREXBNTS A N D FERRICYIELDS
TABLE I DATAWOM I R ~ A D ~ A T E pr,nxIctr,,+S D WavrIcnKth,s9W A., is radial distance from rcnwr of disc; 2 i R distmre from point of entry of tho beam. O W I C A I ~ DENRITY
0 00
dcrn.)
oo
1.138 1.112 1.045 .912 ,604 .276 .io2 .036 .017
,20 .40 .60 .go
1 .00 1.20 1.40 1.60
No. of pulses per min.
Meusiirement
Cal." Cal." Cal." Fe24* Cal." Cal." Fez+' Cal." Cd." Fel*
1
600 900 1200
4.81 7.80 10.15
0.49 0.53 0.515
75 100 150
4.57 6.54 9.41
3.74
Mean instantanooua dose r u b (ev. m1.-1
per
pulse
30 60
9.45 20.6
12.1 13 2
0.607f 0.009 14.6 .
5
20 30.2
3.90 5.24 7.90
14.5 f0.4
4.31
1
11.3 f 0.5
14.1
}
12.2f0.5
18.2
J
* Mean of from 5 to 8 ferroi IS oxidation measurements.
beam, cutting the irradiated block into thin slabs (0.50 cm. thick) and measuring the radial distribution of optical density a t 3900 A. with a Reckman spectrophotometer. Some of t,he optical density data are given in Table I. These and other data were programmcd on an TRM computer to determine a mean optical density Dm for the irradiated volume of D, = f ( D d v ) D / f D d v which leads to an effective irradiated volume VEE. (as distinct from the total volume2h)defined by Vm. =
0.57
I
16.8 16.6 16.5 0 . 8 7 7 5 0.012
Calorimetry measurement.
15.2 f0.4
i
3.83
sec.-I
10-"I
Mean G'(Fe:+)
1]
4.00
Fe¶+b a
pe uiv
0.253 f 0.006
Cal." Gal." ~
temp. rise (pv. rnh-1)
Eoerm abs. in 0.8 N FIzSo4 (ev. m1.-1 per pulse X 10-16)
0.033 f0.005
Cd."
~
Rata of
(fD dv)'lSDa dv
I
01
MEAH
Fig. 1.-Variation
DOSE R A T E
J
I
I
io
io I
(.V
ml-'
00
I~c-')
of G(Fe*+)with mean instantaneous dose
rate. From two independent determinations the effective irradiated volumc was computed to be 15.3 f 1.6 ml. Thus the energy averaged mean instantaneous achieve a temperature rise of 100 ,.molts, they were made alternate to the calorimetric measurements, dose rates are given by but only the mean values are quoted in Table 11. total energy absorbed per pulse (ev.) The rate of energy absorption in the solution is cffcctive irradiated volume (ml.) X duration of pulse (sPc.), calculated from the measured temperature rise, a d are those pertraining to a pulse assumed to he the experimentally determined calibration value, rectangular. and the assumption that the total energy is disResults and Discussion.-Linear temperature us. tributed betwecn the solution and glass walls in time curves were obtained both during electrical ratio of their electron stopping powers. As a check calibration and electron bombardment. In both on the calorimetric techniques used and the ascases the rate of temperature rise was independent sumptions inherent in the oalculations, G(Fe3+) was of the thermocouples used for adiabatic control or determined in a region of dose rate where there is for temperature rise measurement and was the general agreement that the oxidation yield of the
Fricke dosimeter is constant. Our value of G(Fea+) = 15.2 f 0.4 at an instantaneous dose rate of 5.7 X 1021 ev. mL-l sec.-l agrees with the accepted value of 15.G =k 0.35within the limits of experimental error and lends confidence to the calorirnet,ric techniques used throughout this work. The ferric yields determined a t different instantaneous dose rates are plotted in Fig. 1, where it is seen that a t dose rates > 3 X loz2ev. mL-l see.-’, G(Fe3+) decreases rapidly with increasing dose rate while below ev. ml.-l sec.-l G(FeS+) is almost constant and approaches the accepted value for y and electron irradiations a t much lower dose rates. Other evidence from our chemical studies4 supports the implication from the ferrous sulfate measurements that a t dose rates > ev. ml.-l set.-', the homogeneous concentration of radicals throughout the solution is high enough for radical-radical reactions to compete with radicalsolute reactions. The data from the present work are intermediate between those given by Glazunov and Pikayev3 and by Keeneaa and by Rotblat and Sutton.2b I n the dose rate region where we have carried out most of our chemical ~ t u d i e s , ~i.e., from 1.3 to 2.2 X loz3ev. ml.-l sec.-l, our calorimetric data indicate that the oxidation yield of the Fricke dosimeter can be represented adequately as 11.4 f 0.5 ions per 100 ev. ?Vhile these calorimetric data are adequate for our relevant chemical studies it must be emphasized that they are valid only for the particular beam and sample geometry used in this work. More work is required to e s tablish unequivocal radiation dosimetry a t dose rates greater than ev. ml.-l sec.-l. Acknowledgments.-It is a pleasure to acknowledge helpful discussions with Drs. E. J. Hart and S. GordGn and experimental assistance from Miss V. Meyers. The author also thanks the U.K.A.E.A. for support on an exchange scheme during the course of this work. (5) C. J. Hochanadel and J. A. Ghormley, J . Chem. Phya., (1963).
ai,
880
USE OF KRYPTOS FOR SURFACE AREA MEASUREMENTS BYJ. M. IIAYNES* Research Departmen1,En~lzshClay8 Lovering Pochin & St. Austell, Cornmall, &ngland Received July 28, 1961
Although krypton is a solid a t 90 and 78”K., the two most commonly used adsorption temperatures, early users of this adsorbateb-6recommended thatJ the extrapolated liquid vapor prcssurc be adoptcd. It is shown in Table I that the two values differ considerably, and also that agreement between different measurements is not always good. The value of po used influences the slope (and linearity) of a B E T plot, and hence affects both the value of vm and the ease with which it is obtained. This can be demonstrated by calculating an isotherm according to the BET equation, using a “correct” value po for the saturation vapor pressure, and then replotting the calculated data using an I< incorrect” value po’. By suitable choice of PO’, BET plots curving in either direction arc obtainable. Malden and Marsh‘ derived ,Y values from curved BET plots by drawing a tangent to the curve in the region where v,, occurred. It is our purpose to show that although vm values derived in this way satisfy the “internal consistency check” used by Gaines and Cannon,2 they are dependent on the value chosen for po, and the extent of this depcndence varies inversely as the magnitude of the BET c-constant. Theory If a set of adsorption data obeys the BET cquation,’ the volume u of gas adsorbed per gram is related to the pressure p of non-adsorbed gas by thc equation y=-=
v(l
5
- 5)
AX + B
where z is the relative pressure p / p o , and A and B are constants ? n e n x is changed to 3’ = dx, t,he originally straight BET plot becomes curved, the equation to the curve being given by (ii)
A straight line on this graph is of the form (iii)
co. Ltd.,
The BET treatment of krypton adsorption data has been examined by several authors recently. In particular, Gaines and Cannon2 point out that to bring monolayer volumes of krypton into agrecment with those Obtained with other adsorbates, values of n K r , the molecular area of krypton, ranging between 17.7 and 22.0 A.2 have been adopted. We wish to draw attention to the importance of the value selected for po, 1he saturation vapor pressure of krypton at the adsorption temperature.
where A’ and B‘ are new values of t,he const-ants
If the straight line (iii) is to be tangential to the curve (ii) a t the monolayer point defined by (iii), three conditions must be met I. y‘ = y2 11. dy’/dx’ = d?/*/dX’ 111. v = v‘ = v,’
From condition I and equations ii and iii
*
Department of Physical and Inorganic Chemistry, The University. Woodlane Road, Brlbtol 8 , England. (1) P. J. hlalden and J. D. F. Marsh, J . Phya. Cham.. 63, 1309 (1959). (2) G. L. Gaines, Jr., and P. Cannon, %bid.,64, 997 (3) J. M. Thomas, Nafure. 189, 134 (1961). (4) P, Cannon and Gi L. Garnor, Jn, t b t k . 190, 340 (1081)‘
(1960).
(5) R. A. Beebe, J. B. Beckwith and J. M. Honig, J . A m . Chem. Soc., 67, 1554 (1945). ( 6 ) A. J. Rosenberg. ibid., 78, 2929 (1956). (7) 8. Brunaubri P. E, Emmett &nd E. Tdler, ibid,, EO, 300 (1038).
