Determination of Water, Silalnol, and Strained Siloxane on Silica Surfaces Gene E. Kellum and Robert C. Smith Dow Corning Corp., Midlund, Mi&. 48640 The surface of various silicas was characterized by determination of adsorbed water, silanol groups, and strained siloxane linkages. A modified Karl Fischer reagent titration method gave rapid and precise determination of adsorbed water in the presence of silanol and strained siloxane. Results were compared with those from azeotropic distillation of adsorbed water and thermogravimetric analysis. Condensation of silanol catalyzed by boron trifluoride, acetic acid, and pyridine allowed reliable analysis of water and silanol on untreated and treated silicas. These data were compared with those from thermogravimetric and isothermal condensation procedures. Determination of the sum of adsorbed water and strained siloxane rings with reactivity greater than that of hexamethylcyclotrisiloxane was possible by reaction with methanol and conventional Karl Fischer reagent. The course of the reaction was followeij using a reaction rate titration scheme and recording biamperometric apparatus. C H A R A c r E R i z A r I o i v of .:he silica surface in terms of water, silanol, and strained siloxane has been studied by numerous workers (I-j’), Several methods have been reported which allow quantitative determination of adsorbed water (I-3), surface silanol ( I - 6 ) , and strained siloxane ( I , 3, 7). Often these methods are lengtl-y and laborious, have poor precision, and are apparently limited to analysis of untreated silicas. A rapid, modified Karl Fischer reagent (MKFR) titration procedure was recently developed in our laboratory to determine water in the presence of silanol and other interfering materials (8). This technique has been extended to direct titration of water adsorbed o n silica surfaces. The reaction of silanol or strained siloxane with alcohol was minimized or eliminated without appreciably inhibiting the free water reaction by employing high molecular weight linear o r branched alcohols as sample diluents. Adsorbed water concentrations were deterrr ined by thermogravimetric analysis (TGA) and azeotropic distillation (AZD) for comparative data. A new silanol condensation procedure (9) gave rapid and reproducible analysis of total hydroxyl (silanol and water) in monomer silanols, siloxane polymers, and silicone resins. A new catalyst system consisting of boron trifluoride, acetic acid, and pyridine was employed. This system (designated as chemical condensation or CC) was found to be applicable to determination of surfa1:e hydroxyl on silicas. F o r comparison, T G A and a n isothermal condensation (ITC) procedure were performed.
The two-step attack of a partially hydroxylated silica surface by methanol has been described ( I ) . First, an opening of strained siloxane linkages occurs. +ji
\
0
/
+ CH30H+ =SiOCH3 + = S O H
=Si Then the methoxylation of the freshly produced hydroxyl group proceeds in the presence of excess methanol.
=SOH
+ CH30H
+
=SiOCH3
+ H20
(2) We have employed a reaction rate titration scheme for a direct titration of the strained siloxane bonds on silica surfaces utilizing the Karl Fischer reagent (KFR)-methanol diluent system. This method allowed determination of water and strained siloxane in the presence of the slower silanol-alcohol reaction occurring on the remainder of the surface. Results for strained siloxane were obtained by the difference of this titration and the adsorbed water determination. A summary of the three types of analyses and the methods employed for each is given in Table I. The concentration range and observed and expected precision are also presented. EXPERIMENTAL
Apparatus and Reagents. The recording biamperometric apparatus previously described (8) was used for all Karl Fischer reagent titrations. The reaction apparatus and reagents for total hydroxyl determinations were similar to those previously reported (9). The thermobalance for T G A measurements employed an Ainsworth Model “AV” Automatic Balance and “AU” Recording system. Linear temperature programming of a Marshall 1800-watt furnace was accomplished using Harold Beck Co. program controls, triple function relay, and triple
Table I. Summary of Types of Analyses and Methods Employed Concentration Rel. std. Determination Method range dev. Adsorbed water MKFR 0 . 1 4 % H20 5-1 AZD 0.5-8 % H,O 5-1 %a TGA 0.1-8% H,O 40-1 %a Total hydroxyl cc 0.1-1 % OH 10-3 % (water silanol) 1-7z OH 3-1 ITC 0 . 2 - 7 z OH 12-6za TGA 0.1-12z OH 40-1 %“ Strained siloxane KFRb 0.2-3Z as OH 10-5 %a a RSD values estimated from range (10). Reaction rate method using methanol as sample diluent.
z
+
(1) J. A. Hockey, Chern. h d . (London), 1965 p. 57. (2) R. K. Lange, J . Colloid Sci., 20, 231 (1965). (3) W. Noll, K. Damm, a n i R. Fauss, Kolloid Z., 169, 18 (1960). (4) . , H. P. Boehm, M. Schni:ider, and F. Arendt, 2. Atzora. Alkem. Chem., 320, 43 (1963). ( 5 ) H. P. Boehm and M. Sc hneider. Zbid.. 301. 326 (1959). (6j J. J. Fripiat and J. Uyttwhoeven, J. P/zys. Chem.; 66,800 (1962). ( 7 ) G. J. Young, J. Colloid Sci., 13, 67 (1958). (8) R. C. Smith and G. E. Kellum, ANAL.CHEW,38, 67 (1966). (9) Zbid, 39, 338 (1967).
(1)
z
(10) R. B. Dean, and W. J. Dixon, ANAL.CHEM., 23,636 (1951). VOL. 39, NO. 3, MARCH 1967
341
Table 11. Description of Silica Samples Surface area Time and temperature Type mz/gram history Silica Pyrogenic 250 Production lota Cabosil MS-75 375 Production lot" Cabosil S-17b Pyrogenic Treated samples heated to 150" C for 15 minutesa Pyrogenic 260,275 Production lotsa Silica A Production lota Pyrogenic 400 Silica Bb Treated sample heated to 150" C for 16 hoursa QUSO F-20 Precipitated 325 As : oreceived. ' 2 Ludox ASb Wet process 220-235 Both treated and untreated samples evaporated to solids at 60" C and = 5 mm pressure for 10 hoursc Dow Corningd Wet process ~ : 6 0 0 Laboratory synthesis, dried at 100-150" C for 15 hours after treatmentC a
Stored for several months at room temperature.
* Me3SiOl,z-treatedsample was also available. c
d
Stored for approximately 1 week at room temperature. Me3SiOljz-treatedsample only.
Table 111. Determination of Adsorbed Water on Silicas Hz0
Sample Silica A Silica B Cabosil MS-75 Cabosil S-17 Cabosil S-17 (treated) QUSO F-20
Method MKFR TGA AZD MKFR TGA AZD MKFR TGA AZD MKFR TGA MKFR TGA MKFR TGA
wt. 7z 2.89 2.82 2.94 5.83 5.42 5.94 1.52 1.61 1.49 2.76 2.46 1.63 1.30 7.76 8.06
HzO/mp 3.70 3.62 3.76 4.85 4.52 4.95 1.24 1.32 1.22 3.74 3.33 2.20 1.76 7.97 8.27
function motor operator. A Chromel-Alumel Type K Thermocouple was positioned inside the Vycor furnace tube within 1 cm of thc platinum sample holder. Dynamic gas atmospheres were supplied by entrance at the bottom of the furnace tube. Procedures. ADSORBEDWATERDETERMINATIONS. Modified Karl Fischer Reagent Method (MKFR). Silica samples of 0.1 to 0.8 gram, sufficient to give 0.25- to 0.50-ml titration, were added to pretitrated 4 : 1 Lorol 5-pyridine diluent. The end point was similar to that previously described (8). The attenuator of the simplified biamperometric apparatus was set at X4 with a recorder span of 5 mv. Approximately 200 mv was applied to the platinum electrodes. No deadening of current response occurred when silica samples were introduced into the alcohol-pyridine diluent, therefore the base line current increased with K F R addition. About 33 increase in current level was obtained with 1 ml of KFR addition because of introduction of methyl Cellosolve from the reagent, This current increase was easily compensated for in the end pdint of the titrations. Stable end points were generally obtained within 1 to 2 minutes. 342
ANALYTICALCHEMISTRY
3.0
tI
I .o
:
0 0
1.0
:2.0
3.0 4.0 T I M E (MIN)
5.0
6.0
Figure 1. Typical titration curves using KFR titration systems A. KFR water determination on silica, methanol diluent B. MKFR water determination on silica, 4: 1 Lorol S-pyri-
dine diluent C. KFR water and strained siloxane determination on silica after thermal treatment at 200400" C, methanol diluent D. KFR water and strained siloxane detnermination on silica after thermal treatment at 400-800" C,methanol diluent Thermogravimetric Analysis (TGA). Adsorbed water was determined by TGA weight loss in air or helium atmosphere using 300- to 500-mg silica samples, a temperature program of 10" C/minute, and dynamic helium o r air atmosphere flowing at 610 cc/minute. Adsorbed water evolution was assumed complete near 200" C where a sharp break in the weight loss curve occurred. Buoyancy corrections were applied to results at this temperature, Azeotropic Distillation (AZD). Silica samples of 3 to 7 grams were diluted with 75 ml of toluene, and an azeotropic distillation of adsorbed water carried out using the same type moisture traps and water condensers as previously noted (9). The collected water was titrated using K F R and methanol diluent. A blank of toluene was carried through the complete procedure with samples. TOTALHYDROXYL DETERMINATION. Catalytic Condensation (CC). The procedure was similar to that previously reported (9). Silica samples of 3 to 7 grams or roughly 0.2 to 0.5 mole of =Si-0 were dispersed in 75 to 100 ml of dry toluene before boron trifluoride, acetic acid, and pyridine catalyst addition. Heat was applied fast enough to initiate reflux within 15 minutes. Deposits of silica on the flasks above the liquid level were redispersed by swirling several times during the 40- to 50-minute distillations. Duplicate analyses were generally performed. A reagent blank was carried through the complete procedure with the samples. Thermogravimetric Analysis (TGA). Total hydroxyl was determined from the same thermogram as used in adsorbed water analysis above. Weight loss to 1000° C gave a total hydroxyl figure. The weight loss from 200" to 1000" C was attributed to thermal condensation of silanol with release of water. Buoyancy corrections were applied to results a t this temperature. Isothermal Condensation (ITC). Using a method similar to that reported by Noll, Damm, and Fauss (3),total hydroxyl was determined by isothermal condensation of silanol at 1000" C for 4 hours. Silica samples of 0.8 to 12 grams weighed in a platinum boat were introduced into the 1000" C zone after a positive flow was achieved in the apparatus. The water produced was swept into dry methanol with 100 cc,'minute dry nitrogen flow. The water content of the methanol was initially less than 100 ppm as determined by KFR titration.
Table JV. Determination of Surface Silanol Concentration on Silicas Silanol as weight Method OH/mp2 Sample OH Untreated pyrogenic silica:; cc 5.03 4.43 Silica B 3.99 3.53 TGA 4.43 ITC 5.03 cc 2.25 3.05 Silica A 1.70 2.31 TGA 3.05 2.25 ITC cc 2.13 3.01 Cabosil MS-75 TGA 2.00 1.42 ITC 1.84 1.31 cc 3.33 3.53 Cabosil S-17 3.04 3.24 TGA 3.08 3.28 ITC Treated pyrogenic silicas cc 1.58 1.68 Cabosil S-17 4.16 4.42 TGA 1.03 1.09 ITC cc 2.76 3.14 Silica B TGA 6.00 6.80 Untreated precipitated and wet process silicas cc 5.94 5.46 QUSO F-20 8.48 TGA 7.82 5.65-6.02 cc 3.76 Ludox AS ITC 6,42-6.86 4.65 Treated wet process silicas cc 4.29-4.58 2.53 Ludox AS 2.30-2.46 ITC 1.53 cc 6.3C6.80 5.53 DC Silicate 2.10-2.26 ITC 3.69
Samples of 0.2 to STRAINEDSILOXANEDETERMINATION. 1.0 gram were added to pretitrated methanol. The end point was maintained by KFR additions at 1-minute intervals until a linear consumption of K F R was obtained. Volume of K F R added was plotted c's. time, and the curve (similar to "C" in Figure 1) was extrapolated to zero time to determine total adsorbed water and strained siloxane. Strained siloxane content was obtained by difference after subtracting the adsorbed water content as determined above using the M K F R titration or by TGA. THERMAL AGING STUDIES. Ten-gram silica samples were placed in a large platinum dish and heated in a muffle furnace a t 100" to 1000" C for 4-hour periods. The furnace had a static air atmosphere. Slamples were rapidly transferred for above analyses from the hot furnace in a maximum of 2 minutes. RESULTS AND DISCUSSION
Silicas are prepared by a variety of methods and exhibit a range of surface concentration of adsorbed water, surface silanol, and strained sihxane at room temperature. The samples chosen for study were typical in variety and type, and are described in Table II. Ten different silica materials were examined which included untreated and treated pyrogenic and wet process types and untreated precipitated silicas. No heating of the samples was performed in the initial examinations since temperature history after preparation affects the surface activity significantly. Many of the pyrogenic silicas had equilibrated at room temperature in closed bottles for several months before a r alyses were performed. The wide
Table V. Typical Literature Values for Surface Silanol on Silicas Silica type OH/mp2 Ref. A. Pyrogenic silicas 1. Aerosil 2. Silicas A, B, and C B.
1.9-3.2 3.8-4.6 3.5, 4.0, and 3.4
Precipitated silicas and gels 1 . Gels 5.1-7.9 2. Xerogel 5.56 3. Amorphous 5.0 4. Precipitated 6.0 a Values are either experimental or theoretical
(3-5) ( 2 , 6, 11) (2)
( 4 , 12, 13)
(3) (4) (14)
Table VI. Comparison of Water Content after Thermal Aging Apparent Weight Weight Si-0-Si Sample Conditions Method H20 as OH QUSO F-20
Room temperature 300" C, 4 hours
Silica B
Room temperature 200" C, 4 hours
Silica A
Room temperature 100" C , 4 hours
Cabosil S-17 Room temperature
MKFR TGA KFR TGA KFR MKFR TGA KFR TGA KFR MKFR KFR MKFR TGA KFR MKFR KFR
7.76 8.06 8.00
0
0,27 1.89 4.81 4.81 4.75
3.06
0.50
2.08
1.60 1,75 2.08 0.318 0.244 0.640 2.76 2.75
0
0.624
0,747 0
variation in types of silicas and their surface properties have led to much of the confusion in analytical data found in the literature. Determination of adsorbed water on various silicas are given in Table 111. The data are presented in weight per cent water and water molecules per m,u2 of surface area and show excellent agreement of the methods. T G A results were expected to be somewhat high with precipitated QUSO silica and slightly low for pyrogenic type silicas. Advantages of the M K F R titration method include speed, precision, and sensitivity to small amounts of water. It was possible to perform more than one determination with the same solvent aliquot in the titration vessel. Comparative data for determination of surface silanol concentration of a number of silicas are presented in Table IV. Hydroxyl due to adsorbed water has been subtracted from the total hydroxyl analysis. Results obtained with the catalytic
(11) W. Stober, KolloidZ., 145, 17 (1956). (12) C. C. Ballard, E. C. Broge, R. K. Iler, D. S . St. John, and J. R. McWhorter, J . Phys. Chem., 65, 20 (1961). (13) V. A. Dzisko, A. Vishneskaja, and V. S . Chesalova, Zhur. Fir. Khim., 24, 1416 (1950). (14) J. H. deBoer and J. H. Vleeskens, Koninkl. Ned. Akad. Wetenschup. Proc., B61, 3 (1958). VOL. 39, NO. 3, MARCH 1967
343
Sample Silica B QUSO F-20
Silica A
Table VII. Analysis of Water, Silanol, and Strained Siloxane after Thermal Treatment HzO/mp* -Si-O-Si= SiOH Conditions Adsor bed as OH/mp2 OH/mb 2 Room temperature 4.0 Nil 4.43 100" C, 4 hours 0.32 1.97 2.63 300" C, 4 hours 0.23 1.11 1.91 Room temperature 7.88 Nil 5.94 150" C , 4 hours 0.82 3.57 2.32 600" C, 4 hours 0.20 2.74 0.92 Room temperature 3.70 0.80 3.05 150" C , 4 hours 0.68 2.10 1.75 450" C , 4 hours Nil 2.12 0.79
Table VIII. Recovery of Hydroxyl after Trimethylsiloxy Surface OH/mr2lost in treatment Sample Predicted Found Cabosil S-17 1.70 1.75 1.06 1.24 Ludox AS DC Silicate 1.84-2.34 2.15
condensation method generally agreed well with those of isothermal condensation procedure and were slightly higher than found by T G A as expected. Weight losses were incomplete with the time and temperatures employed in the TGA determinations. The isothermal method was performed at 1000" C rather than 1100" C (3),and results were probably somewhat low. The data show that both thermal measurements gave representative results for untreated pyrogenic silicas but produced high results for precipitated types of silicas, undoubtedly from condensation of internal hydroxyl structures. Both thermal methods were unsuitable for analysis of treated silicas. T G A data were consistently high because of treatment removal giving rise to excess weight losses, Results from the isothermal method in which water was recovered and titrated were consistently low and indicated that large hydroxyl losses occurred when treated silicas were decomposed at 1000" C. Noll, Damm, and Fauss (3) have compared data from their isothermal method with those from other proposed hydroxyl methods in the literature. Their results and other typical published data are summarized in Table V with the general classes of silicas. Hydroxyl values obtained with the new catalytic condensation method as given in Table IV compare favorably with quantitative data previously reported. Strained siloxane was purposely introduced on the surface of silicas by thermally condensing silanol groups at temperatures from 100" to 1000" C. The analyses for adsorbed water were performed again after the thermal aging to ascertain whether the strained siloxane formed would react in the K F R system, The K F R titrations employed methanol as solvent with the reaction rate titration scheme described in the experimental section, Typical titration curves from the two K F R methods are shown in Figure 1. The rate of water reaction was similar in both methods as shown in the initial rise of curves A and B. The second sloping portion of the curves is due to the slower silanol-alcohol reaction in which water is formed. When strained siloxane was present, the rate of water reaction in the KFR-methanol system was inhibited slightly as in curves C and D. The rate curves for 344
ANALYTICAL CHEMISTRY
Surface area m2/gram 400
325 275
strained siloxane and water were of similar shape after thermal treatment as in curves C and D. After thermal aging, the water analysis methods did not agree as shown in Table VI. The difference was attributed to the reaction of strained siloxane with methanol (Equations 1 and 2) in the KFR-methanol system which yielded higher water results. We have shown that strained six-member siloxane rings are reactive in the KFR-methanol system, particularly hexamethylcyclotrisiloxane (15). Condensation of two hydroxyl groups produces one water molecule, and the reaction of one strained siloxane linkage with two molecules of methanol yields one molecule of water. Since both reactions involve one water molecule for each two hydroxyl group change, strained siloxane results were calculated as weight per cent hydroxyl condensed to produce siloxane. The difference in water results from the two K F R methods were converted to weight per cent hydroxyl. For example, with QUSO F-20 the deviation in results from the methods was 1.62% H 2 0 which was converted to 3.06% OH from hydroxyl condensation, These tests do not detect strained siloxane on the surface of most silicas at room temperature as would be expected for most equilibrated silica materials. Silica A, which is a high temperature pyrogenic silica, exhibited some strained siloxane at room temperature. After high temperature thermal aging, as above 200" C, strained siloxane was even somewhat reactive with the MKFR titration system. In these instances TGA was employed for surface water determinations. The MKFR titration was still the most useful method for routine adsorbed water determinations, however. Table VI1 presents data taken to ascertain the quantitative value of these independent measurements as a complete silica surface analysis. Silica A, Silica B, and QUSO F-20 silica samples were analyzed for adsorbed water, surface silanol, and strained siloxane before and after four-hour thermal aging periods. The data show that quantitative definition of the surface was possible up to 150" C employing the methods described. The marked decrease in silanol group population upon heating is reflected by a quantitative increase in strained siloxane at temperatures of 150" C and below. Rapid loss of surface water was expected with mild temperatures, but evidence indicated that complete removal of surface water was achieved only at temperatures in excess of 400" C. Strained siloxane linkages were stabilized by thermal effects above 150" C. ~~~~
(15) R. C . Smith and G. E. Kellum, ANAL.CHEM., 38, 647 (1966)
Employing the new methods, water and silanol analysis of silicas which have becn trimethylsiloxy-treated have consistently shown two hydroxyl-group loss per trimethylsiloxy group added to the surface. (This loss, a possible result of steric effects, has not been completely explained.) The data from analysis of three silicas before and after this treatment are given in Table VIII. Carbon analysis for the trimethylsiloxy group was used to predict the decrease in surface hydroxyl groups. After correction for the two hydroxylgroup loss per trimethylsiloxy group added, the experimental values agreed with the predicted values. Observed and estimated precision for the various methods is summarized in Table 1. The lower detection limit of the MKFR system was abcut 0.005 % water with a practical lower limit of approximately 0.01%. The precision of the azeotropic distillation analyses should approach that of the catalytic condensation procedure where a similar separation
of water is made. The practical lower detection limit of the AZD procedure should be 0.01% water. The RSD for TGA adsorbed water analysis was estimated assuming a maximum weighing error of i 0.2 mg in evaluation of weight losses when employing a 300-mg sample. This RSD would apply to the silanol values by TGA also. The lower detection of water loss by TGA is limited to the maximum error in evaluation of weight loss. The practical lower limit of analysis using the catalytic condensation hydroxyl procedure was approximately 0.05 hydroxyl, but this limit could be extended to 0.005 or 0.01% hydroxyl or lower with care. Speed, sensitivity, and precision were identical for both untreated and treated silicas. RECEIVED for review August 17, 1966. Accepted January 9, 1967.
Determination of Boron by Thermal Neutron Capture Gamma-Ray Analysis B. W. Garbrah and J’. E. Whitley Scottish Research Reac,for Center, East Kilbride, Glasgow, Scotland
Thermal neutron capture y r a y analysis is applied to the determination of boron using a 2- by 2-inch NaI(TI) crystal. Analysis is based on the well known 477-keV -pray which i s emitted in 93% of all thermal neutron captures in boron. Correction factors provided for thermal neutron flux depression in boron also make the technique applicablle to the determination of large boron samples.
T H EINHERENT ADVANTAGES of thermal neutron capture y-ray analysis as an analytical technique compared with conventional activation analjsis have been suggested by several authors (1-5). Isenhcur and Morrison have determined boron by thermal neutron capture y-ray analysis using a modulation technique ( 3 ) . Pierce, Peck, and Henry ( 5 ) and Sippel and Glover (6) hz.ve applied the measurement of prompt radiation emitted during charged particle bombardment to the determination of some light elements. The complexity of capture y-ray spectra often makes it difficult to identify uniquely the elements giving rise to the spectra, and also hindtxs satisfactory analysis. The determination of a single element in a matrix is complicated by the production of overlapping peaks due to other elements, in the matrix. In some situations the high background of ~
~
~
(1) R. C . Greenwood and J. H. Reed, “Scintillation Spectrometry Measurements of Capture Gamma Rays from Natural Elements,” U.S. At. Energy Comm ARF 1193/6 (quarterly report) 1962. (2) L-V. Croshev, A-M. Demidov, V. N. Lutsenko, and V. F. Pelekov, “Thermal Neutron Capture ?-Ray Atlas,” Atornizdat (1958). (3) T. L. Isenhour and G. H. Morrison, ANAL. CHEM.,38, 167 (1966). (4) A. F. Para and M. M..Bettoni, Energia Nucleare, 11 (ll), 612 ( 1964). (5) T. B. Pierce, R. F:. Peck, and W. M. Henry, Analyst, 90, 339 (1965). (6) R. F. Sippel and E. D. Glover, Nucl. Znstr. Meihods, 9 , 3 (1960).
y-rays and neutrons around the reactor can also produce undesirable peaks in the spectrum. These difficulties have hindered widespread application of thermal neutron capture y-ray analysis. Because the y-rays arising from other elements in the sample which are not being determined cannot be avoided, a given spectrum can only be improved by a reduction of reactor background spectrum. To this end the methods of Hammermesh and Hummel (7) and of Greenwood and Reed ( I ) have been combined and modified to permit satisfactory quantitative analysis for boron. The method reduces the four-cycle measurements of Hammermesh and Hummel to two, and in some cases the total live counting time is reduced by a factor of four. The reduction in counting time has the additional advantage of reducing the effects of the activity of radioisotopes that build up during irradiation. EXPERIMENTAL
Apparatus. The apparatus is schematically shown in Figure 1 . Irradiations were performed using the UTR lOOKW reactor of the Scottish Research Reactor Centre. The beam tube chosen for the experiments passed through the thermal column of the reactor and so was shielded from fission product y-rays by a lead wall (Figure la). The fraction of epithermal and fast neutrons in the beam tube at this location was considerably less than in other beam tubes in the reactor. At the sample position the thermal neutron flux was 8 X 105 neutrons/cm*/sec; and the beam was 2 cm in diameter. Detector and Shielding. The detector and shielding assembly is shown in Figure 1b. The detector was a 2 in. x 2 in. NaI(T1) crystal detector with built-in photomultiplier (Ecko Type N 609). This was shielded by a lead castle of wall
(7) B. Hammermesh and V. Hummel, Phys. Reu., 88, 916
(1952). VOL. 39, NO. 3 , MARCH 1967
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