Topics in..
.
Edited b y 5. 1. LEWIN. New York University, New York 3, N. Y.
These articles, most of which are to be contribuled by guest authors, are intended to serve the readers of lhis JOURNAL by calling allenlion to new developments i n the theory, design, or auailability of chemical laboratory instncmen/ation, o i by presenting useful insights and explanations of topics :hat are of practical importance to those who use, ,or teach the use % modern instrumentation and instrumental techniyues.
Ill. Activation Analysis E. N. Wise, Department o f Chemistry, University o f Arizona Many elements become radioactive when they are subjected to bombardment with subatomic particles, and the measurement of thisinduced radioactivity may provide both qualitative and quantitative information concerning the composition of a bombarded sample. The sensitivity of the method for many elemente is very high, m d the specificity of identification often makes passihle the elimination of time-consuming chemical separations. Wily was 8 0 attractive an analytical method not more widely used, and what has caused the sharp increase in interest within the last few years? The answer lies in the general availability of the instrumenta required to perform activation source of bombarding analysa-the particles, and the radiation analyzers required to identify the characteristic activation-produced radiation. So long as the sources of homharding particles were cyclotrons, large accelerators, and nuclear reactors, the use of aetivation analysis was largely restricted to those who had ready access to these sources. Radiation analyzers were complex, expensive, and also not accessihle to many research chemists. Conditions have recently dramatically chaneed. Widemread interest in the utilization of radioisotopes in chemistry has made nucleonic instrumentation a common lahoratory tool, and sourres of bombarding particles have been made available a t prices ranging from that of a. good manual spectrophotometer (s1300) to approximately $25,000, depending on the kinetic energy and quantity of particks produced. The most useful subatomic projectile for activating the nucleus of an element is the neutron. Neutrons are effective projectiles because they do not have an electrical charge, and are therefore not
-The
c
o
v
e
y
Tho photograph shows Professor Wise working with Mr. Gary Kozak discussing an wtivatian analysis experiment in Lhe laboratory of the University of Arizona.
repelled by the positively charged nucleus of an atom, as are positively charged protons and alpha particles. Nor are they deflected by the negatively chmged cloud of electrons surrounding the nurleus, as are projectile electrons. When a neutron combines with a target nucleus, there is a rearrangement of energy within the nucleus which usually causes a prompt emission of some form of energy, quite often gamma emission, and then the resulting nucleus is stable or unst.able. We are interested only in those processes which reault in the formation of an unstable nucleus which may then he uniquely identified by the energy of the radiation it emits, and by the average length of time it takes to emit it. The sensitivity of activation analysis for the determination of an element is controlled by five major factors: 1. The numher of bombarding particles provided by the source. This is the product of the fluz, the number of particles per second passing through a square centimeter of the sample normal to the direetianof travel of the particles, and the lime of activation of the sample. 2. The prohahility that an incident particle will combine with the nueleue. This is measured by a parameter called the activation cross-section, and it is expressed in a unit of lO-'* cmR,called a barn. 3. The type of radiation emitted by the activated nucleus following radiation (low energy radiation is inefficiently detected). 4. Tho prohahility that an activated nucleus will emit radiation in a given period of time. This factor is commonly expressed in terms of the half-life of the species, which is the period of time required by onp-half of a large numher of activated nuclei to emit radiation. A very long half-life (months or years) is undesirable hecause only a smell fraction of the activated nuclei emit radiation per unit of time. A very short half-life (seconds) presents problems h o cause rapidly decaying activated nuclei during the homhardment limits effective activation time, and the sample must he very quickly transferred from the activation device to the radiation analyzer.
E. N. Wire is Professor of Chemirtry ot the University of Arizona. He war educated a t Ohio University (B.S., 1937; MS., 19381 ond the University of Kansor (PLD., 19531. He has been o Research Engineer ot Battelle Memorial inrtitvte ond o Staff Member in Andytical Instrumentotion a t the Lor Alomor Scientific Laboratory. His teoching ond research interests ore in electromaiyticol- ond radio-chemistry.
5. The isotopic com.position of the element must he considered, for the values of all hut the first of the nhove factors are different for each i s o t o p ~of an element. Let us now direct our attention to the sources of proj~rtileneutrons which are making acbivation analysis a practical lahoratory method. Srutrans may he produced hy: 1 . Bombarding beryllium with alpha particles from naturally radioactive elements like radium, 2. Romharding deuterium, tritium, or beryllium with high energy deut,erons from ion sccelcrators, or 3. Fission of hcnvy nuclei in atomic reactors. Radioactive Neutron Sources
Whcn a naturally radioactive alpha. emitter is intimately mixed with beryllium, some of the alpha partirles combine n i t h the beryllium-9 nuclei to form excited nuclei which promptly decay to carhon-12 with the emission of neutrons of heterogeneous kinetic energy. The neutron energies range from 1 to 12 lNev (million elertron volts) with the majority having kinetic encrgies of 4 to G Mev. Most elements have a larger cross-section for slow neutrons than for fast ones, so t,he neutrons are slowed hy causing them to undergo multiple elastic collisions with hydrogen atoms. The hydrogcn-bearing compounds used to slow the neutrons to
Volume 39, Number 10, October 1962
/ A771
Chemical lnstrurnentation thermal energy (approximstely 0.02 elmtronvolts) are called moderators. Paraffin and water are common moderators in laboratory neutron sources. Water is slightly more efective as a moderator, due to its greater hydrogen-density. I t is transparent, to permit observation, and it sllows greater flexibility in sample placement. Paraffin is "on-volatile, and is less likely to herome contaminated.
Figure 1.
NH3 NeutronHowitrer
Polonium, radium, and plutonium have been used to provide the alpha. particles. Radium emits powerful gamma rays as well as alpha particles, which requires heavier shielding to protect operating personnel. Polonium and plutonium emit very wcak gnmme rays which do not require extra shielding. Plutonium is preferred to polonium hecause of its longer half-life (24,400 years for P u versus 138 days far Po). Pu-Be sources may be obtained on lorn free of charge from the U. S. Atomic Energy Commission by educational institutions. Examples of commercially-available radioactive neutron sources are the Model
Figure 2. Viiiflux Neutron Howitzer. The motor rotates the source holder to provide a uniform nevtronflux.
(Continued on page A774)
A772 / Journal of Chemical Education
Chemical lnstrurnentation
I
NH3 Neutron Howitser of the NuclearChicago Corp., Des Plaines, Illinn's, and the Visiflux Neutron Howitzer of Reactor Experiments, Inc., Belmont, California. The S H 3 howitzer is paraffin moderated, i t is priced a t $1245, and i t is shown in Figure I . The Visiflux howitzer is watermoderated, its price is $1375, and it is shown in Figure 2. Both howit~ersare designed to use a Pu-Re source containing up to 5 curies of Pu. A 5 curies source will yield 7 X 10' neutrons per second. Instruction and experiment manuals, 38 =-ell as operating supplies, are furnished with both ho~vitners.
Accelerator Neutron Sources The most common reaction for the production of neutrons by arcclerated particles is the tmnhardment of tritium by deuterium, produring a. He-4 nucleus and a 14 Mev neutron. The deuterium is ionized, and the deuterons are acr~lerated by a high potential, striking a titanium target which is loaded with tritium either adsorbed or as the britide. Commerciallyavailable arrelerator sources diRer in the met,hod of ionizing the deuterium, in the method of ohtsining the accelerating potentinl, and in the magnitude of that potentinl and of t,he ion-beam current. These variations result in widely diRering fluxes of neutrons. Thc more powerful accelerator sources have provision for changing the t,arget when its supply of tritium is depleted, while the entire sealed accelerator tube assemhly is exchanged in the other sources. We will disruss the sources in the order of increasing neutron flux that they provide.
I
Sealed Accelerator Sources The Kamn Pz~lsatron,shown in Figure 3, produces deuterium ions I?.causing a n elcrtric discharge t o flow between washers of titanium loaded with deuterium.
Figure 3. Pvltatron NT-60-8 Neutron Generotor and Control Unit.
Coinrident with the discharge between the titanium washem an energy storage raparitor, which is charged to 6 kv, is discharged by a thyratron tube into a 30 : 1 step-up pulse transformer, which puts a 180 k v accelerating potential on a titanium target loaded with 1 curie of t,ritiam. The arcelerated deuterium ions fuse with tritium nuclei, releasing 10' ncudrons per pulse. The pulse ma); be (Continued an page AT761
A774 / Journal o f Chemical Education
repeated a t a rate of up t o 10 pulses per second. The accelerator unit is 4 in. in diameter, 18 in. long, weighs 20 lh, and has an estimated life of over 100,000 pulses, with s guaranteed life of 20,000 pulses. The Pulsxtron NT-60-8 is sold by Kaman Nuclear Carp. of Colorado Springs, Colorado, for $7500 complete with control unit. The exchange price of the accelerator tube is $365. The Pulsatron NT-60-7 has a slightly different accelerator tnhe, and the control unit is supplied in kit form t o be wired hy the purchaser. Its output is 5 X lo8 neutrons per pulse, and i t is priced a t $3950. The P i c k e d h s s e ~$980 Neutron Genemto?, shown in Figure 4, contains a miniature Van de Grsaff electrostatic generator which provides a 130 kv accelerating voltage t o deuterium ions produced from deuterium gas by high-voltage ionization in the center of the tube. The pressure of the deuterium gas, and eonsequently the intensity of the deuteron bombardment heam, is controlled lry electrically heating titanium which is loaded with deuterium. The &curie tritium target is adsorbed on titanium plated on the insido of s copper hand 3/s in. wide and 9 in. in circurnforence placed inside, and concentric with the inside surface, of the source tuhe. The full power output is lo8 "/see. Tho accelerator tuhe has an estimated life of 500 hr of fnllpower operat,ion, and a guarantecd life of 200 hr. The Van de G r a ~ f belt f has a life of 200 hr, and is easily replaced.
Figure 4. Picker-Dresser 2920 Neutron Generator and Control Unit.
The Picker X-Ray Corp. White Plains, New York, is the manufacturer, and the price of the generator, complete with control unit, is $2950. The exchange prico of the accelerator tube ie $495, and a replacement Van de Graitff h d t is 816.25. The Norelco NS-100 Neutmn Generator, shown in Figure 5, features a very versatile power supply which furnishes up t o 125 kv amelcrating potential a t 0.1 ma, either continuously or in 10 to 500 microsceond pulsm. A repetitive pulse rate of GO to 3000 pulses per second is available. A sectional drawing of the ~ccclcrator tuhe is shown in Figure 6. The tnhe is filled with s mixture of deuterium and tritium, and its pressure is regulated hy electricallyheating finely divided zirconium losded with a mixture of deuterium and tritium. The Pcnning ion source produces
A776 / Journal of Chemicol Education
Chemical lnstrumentatien
Figure 5. Norelca NS-100 Neutron Generator and Contml Unit.
ions by means of s. 2 kvionizntian potential and a strong mametic field, which causes electrons to travel in tight helical paths within the ion source cavity. The ions are accelerated in a single step between the ion source and the accelerating electrode. At t,he end of the field-free electrode is a target consisting of a thin layer of titanium plated on a heavy silver base. Silver has n low tritium diffusion
Ion Source-~
Pressure - Regulator
Figure
6.
Norelco Neutron Generator Tube.
rate constant, and it is an excellent heat sink. Tritium adsorbed on the titanium is replenished by tritons from the ion beam, greatly increasing the life of the target. The tube contains a total of 9.5
Volume 39, Number 10, October 1962 / A777
Chemiccrl Instrumentation curics of tritium. At the t : ~ r g ~the t d-t reaction produres 11 kev neutrons with a minimum orrtl~ut of 108 n/ser in continuous operation, and 10Lo n/scr in pulsed operation with a 1% duty ryrlp. The genrrat,or is avnilalk from Phillips
Figure 7. Kaman NT-60-9 Neutron Generator. The ion source and accelerator tube ore in the rpun-metol enclosure at the left. An ion pump is iocoted just right of the center support plate. and the target is a t the far right d t h e drifttube. The dual power supply is underneath the
accelerator.
A778 / Journal o f Chemical Education
Electronic Instruments, Mount Vcrnon, New York, complete with powrr supply and control instrnment,ation, for $12,800. The accelerator tube is available separately from the Amperen Eleet,ranic Corp., Hieksville, New York.
Pumped Accelerator Sources The Kanran AT-fi6.9 Generator, shown in Figure 7, uses a plasma type ion source t,o provide deuterons to homhard titanium
Figure 8.
pleni~hedfrom a bottled supply, and the :tccclerator tube is continually pumped to a pressure of 5 X 10-5 mm H g with an elertric ion pump. An acceleration potential or 150 kv is supplied hy a cascade rectifier, resulting in the pradurtion of 10'on/sec in continuous operation. When dcplcted, t h ~target may be easily replaced in less than one hour. ICaman Suelenr Corp. of Colorado Springs, Colorado, sells the NT-GO-9
Van de Grmff AN-400 Neutron Generator
tritide target which is copper-backed and water-coolcd. The deuterium is re-
(Continued on paye A78I)
Chemical Instrumentation generator, complete with power supply and control unit, for lhlG,000. The H V E Van de @uaf AN-400 N m t m Generator, shown in Figure 8, uses the well-kn0n.n moving-belt electrostatic generation method to obtain a 400 kv accelerating potential. A radiofrequency ion source ~ r o v i d e sdeuterons for a multi-section, grounded cathode, acceleration tube. The evacuation system ineludcs an oil-diffusion pump with liquid nitrogen trap, a fore-pump, and two fast acting high-vacuum valves for quick changing of t a r g ~ t s . The maximum output with a water-cooled tritium target is greater than IOJ%/sec, and target life is significantly increased for a given neutron output by use of a. high accelerating potential and a lav-er beam current. Cont,in~~ous or operation is provided by n control unit. The Model AN-400 is sold by High Voltage Engineering Corp., Burlington, I\lassachusetts, for $20,000. Thc Texas Artcelear 9500 Neutron Generaior, has a high-efficiency (90% atomic yield) radio-frequency ion source which consumes approximately 20 ml of deuterium gas per hour through a palladium leak irom a bottled supply. An eleetros t h c lens focuses the ion heam into a 10 stage accelerator with an accelerating potential of up to 150 kv. The target is a 300 microgram per square centimeter laver of titanium eva~oratedon x 0.025 in.
hombarding the target. At 100 kv accelerating potential, the output with s tritium-loaded target is 1.7 X 10"n/sec, and a 150 kv accelerating potential gives 4 X l0lo n/sec. At 150 kv accelerating potential with a deuterium-loaded target the output is 5 X 10% n/sec of 2 MPVkinetic energy. The Nuclear Chicago Corp. of Des Plaiines, Illinois, sells the Model 9500 for 522,500 complete with power supply and remote-operation console. A Model 9.501 is available a t 524,500 with an addition t o the rontrol console to provide pulsed operztiotion.
Nuclear Reactor Neutron Source Nuclear fission ran provide a tremendous flux of both fast and slow neutrons. The swimming pool reactors, of which General Atomics' TRIGA is the most widely known, use ordinary water moderation of neutrons from the controlled chain-reaction fission of uranium. A uniform thermal neutron flux of up to 8 X 10" n/cm2/sec is available from the 30 kw TRIGA. The first cost of a complete reactor facility is high (approximately 8250,000) and trained personnel are required to operate it.
Nucleonic Detection Instrumentation Radiation from activrtted nuclei can be quantitatively estimated by using a detector which is sensitive to that radiation and a sealer. I t is a feature of artiva(Continued on page A784)
Chemical Instrumentation tion andyais that unique identification and measurement may be made of an element in a sample in which several elements have been activated by the neutron source if a scintillation detector is used with a pulse-height analyzer. This instrumentat,ion will not be further discussed here h e cause it was covered so thoroughly by Professor Lewin in his columns in the March through July 1961, issues of THIS JOITRNAL.
Applications of Activation Analysis Activation analysis is of special intorest bocrtuse of its selectivity. Elements present in a sample have different activation cross sections, these cross sections differ with the kinetic energy of the activating neutrons, and the isotopes of those elements which are significantly activated will emit radiation of differing
types and energies, with different halflives. The possibilities for selectivity are thus grouped into factors producing the activation and factors analyzing the activation-pmduced radiation. We will concern ourselves primarily with the factors affecting activation, and assume that the radiation emittted hill he analyzed by suitable instrumentation. All methods of neutron production yield neutrons with considerable kinetic energy. The hombsrdment of beryllium with alpha particles, for example from a Pu-Be source, gives a spectrum of neutron energies from 1 to 12 Mev with the majority having 4 to 6 Mev kinetic energy. The accelerator bombardment of a. deuterium target with deuterons gives a nearly monochromatic flux of 2 Mev neutrons. The accelerator bombardment of a tritium target with deuterons gives II nearly monochromatic flux of 14 Mev neutrons. The neutrons from any of these sources may he slowed to thermal (Continued on page A786) -
A784
/
Journal o f Chemicd Education
--
-
Chemical instrumentation kinetic energy by passing them through an inch or more of s, moderator having a high concentration of hydrogen atoms to provide numerous elastic collisions. Table 1.
Maximum Flux of Neutrons
Available From Various Sources --
Thermal (n/em2/
Source
sec)
Nucl-Chi NH3 Howitzer Visiflux Howitzer Kaman Pulsatron Picker-Dresser 2U20
N&T&
NSIOO Kaman NT-60-9 Van de Graaff AN-400 Texas Nuclear 9500 General Atomic TRIGA
3 3 6 6
X X X X
104 10' 10' lo4
6 x 10' 2 X 10'
14 Mev ("/ern2/ sec)
-
...
x
5 108 5 X 10' 5
x
lo6 loP
2 X 10'
lo*
108
5 X 10e
5 X 10"
Intense flux of wide spectrum fast neutrons available from the core. Table 2.
A
3 Au B Br
CI Co Cr Cu Dy Eu
F
Fe Ga Ge Hg Ho
I
In Ir
X
Lu Mg Mn Mo N Na. Nb
0 P
Pd Pr Pt Re Rh Sb Sc Se
Si Sm Ta Te V Zn Zr
(Continued on page A788)
Limit of Detection b v Activation Analvsis of Selected Elements
-Thermal Element
The intensity of the thermal neutron obtained from the sources we have considered varies considerably. Table 1 lists the maximum flux of both fast (direct) and thermal (moderated) neutrons available from these sources far t,he rtetivation of smnples. The limit of detection of selected elements is given in Table 2. The deteetion limit is the number of micrograms of the element that will give 1 count per second in a detector having 10% counting efficiency, after activation for the time listed in a flux of 2 X 10' n/cma/sec of thermal neutrons, and 10* n/em'/sec of 14 Mev neutrons. Data. for thermal neutrons were recalculated to correspond to the above conditions from Bulletin AA of the High Voltage Engineering Corp., and for 14 Mev neutrons from Gillespie and Hill (see bibliography). The detection limit weight is inversely proportional to the activation flux, so the data in both Tables 1 and 2 will have to be considered t o determine the detection limits with a given source. These tables indicate that Pu-Be neutron howitzers
nu,
Time of activation
neutrons--Amount detectablen (micrograms)
1 hr 1 min 10 min 1 hr
10 Gin 45 min 1 hr 5 see 30 ndn 1 min 1 hr 20 min 40 min 1 hr 1 hr 2 min 1 hr 1 hr 1 hr 1 hr
...
1 hr 1 hr 45 min 1 hr 20 min 5 min 1 hr 1 min 1 hr 1 hr 1 hr 1 hr 1 hr 1 hr 1 hr 10 min 1 hr 1 hr 45 rnin 1 hr 1 hr 30 sec 1 hr 30 min 2 min 1 hr 1 hr 1 hr 1 hr 1 hr 5 min 1 hr 2 min 1 hr 1 hr 1 hr 1 hr 1hr 15 min 1 hr 1 hr
Mev neutrons---Amount Time of detectableb activation (micrograms)
-14
...
1 hr 1 hr 1 hr 35 min
...
1 hr 20 min 1 hr 45 min 4 min
...
1 min 12 min 1 hr 15 min 1 hr 1 hr 2 min 1 hr 3.8 300 3 . 2 x lo4 58 4 . 5 X lo8 1 . 8 X 108 14 3 . 3 x 10' 2 . 5 X 10'
...
1 hr 12 min 1 hr 1 hr 1 hr 25 mi" 1 hr 20 min
' 2 X 10'n/cm4/sec flux, 1count/secat lo'%counting efficiency. 10% n/cm4/sec flux, 1 count/8ec a t 107' counting efficiency.
A786 / Jaurnol of Chemicol Education
... 10 2.4 3 . 4 X 108 42 24 31 16 41
Chemical Instrumentation may he used to detect 1.3 mg of Ag, 0.13 mg of Dy, 12.6 mg of I, and 9.4 mg of V, and that the method is insensitive to Cr, Fe, K, N, 0,P, Si, and Zr. With the more potent sources of neutrons, many more elements can he determined with useful sensitivity using their greater neutron flux, hut more important is their fast neutron flux which activates many elements not significantly activated by thermal neutrons. The lack of signiticant activation of some elements by fast neutrons may he used to eliminate their interference, thus the choice of appropriate neutron energy for activation is an important factor in the selectivity of activation rtndysis. The reader is referred to the Leddicotte, and the Berl references in the bibliography for hundreds of specific applications. The important determinations now being made hy activation analysis, far example the direct, rapid determination of oxygen in organic compounds, make interesting reading, and explain the adoption of activation analysis in many laboratories as both a research and routine analytical too.
of this method can be made with a thermal neutron source consisting of Pu-Be moderated with water or paraffin, a Geieer counter and a scalor. To exnand
Bibliography
Conclusion
permit counting the gamma radiation of the desired isotope in the presence of other energies of gamma radiation from the sample matrix. An accelerator-type neutron generator would logically be the next item purchased, getting the most powerful one that the hudget will afford, especially if an interest in short half-life nuclides which are determined with only moderate sensitivity is anticipated. Simple shielding is required with the sealed accelerator generators, and more complete shielding with the pumped accelerators, For the latter, an expenditure of $2000 far shielding (or more, for reasons of convenience) should he anticipated. Finally, for the very short half-life isotopes, a pneumatic ssmple-transfer system is required, and a multi-channel pulse height analyzer will permit the simultaneous determination of two or more elements with very short half-lives in a sample, or give background information to permit allowances to he made for an
The sensitivity and selectivity of activation analysis were recognized many years ago, hut powerful moderately priced neutron sources have only recently been marketed. A modest beginning in the application
before making large expenditures for equipment, for activation analysis does not provide the ansurer to all analytical problems. Careful attention should he
A788 / Journal o f Chemical Educofion
given to the health hazards of high energy radiation, and of thermal neutrons, hut these necessary precautions should not unduly defer the use of the method.
BERL, W. G., ed. "Physical Methods in Chemical Analysis," vol. 111, pp. 449-621, "Neutron Spectroscopy and Neutron Interactions in Chemical Analysis," by T. I. Taylor and W. W. Havens, Jr., Academic Press (1956). Very good general reference with over 100 abstracts of amlications of activa.. tion analysis. GILLESPIE,A. S., Jr., AXD HILL, W. W., "Sensitivities for Activation Analvsin with 14 Mev Neutrons," Nucleonics, 19,170 (Nov. 1961). High Voltage Engineering Corp., "Activation Analysis with Van de Graaff Neutron Sources," Bulletin AA (1961), Burlington, Messachusetts. Tabulation of thermal-neutron artivation characteristics of isotopes. KOCH,R. C., "Activation Analysis Handbook," Academic Press, New Yark (1960). , , LEDDICOTTE, G. W., "Nucleonics," Anal. Chem., 34 143R (1962). Excellent review article including Activation Analysis and Activation Analvsis ADplications. Many references. VORRES,K. S., "Neutron Activation Experiments in Radiochemistry, " T H ~ JOURNAL, 37, 391, (1960). Very good classroom-orienated experiments using radioactive neutron sources.
