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Production of Mg28 in the pile and cyclotron at Harwell
International Journal of Applied Radiation and Isotopes, 1957, Vol. I, pp. 299-304, Pergamon Pre~ Ltd., London
Production of Mg in the Pile and Cyclotron at Harwell C. E. M E L L I S H and G. W. C R O C K F O R D Atomic Energy Research Establishment, Harwell, Didcot, Berks
(Received 1 June 1956) T w o methods of production of M g 2s are investigated. By b o m b a r d m e n t in the reactor the t, p reaction on magnesium in an alloy of lithium and magnesium is used and the dependence of the cross-section on the lithium content is determined. I n the cyclotron method the spallation of chlorine by proton b o m b a r d m e n t is examined and the dependence on proton energy and target size is measured. Production de aaMg dans la pile et le cyclotron de Harwell O n 6tudie deux modes de production de 2SMg. Par bombardement d a m le r~acteur e n utilise la r6action t, p sur le magn6sium d ' u n aUiage lithium-magndsium et on d&ermine la section efficace en fonction de la teneur en lithium. D a m la m6thode du cyclotron on examine la fission du chlore par b o m b a r d e m e n t protonique et on mesure l'effet de l'6nergie des protons et de la dimension de la cible. ]-loHyqeHHe M g 's B HOTHe H Ha KHRffiOTpOHe B XapBeHH PaeCMOTpeHI,I Rva l~IeToRa noayaeHrxn M g 'B. 13 I~OTHe 6/~y[a HcnoH~,3ovaHa peaH~an (t,p) Ha MaFHHH C blHHIeHbIO H3 enHaBa ffiHTHH C MaPHHeM H onpe~effieHa 3aBHCHMOCT/~eeqeHHn peaH~rm OT co~eprKaHHH HrrrHn. Ha ~HaffiOTpOHe 6I~Iffia aayqesa pea~t~aH pae~eHHeHHH xffiopa HpH 60M6apRHpoBKe HpOTOHaMH B 3aBHCHMOCTHOT oHepFHH HpOTOHa H paaMepoB MHIIIeHH.
Erzeugung von M g 2s im Reaktor und im Zyklotron in Harwell Zwei Methoden zur Herstellung yon M g zs werden geprfift: Durch Bestrahlung von Magnesiurn-Lithium-Legierungen im Reaktor wird die (t,p) Reaktion ausgeniitzt, die Abh/ingigkeit der Ausbeute yon Lithiumgehalt wird bestimmt. Bei der Zyld0tron-methode wird die Spalliation von Chlor durch Protonenbeschuss untersucht und deren Abh~ingigkeit yon Protonenenergie und Targetgr6sse wird gemessen.
INTRODUCTION Neutron irradiation of magnesium spallation of chlorine with 340-MeV protons compounds gives only an active isotope M g 27 in a cyclotron ~s) and by irradiation of a which has a half-life of 10 minutes. Mg 2s, lithium-magnesium alloy in the pile,~ 4) when half-life 21 hours, is therefore of considerable the following reactions occur :interest as being the only isotope of mag+ ."Li + 'ne nesium suitable for tracer application. It ~H -t~ M g ---> ~H + ~Mg decays by/~-emission to 2-minute A12s. The decay scheme is shown ~lJ in Fig. 1. At Hal-well, the isotope is made both in The isotope was first discovered ~z~ as a t h e reactor and in the 160-MeV-proton product of the reactions Si 3° (y, 2p)Mg 2s and cyclotron; a carrier-free product can be Mg26(~, 2p)Mg~S; it has been made Since by made f r o m cyclotron irradiation. 299
C. E. Mellish and G. W. Crockford
300
REACTOR M g 28
TV2 =21 hr'
/5,-i,28
PRODUCTION
S i 28
T1/2=2"3rain
Stable
/8-.49 0.4
PROCESS
efficient as the percentage of lithium is increased. The graphs are based on the irradiation of very small samples with negligible selfshielding; 1 g of alloy containing 25% lithium yields at saturation, in a flux of 1.5 ×1 012 n/cm2/sec, about 35 #c of M g 2 S + A1~8 in equilibrium, at a specific activity of 45 /~c/g Mg. Alternatively, 12% alloy containing the same amount of lithium gives 44#c of the mixture at a specific activity of 25 #c/g Mg. These are typical 60
~-2.87M e v ~
_j278
55
/ /
50 FIG. 1. Decay scheme of M g 28. 45
Lithium-magnesium alloy, of which the two kinds readily available contain 12 and 25% lithium by weight, is rolled into thin strips to minimize self-shielding from the neutron flux, and irradiated with neutrons for a few days. After irradiation, the main activities present are Na 24, Zn 6~, Cu 64, and Mn 56, produced by neutron capture in impurities. The irradiated material is therefore purified by three H~S precipitations in alkaline ammoniacal solution, using fresh carrier for the impurities for each precipitation. This is followed by two precipitations of Mg(OH)2 with caustic soda. Yield
A range of Li-Mg alloys was studied, containing from 2-25% lithium, and the results are shown in Figs. 2 and 3. Up to 12% lithium the specific activity (activity per g of Mg) is very nearly proportional to the percentage of lithium in the alloy (Fig. 2), but the result for a 25% alloy shows a marked departure from linearity. From Fig. 3, showing the activity obtained per gram of lithium irradiated, it follows that the process of using the tritons for reaction with magnesium becomes steadily less
40
/
/
/"
35
oi
/
~L 25
20
i
105 /o 5
10
15 20 % Li in olloy
25
FIG. 2. Saturation activity of (Mg *s + AP s) in microcuries/g Mg as a function of lithium content of the alloy. (Neutron flux 1.5 x l01S n[cm2[sec.)
301
Production of Mg 2s in the pile and cyclotronat Harwell
energy (about 2-2 MeV) to react with a Mg 2e nucleus; X represents the other body in an unproductive collision, which deprives the triton of its kinetic energy of recoil and does not produce Mg 28. Now, assuming a steady concentration of
300
.\ =L
~H*: Kl(ln) (~Li) = K 2\l/aH*~J \12{26M°"~5/ + Ka(~H* ) (X)
\
i.e.
(~H*) ---gl(~n) (~ai) K~(~Mg) ÷ K 3 ( X )
.'. rate of production of~2SMg
20C
\
3 * )(12Mg) 26 = K2(1H glK2(~n)(a~Li)(l~Mg) g~(~Mg) ÷ Kz(X ) _
At saturation, this rate of production will be equal to the disintegration rate. The denominator is approximately K s ( X ), since K2(a2~Mg) is small in comparison. Also the neutron concentration (01n) is proportional to .10o
Fro. 3.
5
10 15 20 % Li in elloy
25
Saturation activity of (Mg 's + A1~') in
microcuries/gLi as a function of lithium content of the alloy. (Neutronflux 1.5 × 10azn/cm2]sec). figures based on the irradiation of alloy rolled to foil so as just to fit into a 3" long, 1" diameter, cylindrical standard irradiation can. The linear dependence of specific activity on lithium percentage up to 12% Li can be explained simply by postulating a steady concentration of tritons in the irradiated specimens in the usual kinetic way, thus :0an + ~Li x,_+ ~H* + ~He ~H* + l~Mg 26 2s x,_+ 12Mg ÷ 1H ~H* + X % - + a a H + X * Kx, K2, Ks, are the three rate constants, laH* is a triton possessing enough of its original
(%
the flux ¢, so that we may put o
= K4(¢). It then follows that: Specific activity of ~ M g = Activity of ~S2Mg Mass of Mg = K,f(~Li)(~Mg) (X) (Mg) = gs¢(%Li) The yield of Mg 2s from mixtures of fused L i C l - k - M g O was investigated, but even with a very high lithium-magnesium ratio much lower specific activities were obtained than with the alloys. This is not surprising, as foreign atoms decrease the chance of a recoil triton colliding with an Mg 26 nucleus in its short path. Even in the alloy the reacting nuclei Li 6 and Mg *s are only 7% and 1 1-1% abundant respectively in their parent elements.
CYCLOTRON PRODUCTION PROCESS The nuclear reactions used for this process CI ~ (p;6p,2n) Mg ~s are those of 160-MeV protons with the two C137 (p;6n,4p) Mg ~s stable chlorine isotopes, the reactions being written formally As a convenient form for the chlorine 5---(20 pp.)
302
C. E. MeUish and G. W. Crockford
target, fused potassium chloride is used in the shape of a small vertical cylinder, which is wrapped in thin aluminium foil to conduct away heat during the irradiation. The use of potassium rather than a n o t h e r metal increases the yield of Mg 2s by about 10% by spallation of this element. The magnesium is produced in the target in a carrier-free state, as are also the unwanted activities F 18, Na~4, Si31, p32, $35. These together with chlorine and potassium activities, are removed on ion-exchange columns in the following way: The target is dissolved in 10% nitric acid and boiled for five minutes to convert Si 3a, p32, and S 85 to silicate, phosphate and sulphate respectively. Several grams of sodium hydrogen phosphate and a few milligrams of Fe +++ are added, and the solution then made alkaline. Nearly all the Mg 28 is carried down on the ferric hydroxide precipitate, which is dissolved in dilute HC1 and reprecipitated. Most of the activity in the target, the Na 24, S 85 C134, K 42, and most of the p32, is left at this stage in the supernate. The precipitate is then dissolved in .05 N HC1 and the solution added to a column of Zeokarb 225 cation-exchange resin. On elution with -05 N HCI, all anionic activities present pass through, leaving Mg 28 and the inactive iron at the top. These cations are eluted with 6 N HCI; the product solution is evaporated to dryness, taken up in 10 N HC1, and added to a column of De Acidite FF anion-exchange resin. The magnesium passes through and the iron is retained. Thus the Mg 28 is separated carrier-free and with no inactive contaminants, the chemical processing taking 2-3 hours. For research experiments, carrier magnesium was added, and the isotope purified by ferric-hydroxide scavenges in ammoniacal solution. It was finally precipitated as oxinate for determination of fi-particle activity. Yields The variations of yield with incident proton energy (Fig. 4) and with target size (Fig. 5) have been studied. To investigate the effect of proton energy, thin targets of
NaC1 were used sandwiched between thin A1 foils. Sodium chloride was used for these experiments so that the absolute cross-section could be obtained for spallation of the chlorine isotopes only. The intensity of the proton beam was assessed by measuring the Na 24 produced in the aluminium with an end-window Geiger counter after leaving the foils for a day to allow short-lived activity to decay. Using the cross-section of the reaction A127(p, 3pn)Na 24 given by STEVENSON, Hlcr:s, and F O L O E R , (5) the cross-section for formation of Mg 28 from chlorine could then be calculated. The results show a threshold for the reaction near 60 MeV, and a rise of cross-section with energy which persists up to the highest energy available. Fig. 5, showing the variation of yield with target mass at 160 MeV demonstrates the importance of using a fairly large target to obtain a good yield in reactions with highenergy protons (protons of 160-MeV energy have a range in aluminium of 22 g/cm2). In
/
1.0
1
/
1,.,
-~ o . s - - -
/
/ 0
20
40
60
8 0 100 120 140 160 MeV FIo. 4. Cross-section of chlorine for production of M g 2s as a function of proton energy.
Production of Mg ~s in the pile and cyclotron at Harwell
//
303
by preparing sources of magnesium oxinate and counting them under an "end-window" Geiger counter. A correction for backscattering was applied from the figures of BURTT;~s) the counts were corrected to zero 4C absorber and for self absorption in the source by plotting absorber curves and making up sources of different weights. The geometrical efficiency of the counting arrangement was determined by counting a calibrated ps~ source (also correcting for backscattering and absorption). 02 The result arrived at in this way gives the total activity of Mg ~8 + A1~s in the source, 2O and the figures quoted in this paper refer to equilibrium activities of this kind, with the exception of the cross-sections, in Fig. 4 which refer to Mg ~s only. 10 The accuracy of the figures for reactor produced Mg 2s is largely that of the counting method, the spread of results between experiments being about -4-3%. When allowance is made for possible systematic 0 10 20 3o errors, the overall accuracy of the points in g KC~ in torget Fig. 2 is probably not better than ± 1 0 % . FIG. 5. Yields of (IVIg2s + AI 2s) in the cyclotron from 160 M e V protons with K C I targets of different masses. The cross-sections for cyclotron productions general, targets of more than 15-20 grams in Fig. 4 are considerably less accurate; this are inconvenient to handle, and 40/~c/hr of is mainly due to difficulty in obtaining exact the isotope mixture is the figure aimed at in alignment of the edges of the sodium-chloride targets and the aluminium monitor foils. production. Thus it is considered that absolute values Counting for the cross-section may be up to 25% in The combined Mg ~s + A12s activity from error. both methods of production was measured 5C
/
I/
f
/
f
FUTURE PROSPECTS
(a) The Reactor Process. The approximate overall cross-section for Mg ~e to produce Mg ~s in the 25% alloy is 0-5 millibarn. This gives low yields with the present reactor facilities, but should provide specific activities in the region of millicuries per gram in the high flux reactors which will soon be available in this country. Under these conditions other (t, p) reactions may possibly be made to give useful yields, e.g. for the production of the long-lived Si3~, and for the as yet undiscovered O 2°. We have not been able to obtain either of these isotopes with our present facilities, and (t, p) reaction studies by other authors
gave no result for the reaction P31(t,p)P33, in lithium phosphate. (b) The Cyclotron Process The cross-sections of spallation reactions usually vary little with increasing energy of the incident particles after a sharp initial rise from their thresholds; the figures obtained here show the limiting cross-section has not been reached at 160 MeV for this reaction. Considerably higher yields could probably be obtained, therefore, from cyclotrons of higher energy, provided that the beam current is not significantly smaller than the nominal 1 /~amp of the Harwell machine.
304
C. E. Mellish and G. W. Crockford: Production of Mg ~8 in the pile and cyclotron at Harwell REFERENCES
1. SHELINER. K. and JOHNSONN . R . Phys. Rev. 94, 1642 (1954). 2. SHELINER. K. and JOHNSON N . R . Phys. Rev. 89, 520 (1953). 3. LXNDNERM. Phys. Rev. 89~ 1150 (1953).
4. IW~RS~NE., KOSKIW. S., and RASETTI F. Phys. Rev. 91, 1229 (1953). 5. STEVENSONP. C., HIcKs H. G., and FOLGER R. L. Report UCRL 4371. 6. BURTT E. Nucleonics 5, No. 2, 28 (1949).
Production of Mg in the Pile and Cyclotron at Harwell C. E. M E L L I S H and G. W. C R O C K F O R D Atomic Energy Research Establishment, Harwell, Didcot, Berks
(Received 1 June 1956) T w o methods of production of M g 2s are investigated. By b o m b a r d m e n t in the reactor the t, p reaction on magnesium in an alloy of lithium and magnesium is used and the dependence of the cross-section on the lithium content is determined. I n the cyclotron method the spallation of chlorine by proton b o m b a r d m e n t is examined and the dependence on proton energy and target size is measured. Production de aaMg dans la pile et le cyclotron de Harwell O n 6tudie deux modes de production de 2SMg. Par bombardement d a m le r~acteur e n utilise la r6action t, p sur le magn6sium d ' u n aUiage lithium-magndsium et on d&ermine la section efficace en fonction de la teneur en lithium. D a m la m6thode du cyclotron on examine la fission du chlore par b o m b a r d e m e n t protonique et on mesure l'effet de l'6nergie des protons et de la dimension de la cible. ]-loHyqeHHe M g 's B HOTHe H Ha KHRffiOTpOHe B XapBeHH PaeCMOTpeHI,I Rva l~IeToRa noayaeHrxn M g 'B. 13 I~OTHe 6/~y[a HcnoH~,3ovaHa peaH~an (t,p) Ha MaFHHH C blHHIeHbIO H3 enHaBa ffiHTHH C MaPHHeM H onpe~effieHa 3aBHCHMOCT/~eeqeHHn peaH~rm OT co~eprKaHHH HrrrHn. Ha ~HaffiOTpOHe 6I~Iffia aayqesa pea~t~aH pae~eHHeHHH xffiopa HpH 60M6apRHpoBKe HpOTOHaMH B 3aBHCHMOCTHOT oHepFHH HpOTOHa H paaMepoB MHIIIeHH.
Erzeugung von M g 2s im Reaktor und im Zyklotron in Harwell Zwei Methoden zur Herstellung yon M g zs werden geprfift: Durch Bestrahlung von Magnesiurn-Lithium-Legierungen im Reaktor wird die (t,p) Reaktion ausgeniitzt, die Abh/ingigkeit der Ausbeute yon Lithiumgehalt wird bestimmt. Bei der Zyld0tron-methode wird die Spalliation von Chlor durch Protonenbeschuss untersucht und deren Abh~ingigkeit yon Protonenenergie und Targetgr6sse wird gemessen.
INTRODUCTION Neutron irradiation of magnesium spallation of chlorine with 340-MeV protons compounds gives only an active isotope M g 27 in a cyclotron ~s) and by irradiation of a which has a half-life of 10 minutes. Mg 2s, lithium-magnesium alloy in the pile,~ 4) when half-life 21 hours, is therefore of considerable the following reactions occur :interest as being the only isotope of mag+ ."Li + 'ne nesium suitable for tracer application. It ~H -t~ M g ---> ~H + ~Mg decays by/~-emission to 2-minute A12s. The decay scheme is shown ~lJ in Fig. 1. At Hal-well, the isotope is made both in The isotope was first discovered ~z~ as a t h e reactor and in the 160-MeV-proton product of the reactions Si 3° (y, 2p)Mg 2s and cyclotron; a carrier-free product can be Mg26(~, 2p)Mg~S; it has been made Since by made f r o m cyclotron irradiation. 299
C. E. Mellish and G. W. Crockford
300
REACTOR M g 28
TV2 =21 hr'
/5,-i,28
PRODUCTION
S i 28
T1/2=2"3rain
Stable
/8-.49 0.4
PROCESS
efficient as the percentage of lithium is increased. The graphs are based on the irradiation of very small samples with negligible selfshielding; 1 g of alloy containing 25% lithium yields at saturation, in a flux of 1.5 ×1 012 n/cm2/sec, about 35 #c of M g 2 S + A1~8 in equilibrium, at a specific activity of 45 /~c/g Mg. Alternatively, 12% alloy containing the same amount of lithium gives 44#c of the mixture at a specific activity of 25 #c/g Mg. These are typical 60
~-2.87M e v ~
_j278
55
/ /
50 FIG. 1. Decay scheme of M g 28. 45
Lithium-magnesium alloy, of which the two kinds readily available contain 12 and 25% lithium by weight, is rolled into thin strips to minimize self-shielding from the neutron flux, and irradiated with neutrons for a few days. After irradiation, the main activities present are Na 24, Zn 6~, Cu 64, and Mn 56, produced by neutron capture in impurities. The irradiated material is therefore purified by three H~S precipitations in alkaline ammoniacal solution, using fresh carrier for the impurities for each precipitation. This is followed by two precipitations of Mg(OH)2 with caustic soda. Yield
A range of Li-Mg alloys was studied, containing from 2-25% lithium, and the results are shown in Figs. 2 and 3. Up to 12% lithium the specific activity (activity per g of Mg) is very nearly proportional to the percentage of lithium in the alloy (Fig. 2), but the result for a 25% alloy shows a marked departure from linearity. From Fig. 3, showing the activity obtained per gram of lithium irradiated, it follows that the process of using the tritons for reaction with magnesium becomes steadily less
40
/
/
/"
35
oi
/
~L 25
20
i
105 /o 5
10
15 20 % Li in olloy
25
FIG. 2. Saturation activity of (Mg *s + AP s) in microcuries/g Mg as a function of lithium content of the alloy. (Neutron flux 1.5 x l01S n[cm2[sec.)
301
Production of Mg 2s in the pile and cyclotronat Harwell
energy (about 2-2 MeV) to react with a Mg 2e nucleus; X represents the other body in an unproductive collision, which deprives the triton of its kinetic energy of recoil and does not produce Mg 28. Now, assuming a steady concentration of
300
.\ =L
~H*: Kl(ln) (~Li) = K 2\l/aH*~J \12{26M°"~5/ + Ka(~H* ) (X)
\
i.e.
(~H*) ---gl(~n) (~ai) K~(~Mg) ÷ K 3 ( X )
.'. rate of production of~2SMg
20C
\
3 * )(12Mg) 26 = K2(1H glK2(~n)(a~Li)(l~Mg) g~(~Mg) ÷ Kz(X ) _
At saturation, this rate of production will be equal to the disintegration rate. The denominator is approximately K s ( X ), since K2(a2~Mg) is small in comparison. Also the neutron concentration (01n) is proportional to .10o
Fro. 3.
5
10 15 20 % Li in elloy
25
Saturation activity of (Mg 's + A1~') in
microcuries/gLi as a function of lithium content of the alloy. (Neutronflux 1.5 × 10azn/cm2]sec). figures based on the irradiation of alloy rolled to foil so as just to fit into a 3" long, 1" diameter, cylindrical standard irradiation can. The linear dependence of specific activity on lithium percentage up to 12% Li can be explained simply by postulating a steady concentration of tritons in the irradiated specimens in the usual kinetic way, thus :0an + ~Li x,_+ ~H* + ~He ~H* + l~Mg 26 2s x,_+ 12Mg ÷ 1H ~H* + X % - + a a H + X * Kx, K2, Ks, are the three rate constants, laH* is a triton possessing enough of its original
(%
the flux ¢, so that we may put o
= K4(¢). It then follows that: Specific activity of ~ M g = Activity of ~S2Mg Mass of Mg = K,f(~Li)(~Mg) (X) (Mg) = gs¢(%Li) The yield of Mg 2s from mixtures of fused L i C l - k - M g O was investigated, but even with a very high lithium-magnesium ratio much lower specific activities were obtained than with the alloys. This is not surprising, as foreign atoms decrease the chance of a recoil triton colliding with an Mg 26 nucleus in its short path. Even in the alloy the reacting nuclei Li 6 and Mg *s are only 7% and 1 1-1% abundant respectively in their parent elements.
CYCLOTRON PRODUCTION PROCESS The nuclear reactions used for this process CI ~ (p;6p,2n) Mg ~s are those of 160-MeV protons with the two C137 (p;6n,4p) Mg ~s stable chlorine isotopes, the reactions being written formally As a convenient form for the chlorine 5---(20 pp.)
302
C. E. MeUish and G. W. Crockford
target, fused potassium chloride is used in the shape of a small vertical cylinder, which is wrapped in thin aluminium foil to conduct away heat during the irradiation. The use of potassium rather than a n o t h e r metal increases the yield of Mg 2s by about 10% by spallation of this element. The magnesium is produced in the target in a carrier-free state, as are also the unwanted activities F 18, Na~4, Si31, p32, $35. These together with chlorine and potassium activities, are removed on ion-exchange columns in the following way: The target is dissolved in 10% nitric acid and boiled for five minutes to convert Si 3a, p32, and S 85 to silicate, phosphate and sulphate respectively. Several grams of sodium hydrogen phosphate and a few milligrams of Fe +++ are added, and the solution then made alkaline. Nearly all the Mg 28 is carried down on the ferric hydroxide precipitate, which is dissolved in dilute HC1 and reprecipitated. Most of the activity in the target, the Na 24, S 85 C134, K 42, and most of the p32, is left at this stage in the supernate. The precipitate is then dissolved in .05 N HC1 and the solution added to a column of Zeokarb 225 cation-exchange resin. On elution with -05 N HCI, all anionic activities present pass through, leaving Mg 28 and the inactive iron at the top. These cations are eluted with 6 N HCI; the product solution is evaporated to dryness, taken up in 10 N HC1, and added to a column of De Acidite FF anion-exchange resin. The magnesium passes through and the iron is retained. Thus the Mg 28 is separated carrier-free and with no inactive contaminants, the chemical processing taking 2-3 hours. For research experiments, carrier magnesium was added, and the isotope purified by ferric-hydroxide scavenges in ammoniacal solution. It was finally precipitated as oxinate for determination of fi-particle activity. Yields The variations of yield with incident proton energy (Fig. 4) and with target size (Fig. 5) have been studied. To investigate the effect of proton energy, thin targets of
NaC1 were used sandwiched between thin A1 foils. Sodium chloride was used for these experiments so that the absolute cross-section could be obtained for spallation of the chlorine isotopes only. The intensity of the proton beam was assessed by measuring the Na 24 produced in the aluminium with an end-window Geiger counter after leaving the foils for a day to allow short-lived activity to decay. Using the cross-section of the reaction A127(p, 3pn)Na 24 given by STEVENSON, Hlcr:s, and F O L O E R , (5) the cross-section for formation of Mg 28 from chlorine could then be calculated. The results show a threshold for the reaction near 60 MeV, and a rise of cross-section with energy which persists up to the highest energy available. Fig. 5, showing the variation of yield with target mass at 160 MeV demonstrates the importance of using a fairly large target to obtain a good yield in reactions with highenergy protons (protons of 160-MeV energy have a range in aluminium of 22 g/cm2). In
/
1.0
1
/
1,.,
-~ o . s - - -
/
/ 0
20
40
60
8 0 100 120 140 160 MeV FIo. 4. Cross-section of chlorine for production of M g 2s as a function of proton energy.
Production of Mg ~s in the pile and cyclotron at Harwell
//
303
by preparing sources of magnesium oxinate and counting them under an "end-window" Geiger counter. A correction for backscattering was applied from the figures of BURTT;~s) the counts were corrected to zero 4C absorber and for self absorption in the source by plotting absorber curves and making up sources of different weights. The geometrical efficiency of the counting arrangement was determined by counting a calibrated ps~ source (also correcting for backscattering and absorption). 02 The result arrived at in this way gives the total activity of Mg ~8 + A1~s in the source, 2O and the figures quoted in this paper refer to equilibrium activities of this kind, with the exception of the cross-sections, in Fig. 4 which refer to Mg ~s only. 10 The accuracy of the figures for reactor produced Mg 2s is largely that of the counting method, the spread of results between experiments being about -4-3%. When allowance is made for possible systematic 0 10 20 3o errors, the overall accuracy of the points in g KC~ in torget Fig. 2 is probably not better than ± 1 0 % . FIG. 5. Yields of (IVIg2s + AI 2s) in the cyclotron from 160 M e V protons with K C I targets of different masses. The cross-sections for cyclotron productions general, targets of more than 15-20 grams in Fig. 4 are considerably less accurate; this are inconvenient to handle, and 40/~c/hr of is mainly due to difficulty in obtaining exact the isotope mixture is the figure aimed at in alignment of the edges of the sodium-chloride targets and the aluminium monitor foils. production. Thus it is considered that absolute values Counting for the cross-section may be up to 25% in The combined Mg ~s + A12s activity from error. both methods of production was measured 5C
/
I/
f
/
f
FUTURE PROSPECTS
(a) The Reactor Process. The approximate overall cross-section for Mg ~e to produce Mg ~s in the 25% alloy is 0-5 millibarn. This gives low yields with the present reactor facilities, but should provide specific activities in the region of millicuries per gram in the high flux reactors which will soon be available in this country. Under these conditions other (t, p) reactions may possibly be made to give useful yields, e.g. for the production of the long-lived Si3~, and for the as yet undiscovered O 2°. We have not been able to obtain either of these isotopes with our present facilities, and (t, p) reaction studies by other authors
gave no result for the reaction P31(t,p)P33, in lithium phosphate. (b) The Cyclotron Process The cross-sections of spallation reactions usually vary little with increasing energy of the incident particles after a sharp initial rise from their thresholds; the figures obtained here show the limiting cross-section has not been reached at 160 MeV for this reaction. Considerably higher yields could probably be obtained, therefore, from cyclotrons of higher energy, provided that the beam current is not significantly smaller than the nominal 1 /~amp of the Harwell machine.
304
C. E. Mellish and G. W. Crockford: Production of Mg ~8 in the pile and cyclotron at Harwell REFERENCES
1. SHELINER. K. and JOHNSONN . R . Phys. Rev. 94, 1642 (1954). 2. SHELINER. K. and JOHNSON N . R . Phys. Rev. 89, 520 (1953). 3. LXNDNERM. Phys. Rev. 89~ 1150 (1953).
4. IW~RS~NE., KOSKIW. S., and RASETTI F. Phys. Rev. 91, 1229 (1953). 5. STEVENSONP. C., HIcKs H. G., and FOLGER R. L. Report UCRL 4371. 6. BURTT E. Nucleonics 5, No. 2, 28 (1949).