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Cavitation nucleated by 10B(n, α)7Li
NUCLEAR
INSTRUMENTS
AND METHODS
82
(197o) 3 1 o - 3 1 2 ;
© NORTH-HOLLAND
PUBLISHING
CO.
CAVITATION NUCLEATED BY l°B(n#)VLi M. G R E E N S P A N and C. E. T S C H I E G G
National Bureau of Standards, Institute of Basic Standards, Washington, D.C. 20234, U.S.A.
Received 8 December 1969 Acoustic cavitation has been nucleated on the reaction 10B(n,~)TLi in methanol solution. Implications for a slow neutron detector or counter are discussed. Acoustic cavitation, which is the rupture (or boiling) of a liquid initiated during the tensile phase of an alternating stress, occurs only on pre-existing nuclei1). The naturally occuring nuclei, presumed to be gas or vapor bubbles somehow stabilized on motes, or poorly wetted motes, can be removed to such a degree that for a particular amplitude of sound pressure, the liquid will cavitate only when exposed to a suitable source of ionizing radiation. Neutrons, c~-disintegrations, and fission fragments (fast, slow and spontaneous) have been used. In the case of neutrons, it is supposed that the nucleation is connected with the deposition, in the liquid, of the kinetic energy acquired by an atomic nucleus of one of the heavier elements in the liquid as a result of an elastic collision with a neutron. In the cases of c~-disintegration and of fission, the sources are in solution in the test liquid, and the heavy recoil nuclei are responsible. It should be emphasized that a cavitation event is in no sense collective; it arises from the action of a single neutron, for instance. These matters are discussed in ref. 1), which gives also a complete set of references. In 1967, West 2) suggested that a cavitation cell containing a dissolved boron compound might be used as a slow-neutron detector or counter. The reaction proposed was l°B(n,~)7Li. An enriched organic boron compound was dissolved in tetrachloroethylene (C2C14) and exposed to thermal neutrons. The effect was not observed in CzC14 nor in the "weaker" liquid, methanol, although both were sensitive to fast neutrons from a Pu-Be source. West surmised that the negative pressures which he could achieve were too small. It turns out that West was correct. We have observed the effect in methanol at 29 ° C. The "threshold" negative pressure is about 12 bar but greater stresses are needed to make the result unambiguous. At 20 bar, the effect is very pronounced. For comparison we note that the threshold of methanol at 29°C to Pu-Be neutrons (presumably to the 10 MeV neutrons) is about 5 bar, and to natural Th a-recoils (presumably the 8.95 MeV c(s from ThC'), about 11 bar1). As the work progressed, it became apparent that a
thermal neutron counter based on the effect would have several disadvantages relative to those now in use. Therefore we confined our efforts to a determination of the circumstances under which the effect in question can be produced. Perhaps other, more attractive reactions can be found. An obvious one, fission is discussed later on. The cavitation cell was a 10 cm OD by 15 cm high barium titanate cylinder (volume 1.2 1)fitted with sheet brass ends 0.075 m m thick. It was self driven in the (3, 0, 1)-mode at about 28.7 kHz. Details are given in ref. 1). The cell was placed about 30 cm from a 244Cm fission neutron source emitting l a x 104 n/s into 4z~. The source was surrounded by 4 1-1b blocks of paraffin to help thermalize the neutrons. The geometry was not disturbed during the set of tests. The cell was removed when necessary to change the sample and replaced carefully. Measurements taken at the location of the cell with an ordinary BF3 neutron dosimeter showed 5 to 6 slow (no shield), and 3 to 4 fast (paraffin plus Cd shield), neutrons per cm z per second. These figures are probably correct within a factor of two. Boric acid was selected for the solute because it is readily available. The first tests were made with the natural material, in which the abundance of I°B is about 20%. Although the results were positNe, it was felt necessary to repeat the tests with samples b o t h enriched and depleted in t°B ill order to rule out artifacts. The compositions of the samples are given in table 1. In all cases the solvent was methanol, and the solute (if any) was H3BO3. For each sample, the cavitation events were counted TABLE 1 Composition of samples.
310
Sample
Symbol (fig. 1)
A B C D
© • @ @
Concentration (g/l) Total B l°B 0.0 1.69 1.82 0.228
0.0 0.025 1.67 0.209
CAVITATION NUCLEATED BY I00
/ I
10
~..
Z
I
_
-
~/~@ o
O.
8
I0
12
14 16 -P, BAR ABS
18
20
22
Fig. 1. Cavitation rate vs negative pressure in methanol at 29°C containing 10B. Negative pressure is amplitude at 28.7 kHz minus steady pressure (1 bar). See table 1 for key.
for 10 min for each of several sound pressures. Four runs were made for each sample. All the data, corrected for dead time (0.01 min) are shown in fig. 1. The broken lines, where shown, connect the averages of four points. The data are too few to be worth statistical analysis* but the effect is obvious. Curve A represents the "background", i.e. the count arising from the action of fast neutrons on methanol. The increased counts for the other curves arise from the l°B(n,~.)TLi reaction induced by slow neutrons. Of course, some of these are owing to slowing down of fast neutrons by the methanol itself. The excess count rate does not increase as fast as the concentration of *°B because the liquid in the cell is a thick target. That is, as the concentration of I°B is increased a larger faction of the thermal neutrons is absorbed in the peripheral region where the negative pressure is below cavitation threshold. Still, at - 20 bar, the cavitation rate for 1.67 g/1 l°B (curve C) is four times the background (curve A). A comparison of curves B and C, for which the concentrations of H3BO 3 were about the same, indicates that the effect is real. As a further check, a run on a solution of 100 g of LiNO3 per liter of methanol was made. The results were not distinguishable from those for pure methanol. To see whether or not the results are reasonable, one * The total count is about 10 times the ordinate.
t°B(n, @7Li
311
can calculate the stopping power of methanol for (a) 12C and 160 ions for various energies they would get from neutron impact, (b) Z°sPb recoils from e-disintegration of ThC', (c) 7Li ions from the reaction in question and (d) a-particles and 3H ions from the reaction 6Li(n,c03H. This was done by the method of Booth and Grant 3) for cases (a), (c) and (d) and that of Lindhard, Scharff and Schiott, taken from Northcliffe4), for case (b). The necessary proton stopping powers were calculated from data given in Whaling's review 5) on the assumption that the atomic stopping powers are additive. At the low energies with which we have to deal, both the BG method and the additivity above mentioned are poor, but the calculations are nevertheless roughly consistent with the experimental results of ref. 1) on the basis of the obvious notion, due to Lieberman6), that the higher the m a x i m u m stopping powe~ for an ion as it slows from its initial energy, the lower the cavitation threshold. The most obvious disadvantage that a thermal neutron detector or counter based on the present effect would have is its high sensitivity to fast neutrons. Even if the fast flux were relatively low, a blank would have to be run. Another disadvantage is the long dead time. This is not as bad as it looks; the 0.6 sec dead time was used as a laboratory convenience. We have operated some cells with a dead time of 20 ms and West and Howlett 7) have achieved 8 ms. Also, the cell is temperature sensitive. For good results the temperature should be held to _ 1° C. This is not much of a problem. We enclose the cell in a foamed-polystyrene box (such as is sold as a picnic box) equipped with an ~ncandescent lamp as as heater and with a simple thermostat. This arrangement works well if the operating temperature is a few degrees above ambient. The device is rather inefficient. Rough calculations based on the overall cell dimensions give an efficiency of 10 .3 to 10 . 4 at the highest negative pressure used, 20 bars. Poor efficiency is inherent in the device itself and probably is not susceptible to much improvement. There are two aspects to this. First, the "effective" volume 1) of the cell, which is the fraction of the total volume within which the negative pressure is above threshold (suitably weighted for time), is small. Second, a large fraction of the incident neutrons is absorbed in the peripheral, insensitive regions of the cell as already explained. The scheme has some advantages. The apparatus is fairly simple, and completely 7-ray insensitive. The cable from the cell itself to the rest of the apparatus and to the observer can be up to a hundred meters long.
312
M. G R E E N S P A N AND C. E. T S C H I E G G
A n obvious way to separate the effects o f slow and fast neutrons is to a b a n d o n the l°B(n,~)VLi reaction in favor of fission, as the cavitation threshold on fission events is very low1). Thus solutions containing 235U o r 2 3 9 p u would have high sensitivity to thermal neutrons, and those containing 2 3 s u or 232Th would be sensitive to fast neutrons. So far our experiments with fission in the lower alcohols, which are " w e a k " liquids are not encouraging. There is a tendency for the cavitation bubbles to persist so long that a dead time of several seconds would have to be built in. The most promising c o m b i n a t i o n so far is uranyl nitrate in water. Recent w o r k indicates that at 80°C the threshold is only a b o u t 2 bar, so that at (say) 4 bar the count rate should be fairly high. We hope to pursue this further. In any case this letter is directed principally at the
demonstration o f a new way in which the interaction o f a n e u t r o n with a nucleus gives rise to a macroscopic event.
References 1) M. Greenspan and C. E. Tschiegg, J. Res. Nat. Bur. Std. 71C (1967) 299. e) C. West, Cavitation nucleation by energetic particles, U. K. Atomic Energy Authority Research Group Report AERER5486 (1967). 8) W. Booth and I S. Grant, Nucl. Phys. 63 (1965) 481. 4) L. C. Northcliffe, in Studies in penetration of charged particles in matter, NAS-NRC Publ. 1133, Nucl. Sci. Ser. Report no. 39 (1964). 5) W. Whaling, in Encyclopedia of physics 34 (ed. S. Fliigge; Springer-Verlag, Berlin, 1958) p. 193. 6) D. Lieberman, Phys. Fluids 2 (1959) 466. 7) C. West and R. Howlett, Brit. J. Appl. Phys. (J. Phys. D) Ser. 2, 1 (1968) 247.
INSTRUMENTS
AND METHODS
82
(197o) 3 1 o - 3 1 2 ;
© NORTH-HOLLAND
PUBLISHING
CO.
CAVITATION NUCLEATED BY l°B(n#)VLi M. G R E E N S P A N and C. E. T S C H I E G G
National Bureau of Standards, Institute of Basic Standards, Washington, D.C. 20234, U.S.A.
Received 8 December 1969 Acoustic cavitation has been nucleated on the reaction 10B(n,~)TLi in methanol solution. Implications for a slow neutron detector or counter are discussed. Acoustic cavitation, which is the rupture (or boiling) of a liquid initiated during the tensile phase of an alternating stress, occurs only on pre-existing nuclei1). The naturally occuring nuclei, presumed to be gas or vapor bubbles somehow stabilized on motes, or poorly wetted motes, can be removed to such a degree that for a particular amplitude of sound pressure, the liquid will cavitate only when exposed to a suitable source of ionizing radiation. Neutrons, c~-disintegrations, and fission fragments (fast, slow and spontaneous) have been used. In the case of neutrons, it is supposed that the nucleation is connected with the deposition, in the liquid, of the kinetic energy acquired by an atomic nucleus of one of the heavier elements in the liquid as a result of an elastic collision with a neutron. In the cases of c~-disintegration and of fission, the sources are in solution in the test liquid, and the heavy recoil nuclei are responsible. It should be emphasized that a cavitation event is in no sense collective; it arises from the action of a single neutron, for instance. These matters are discussed in ref. 1), which gives also a complete set of references. In 1967, West 2) suggested that a cavitation cell containing a dissolved boron compound might be used as a slow-neutron detector or counter. The reaction proposed was l°B(n,~)7Li. An enriched organic boron compound was dissolved in tetrachloroethylene (C2C14) and exposed to thermal neutrons. The effect was not observed in CzC14 nor in the "weaker" liquid, methanol, although both were sensitive to fast neutrons from a Pu-Be source. West surmised that the negative pressures which he could achieve were too small. It turns out that West was correct. We have observed the effect in methanol at 29 ° C. The "threshold" negative pressure is about 12 bar but greater stresses are needed to make the result unambiguous. At 20 bar, the effect is very pronounced. For comparison we note that the threshold of methanol at 29°C to Pu-Be neutrons (presumably to the 10 MeV neutrons) is about 5 bar, and to natural Th a-recoils (presumably the 8.95 MeV c(s from ThC'), about 11 bar1). As the work progressed, it became apparent that a
thermal neutron counter based on the effect would have several disadvantages relative to those now in use. Therefore we confined our efforts to a determination of the circumstances under which the effect in question can be produced. Perhaps other, more attractive reactions can be found. An obvious one, fission is discussed later on. The cavitation cell was a 10 cm OD by 15 cm high barium titanate cylinder (volume 1.2 1)fitted with sheet brass ends 0.075 m m thick. It was self driven in the (3, 0, 1)-mode at about 28.7 kHz. Details are given in ref. 1). The cell was placed about 30 cm from a 244Cm fission neutron source emitting l a x 104 n/s into 4z~. The source was surrounded by 4 1-1b blocks of paraffin to help thermalize the neutrons. The geometry was not disturbed during the set of tests. The cell was removed when necessary to change the sample and replaced carefully. Measurements taken at the location of the cell with an ordinary BF3 neutron dosimeter showed 5 to 6 slow (no shield), and 3 to 4 fast (paraffin plus Cd shield), neutrons per cm z per second. These figures are probably correct within a factor of two. Boric acid was selected for the solute because it is readily available. The first tests were made with the natural material, in which the abundance of I°B is about 20%. Although the results were positNe, it was felt necessary to repeat the tests with samples b o t h enriched and depleted in t°B ill order to rule out artifacts. The compositions of the samples are given in table 1. In all cases the solvent was methanol, and the solute (if any) was H3BO3. For each sample, the cavitation events were counted TABLE 1 Composition of samples.
310
Sample
Symbol (fig. 1)
A B C D
© • @ @
Concentration (g/l) Total B l°B 0.0 1.69 1.82 0.228
0.0 0.025 1.67 0.209
CAVITATION NUCLEATED BY I00
/ I
10
~..
Z
I
_
-
~/~@ o
O.
8
I0
12
14 16 -P, BAR ABS
18
20
22
Fig. 1. Cavitation rate vs negative pressure in methanol at 29°C containing 10B. Negative pressure is amplitude at 28.7 kHz minus steady pressure (1 bar). See table 1 for key.
for 10 min for each of several sound pressures. Four runs were made for each sample. All the data, corrected for dead time (0.01 min) are shown in fig. 1. The broken lines, where shown, connect the averages of four points. The data are too few to be worth statistical analysis* but the effect is obvious. Curve A represents the "background", i.e. the count arising from the action of fast neutrons on methanol. The increased counts for the other curves arise from the l°B(n,~.)TLi reaction induced by slow neutrons. Of course, some of these are owing to slowing down of fast neutrons by the methanol itself. The excess count rate does not increase as fast as the concentration of *°B because the liquid in the cell is a thick target. That is, as the concentration of I°B is increased a larger faction of the thermal neutrons is absorbed in the peripheral region where the negative pressure is below cavitation threshold. Still, at - 20 bar, the cavitation rate for 1.67 g/1 l°B (curve C) is four times the background (curve A). A comparison of curves B and C, for which the concentrations of H3BO 3 were about the same, indicates that the effect is real. As a further check, a run on a solution of 100 g of LiNO3 per liter of methanol was made. The results were not distinguishable from those for pure methanol. To see whether or not the results are reasonable, one * The total count is about 10 times the ordinate.
t°B(n, @7Li
311
can calculate the stopping power of methanol for (a) 12C and 160 ions for various energies they would get from neutron impact, (b) Z°sPb recoils from e-disintegration of ThC', (c) 7Li ions from the reaction in question and (d) a-particles and 3H ions from the reaction 6Li(n,c03H. This was done by the method of Booth and Grant 3) for cases (a), (c) and (d) and that of Lindhard, Scharff and Schiott, taken from Northcliffe4), for case (b). The necessary proton stopping powers were calculated from data given in Whaling's review 5) on the assumption that the atomic stopping powers are additive. At the low energies with which we have to deal, both the BG method and the additivity above mentioned are poor, but the calculations are nevertheless roughly consistent with the experimental results of ref. 1) on the basis of the obvious notion, due to Lieberman6), that the higher the m a x i m u m stopping powe~ for an ion as it slows from its initial energy, the lower the cavitation threshold. The most obvious disadvantage that a thermal neutron detector or counter based on the present effect would have is its high sensitivity to fast neutrons. Even if the fast flux were relatively low, a blank would have to be run. Another disadvantage is the long dead time. This is not as bad as it looks; the 0.6 sec dead time was used as a laboratory convenience. We have operated some cells with a dead time of 20 ms and West and Howlett 7) have achieved 8 ms. Also, the cell is temperature sensitive. For good results the temperature should be held to _ 1° C. This is not much of a problem. We enclose the cell in a foamed-polystyrene box (such as is sold as a picnic box) equipped with an ~ncandescent lamp as as heater and with a simple thermostat. This arrangement works well if the operating temperature is a few degrees above ambient. The device is rather inefficient. Rough calculations based on the overall cell dimensions give an efficiency of 10 .3 to 10 . 4 at the highest negative pressure used, 20 bars. Poor efficiency is inherent in the device itself and probably is not susceptible to much improvement. There are two aspects to this. First, the "effective" volume 1) of the cell, which is the fraction of the total volume within which the negative pressure is above threshold (suitably weighted for time), is small. Second, a large fraction of the incident neutrons is absorbed in the peripheral, insensitive regions of the cell as already explained. The scheme has some advantages. The apparatus is fairly simple, and completely 7-ray insensitive. The cable from the cell itself to the rest of the apparatus and to the observer can be up to a hundred meters long.
312
M. G R E E N S P A N AND C. E. T S C H I E G G
A n obvious way to separate the effects o f slow and fast neutrons is to a b a n d o n the l°B(n,~)VLi reaction in favor of fission, as the cavitation threshold on fission events is very low1). Thus solutions containing 235U o r 2 3 9 p u would have high sensitivity to thermal neutrons, and those containing 2 3 s u or 232Th would be sensitive to fast neutrons. So far our experiments with fission in the lower alcohols, which are " w e a k " liquids are not encouraging. There is a tendency for the cavitation bubbles to persist so long that a dead time of several seconds would have to be built in. The most promising c o m b i n a t i o n so far is uranyl nitrate in water. Recent w o r k indicates that at 80°C the threshold is only a b o u t 2 bar, so that at (say) 4 bar the count rate should be fairly high. We hope to pursue this further. In any case this letter is directed principally at the
demonstration o f a new way in which the interaction o f a n e u t r o n with a nucleus gives rise to a macroscopic event.
References 1) M. Greenspan and C. E. Tschiegg, J. Res. Nat. Bur. Std. 71C (1967) 299. e) C. West, Cavitation nucleation by energetic particles, U. K. Atomic Energy Authority Research Group Report AERER5486 (1967). 8) W. Booth and I S. Grant, Nucl. Phys. 63 (1965) 481. 4) L. C. Northcliffe, in Studies in penetration of charged particles in matter, NAS-NRC Publ. 1133, Nucl. Sci. Ser. Report no. 39 (1964). 5) W. Whaling, in Encyclopedia of physics 34 (ed. S. Fliigge; Springer-Verlag, Berlin, 1958) p. 193. 6) D. Lieberman, Phys. Fluids 2 (1959) 466. 7) C. West and R. Howlett, Brit. J. Appl. Phys. (J. Phys. D) Ser. 2, 1 (1968) 247.