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Self-irradiation damage in the actinide metals
LINERSTOTHEEDITORS Self-irradiation damage in the actinide metals
changes progressively through uranium and neptunium to plutonium, the heaviest metal investigated so far. In particular, the linear rise which normally follows the gradual increase at the lowest temperatures is replaced by departures from linearity which, in the case of ~-plutonium have developed into a resistivity maximum at about 100°K; an even steeper rise was observed in fl-quenched plutonium, with a maximum at ~ 30°K (see Figure 1 (e)). 2 More information on the anomaly was obtained by observations at helium temperatures, lasting for several
earlier communications on the low temperature electrical resistivities of the actinide metals, 1 we have reported on anomalous behaviour which develops with increasing atomic number. The usual pattern of metallic resistivity which can still be found in thorium I N A N U M B E R Of
E
25
25
35
20
20
28
15
15
21
r
+ co~
or,,,,l,,,Jl,,~,l 0
100
200
F 0 1 , , , , I , I , , I , , , , I
300
0
100
Temperature,° K
(~)
200
i
300
, j,
j
0
100
Temperature~° K
200
300
Tempera t ure, ° K
125!
~06 100
140
-
120 E
75 -
r
E
iii
r l l
i
l
I00
t~
+
";
8O
5C -
::%
(z
Ul
25 -
4¢ 2C
O',l
0
== , l l
,,I
100
t=
200
Temperature,° K
t
(e)
, ,I
300
100
200
300
400
500
Temperature, ° K
Figure 1. Temperature dependence of the electrical resistivity of (a) thorium, (b) protoactinium, (c) uranium, (d) neptunium a n d (e) plutonium. t10
CRYOGENICS
- APRIL
1968
thousand hours, of the rise of resistivity with time due to the accumulation of self damage by or-activity. 3 While these observations led to very clear results in the case of plutonium, the much lower activities of Np 237 and U 233 give rise to fairly small resistivity increases. However, analysis of our earlier results showed again an increase with atomic number when the ratio of the initial rate of resistivity increase to self-heat was considered. From this we concluded that the anomalous term in the electrical resistivity was far more sensitive to self-irradiation than were those due to scattering of electrons by impurities or lattice vibrations. It was emphasized, however, that confirmation of this conclusion would have to wait until much longer observations on uranium and neptunium had been made. In addition, it was realized that results on protoactinium would be most valuable, since this metal could be expected to show normal resistive behaviour and, at the same time, have an activity of the same order of magnitude as plutonium. Observation on samples of Np 237 and U 233 have now been carried out for more than 6 000 hours and the accurate results obtained are in good agreement with our preliminary measurements. In addition protoactinium metal has become available in sufficient quantity for resistivity work. The temperature dependence (Figure 1 (b)) confirms that this metal shows normal characteristics, indicating that the anomalous behaviour starts with uranium. The effect of accumulation of self damage at ~ 5°K on the resistivity has been measured for ~ 5 000 hours and the results show that, in spite of the high activity, the ratio of the initial rate of resistivity increase to self heat is smaller than in either neptunium or uranium. Comparison values are given in Table 1. (It is hoped shortly to obtain an approximate value of this ratio for thorium, by enriching a sample with a small amount of the highly active isotope pu238.)
Measurements of the vapour pressure of helium-3 in high magnetic fields I N A P R E V I O U S paper I o n e of us predicted a reduction in the vapour pressure of helium-3 due to a magnetic field. Experiments were carried out to investigate this effect, and it was found that the vapour pressure in fact increased in a field. We were therefore led to re-examine the theory and found that the original calculation was incorrect. We cannot explain the observed effect, but as it may turn out to be significant we are reporting the observations and take the opportunity of pointing out the error in the previous paper. To look for a field effect, a copper helium-3 vapour pressure bulb was soldered to a copper vessel whose temperature could be controlled by pumping liquid helium-4. The temperature was maintained constant at about 3°K, and both vapour pressures were measured continuously. As the temperature drifted, the helium-3 vapour pressure changed about three times as much as the CRYOGENICS
• APRIL
1968
TABLE 1
Element
Initial rate of resistivity rise,
Self-heat, m W/g
lafZ. cm/h Pa 2ax* U 2= Np 23; ~Pu ~22 flPu 229
1.0 x 10 a 7.7 x 10 -4 2.15 x 10 -~ 6"0 x 10 2 1"0 x 10 -~
1"21 2.75 x 10-x 1.97 × 10-8 2"50 2.50
(Initial rate of resivity rise) "-- (self-heat) 0.826 x 10 -3 2.80 x 10 -a 10.9 x 10 2 24.0 × 10-3 40.0 × 10-3
* B e c a u s e of t h e i r r e g u l a r shape of t h i s specimen t h e a b s o l u t e values are uncertain t o + 10~o.
The observations on uranium and neptunium thus show clearly that self-damage by a-activity affects the anomalous resistivity far more strongly than the normal resistivity in protoactinium. The data further emphasize that the anomalous behaviour starts with uranium and rises with atomic number to plutonium. A detailed account of the results and their discussion will be left to a fuller communication.
Clarendon Laboratory, Parks Road, Oxford, U.K.
C. S. GR1FF1N K. MENDELSSOHN
A ERE,
M. J. MORTIMER
Harwell, UK.
(8 February 1968)
REFERENCES 1. MEADEN, G. T. Proc. Roy. Soc. A276, 553 (1963) 2. KING, E., and LEE, J. A. Cryogenics 3, 117 (1963) 3. KING, E., LEE, J. A., MENDELSSOHN, K., and WIGLEY, D. A. Proc. Roy. Soc. A284, 325 (1965)
helium-4, in agreement with the relative values of dP/dT. In a magnetic field the helium-3 pressures were consistently higher than in zero-field, as shown in Figure 1. In order to see whether the effect was due to eddy current heating (because of magnet current ripple or apparatus vibrating in the field) we substituted helium-4 for helium-3 and worked at 3.8°K, so that the vapour pressures were comparable. It can be seen from Figure 1 that, within experimental error, a field .does not produce any difference between the helium-4 in the two vessels. The error in the previous paper can be seen by considering the integrated form of the Clausius-Clapeyron equation T
P
f (sv - s,) dT = f (Vv - V,) dP 0
0
The entropies are certainly affected by a magnetic field, and the integration over T suggests that the difference in nuclear susceptibilities of liquid and vapour below 1°K can influence the vapour pressure at higher temperatures. This was the basis of the previous calculation. However, one should take into account the effect of field on molar 11t
changes progressively through uranium and neptunium to plutonium, the heaviest metal investigated so far. In particular, the linear rise which normally follows the gradual increase at the lowest temperatures is replaced by departures from linearity which, in the case of ~-plutonium have developed into a resistivity maximum at about 100°K; an even steeper rise was observed in fl-quenched plutonium, with a maximum at ~ 30°K (see Figure 1 (e)). 2 More information on the anomaly was obtained by observations at helium temperatures, lasting for several
earlier communications on the low temperature electrical resistivities of the actinide metals, 1 we have reported on anomalous behaviour which develops with increasing atomic number. The usual pattern of metallic resistivity which can still be found in thorium I N A N U M B E R Of
E
25
25
35
20
20
28
15
15
21
r
+ co~
or,,,,l,,,Jl,,~,l 0
100
200
F 0 1 , , , , I , I , , I , , , , I
300
0
100
Temperature,° K
(~)
200
i
300
, j,
j
0
100
Temperature~° K
200
300
Tempera t ure, ° K
125!
~06 100
140
-
120 E
75 -
r
E
iii
r l l
i
l
I00
t~
+
";
8O
5C -
::%
(z
Ul
25 -
4¢ 2C
O',l
0
== , l l
,,I
100
t=
200
Temperature,° K
t
(e)
, ,I
300
100
200
300
400
500
Temperature, ° K
Figure 1. Temperature dependence of the electrical resistivity of (a) thorium, (b) protoactinium, (c) uranium, (d) neptunium a n d (e) plutonium. t10
CRYOGENICS
- APRIL
1968
thousand hours, of the rise of resistivity with time due to the accumulation of self damage by or-activity. 3 While these observations led to very clear results in the case of plutonium, the much lower activities of Np 237 and U 233 give rise to fairly small resistivity increases. However, analysis of our earlier results showed again an increase with atomic number when the ratio of the initial rate of resistivity increase to self-heat was considered. From this we concluded that the anomalous term in the electrical resistivity was far more sensitive to self-irradiation than were those due to scattering of electrons by impurities or lattice vibrations. It was emphasized, however, that confirmation of this conclusion would have to wait until much longer observations on uranium and neptunium had been made. In addition, it was realized that results on protoactinium would be most valuable, since this metal could be expected to show normal resistive behaviour and, at the same time, have an activity of the same order of magnitude as plutonium. Observation on samples of Np 237 and U 233 have now been carried out for more than 6 000 hours and the accurate results obtained are in good agreement with our preliminary measurements. In addition protoactinium metal has become available in sufficient quantity for resistivity work. The temperature dependence (Figure 1 (b)) confirms that this metal shows normal characteristics, indicating that the anomalous behaviour starts with uranium. The effect of accumulation of self damage at ~ 5°K on the resistivity has been measured for ~ 5 000 hours and the results show that, in spite of the high activity, the ratio of the initial rate of resistivity increase to self heat is smaller than in either neptunium or uranium. Comparison values are given in Table 1. (It is hoped shortly to obtain an approximate value of this ratio for thorium, by enriching a sample with a small amount of the highly active isotope pu238.)
Measurements of the vapour pressure of helium-3 in high magnetic fields I N A P R E V I O U S paper I o n e of us predicted a reduction in the vapour pressure of helium-3 due to a magnetic field. Experiments were carried out to investigate this effect, and it was found that the vapour pressure in fact increased in a field. We were therefore led to re-examine the theory and found that the original calculation was incorrect. We cannot explain the observed effect, but as it may turn out to be significant we are reporting the observations and take the opportunity of pointing out the error in the previous paper. To look for a field effect, a copper helium-3 vapour pressure bulb was soldered to a copper vessel whose temperature could be controlled by pumping liquid helium-4. The temperature was maintained constant at about 3°K, and both vapour pressures were measured continuously. As the temperature drifted, the helium-3 vapour pressure changed about three times as much as the CRYOGENICS
• APRIL
1968
TABLE 1
Element
Initial rate of resistivity rise,
Self-heat, m W/g
lafZ. cm/h Pa 2ax* U 2= Np 23; ~Pu ~22 flPu 229
1.0 x 10 a 7.7 x 10 -4 2.15 x 10 -~ 6"0 x 10 2 1"0 x 10 -~
1"21 2.75 x 10-x 1.97 × 10-8 2"50 2.50
(Initial rate of resivity rise) "-- (self-heat) 0.826 x 10 -3 2.80 x 10 -a 10.9 x 10 2 24.0 × 10-3 40.0 × 10-3
* B e c a u s e of t h e i r r e g u l a r shape of t h i s specimen t h e a b s o l u t e values are uncertain t o + 10~o.
The observations on uranium and neptunium thus show clearly that self-damage by a-activity affects the anomalous resistivity far more strongly than the normal resistivity in protoactinium. The data further emphasize that the anomalous behaviour starts with uranium and rises with atomic number to plutonium. A detailed account of the results and their discussion will be left to a fuller communication.
Clarendon Laboratory, Parks Road, Oxford, U.K.
C. S. GR1FF1N K. MENDELSSOHN
A ERE,
M. J. MORTIMER
Harwell, UK.
(8 February 1968)
REFERENCES 1. MEADEN, G. T. Proc. Roy. Soc. A276, 553 (1963) 2. KING, E., and LEE, J. A. Cryogenics 3, 117 (1963) 3. KING, E., LEE, J. A., MENDELSSOHN, K., and WIGLEY, D. A. Proc. Roy. Soc. A284, 325 (1965)
helium-4, in agreement with the relative values of dP/dT. In a magnetic field the helium-3 pressures were consistently higher than in zero-field, as shown in Figure 1. In order to see whether the effect was due to eddy current heating (because of magnet current ripple or apparatus vibrating in the field) we substituted helium-4 for helium-3 and worked at 3.8°K, so that the vapour pressures were comparable. It can be seen from Figure 1 that, within experimental error, a field .does not produce any difference between the helium-4 in the two vessels. The error in the previous paper can be seen by considering the integrated form of the Clausius-Clapeyron equation T
P
f (sv - s,) dT = f (Vv - V,) dP 0
0
The entropies are certainly affected by a magnetic field, and the integration over T suggests that the difference in nuclear susceptibilities of liquid and vapour below 1°K can influence the vapour pressure at higher temperatures. This was the basis of the previous calculation. However, one should take into account the effect of field on molar 11t