Exposure of NaCl to high doses of ionizing radiation leads to the formation of nano-particles of metallic Na, very small chlorine precipitates, vacancy voids, and dislocations. A useful way to monitor the stage of the damage formation process is measuring the latent heat of melting of the Na-particles (~100°C) and chlorine precipitates (-101°C). In this paper we will present data, showing that for doses in the range of TRad (1010 Gy) the concentration of radiolytic Na may become very large. Even in pure samples, we have converted more than 20% of all NaCl molecules into metallic Na and chlorine, but often higher percentages can be achieved. In this paper we will present new data obtained for ultra-high irradiation doses and a first attempt will be made to understand the results.

Hartog, H.P. den,

Hartog, H.W. den,

Sugonyako, A.V.,

Turkin, A.A.,

Vainshtein, D.I.,

English

Sugonyako, A. V.

Vainshtein, D. I.

Turkin, A. A.

den Hartog, H. P.

University of Groningen Research Database

  
 University of Groningen
Melting properties of radiation-induced Na and Cl2 precipitates in ultra-heavily irradiated NaCl
Sugonyako, A.V.; Vainshtein, D.I.; Hartog, Hermien; Turkin, A.A.; Hartog, Hermien
Published in:
Physica Status Solidi (C)
DOI:
10.1002/pssc.200460185
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Sugonyako, A. V., Vainshtein, D. I., Hartog, H. P. D., Turkin, A. A., & Hartog, H. W. D. (2005). Melting
properties of radiation-induced Na and Cl2 precipitates in ultra-heavily irradiated NaCl. Physica Status
Solidi (C), 2(1), 362-366. DOI: 10.1002/pssc.200460185
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phys. stat. sol. (c) 2, No. 1, 362–366 (2005) / DOI 10.1002/pssc.200460185 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
Melting properties of radiation-induced Na and Cl2 precipitates 
in ultra-heavily irradiated NaCl 
A. V. Sugonyako1, D. I. Vainshtein1, H. P. den Hartog1, A. A. Turkin2,  
and H. W. den Hartog*, 1  
1
 Solid State Physics Laboratory, University of Groningen, Nijenborgh 4, NL-9747 AG Groningen, 
The Netherlands 
2
 National Science Center Kharkov Institute of Physics and Technology, 310108 Kharkov, Ukraine 
Received 11 July 2004, revised 3 September 2004, accepted 17 September 2004 
Published online 20 January 2005 
PACS 61.46.+w, 82.50.Kx 
Exposure of NaCl to high doses of ionizing radiation leads to the formation of nano-particles of metallic 
Na, very small chlorine precipitates, vacancy voids, and dislocations. A useful way to monitor the stage of 
the damage formation process is measuring the latent heat of melting of the Na-particles (~100˚C)  and 
chlorine precipitates (-101˚C). In this paper we will present data, showing that for doses in the range of 
TRad (1010 Gy) the concentration of radiolytic Na may become very large. Even in pure samples, we 
have converted more than 20% of all NaCl molecules into metallic Na and chlorine, but often higher per-
centages can be achieved. In this paper we will present new data obtained for ultra-high irradiation doses 
and a first attempt will be made to understand the results. 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
1 Introduction 
In the earliest stages of the irradiation primary F and H-centers dominate the defect production. The next 
stage is governed by the mobility and reaction rates of the above-mentioned defects, leading to: (i) anni-
hilation of F and H-centers, (ii) production of aggregates (M, R and N centers), chlorine molecules and 
chlorine precipitates, and (iii) formation of defects associated with trace impurities. Subsequently iso-
lated extended defects are formed, which are distributed randomly over the crystal. Extended defects of 
Cl-atoms have not been investigated in detail, because these precipitates can be studied only with a very 
few techniques. This is unfortunate, because chlorine nano-bubbles play a crucial role in advanced stages 
of radiolysis. 
Apart from the above-mentioned defects also dislocations and vacancy voids are created [1]. The efforts 
in the 1970’s and 1980’s were made to explain the formation of damage in NaCl, but vacancy voids were 
not taken into account [2, 3]. The formation of voids is far from trivial, because radiolytic processes in 
NaCl take place in the halide sub-lattice. To create voids it is necessary to displace Na+ ions and these 
displacements should be incorporated as natural steps in the process. We have proposed a new mecha-
nism describing the formation of neutral vacancy voids [4] to explain the observations for heavily dam-
aged NaCl. 
Heavily irradiated NaCl samples may become unstable, and in some cases they even decompose explo-
sively. This has been explained by interactions between Na-particles, chlorine precipitates and voids [5]. 
In some heavily irradiated NaCl:K appreciable differences may exist between the stored energy and the 
 
 
*
 Corresponding author: e-mail: h.w.den.hartog@phys.rug.nl, Phone: +3150-363-4789, Fax: +3150-363-4879 
phys. stat. sol. (c) 2, No. 1 (2005) / www.pss-c.com 363 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
value expected on the basis of the concentrations of radiolytic Na and chlorine [6]. We assume that the 
lost stored energy is released during instantaneous reactions after measuring the LHM of Na at about 
100 ºC and prior to the stored energy measurement at 300–450 ºC. Depending on the presence of small 
concentrations of impurities (a few 100 ppm) some materials showed a much milder behavior. 
We will present results showing non-saturating behavior of pure and doped NaCl samples up to doses as 
high as 1010 Gy (1012 Rad). In these samples new stages of damage formation are reached. It is remark-
able that also in these extremely heavily damaged materials a few 100 ppm impurities affect the radioly-
sis decisively. In agreement with earlier results we have found at ultra-high doses a drastic reduction of 
the rate of production of radiation damage in NaCl samples due to doping with 400 ppm NaBr. On the 
other hand full saturation of the amount of damage in NaCl samples doped with only 400 ppm NaBr as a 
function of the dose, indicated by our earlier observations up to 1500 MGy, could not be confirmed, 
because between 1500 and 104 MGy a small but consistent increase of the amount of damage is ob-
served. The reduced production of radiation damage due to NaBr, which has been found earlier, is con-
firmed in this investigation. This might have important applications in the development of radiation re-
sistant overpack materials for storage of high-level nuclear waste. 
The other samples did not show saturation up to 104 Mgy. In some samples (including pure NaCl) the 
concentration of metallic Na can be higher than 20 at%. The following question should be dealt with: 
What is the reason why a sample accommodates more than 20% metallic Na plus the same amounts of 
chlorine and large numbers of electro-neutral vacancy voids. The observations for extremely heavily 
damaged samples suggest that in these materials entirely new stages of damage formation develop, 
which are characterized by dense and well-organized, structured ‘fabrics’ of: (i) Na-precipitates, (ii) Cl2 
nano-bubbles, (iii) dislocation loops, and (iv) vacancy voids.  
2 Experimental results and discussion 
The latent heat of melting of Na-precipitates and chlorine bubbles is measured by means of differential 
scanning calorimetry (DSC). We have observed a variety of latent heat peaks due to Na-precipitates. 
Some peaks are well defined (peaks 1 and 2, [7]) while sometimes they vary with the dose, dose rate, 
dopant, irradiation temperature, thermal treatment, etc. The latent heat curves for chlorine are well de-
fined. 
Pure NaCl 
Pure NaCl, latent heat
Irradiation dose (MGy)
0 2000 4000 6000 8000 10000
La
te
n
th
e
a
to
fm
e
lti
ng
o
fN
a
(J/
g)
0
2
4
6
8
10
12
NaCl:Br, latent heat
Irradiation dose (MGy)
0 2000 4000 6000 8000 10000
La
te
n
th
e
a
to
fm
el
tin
g
o
fN
a
(J/
g)
0
2
4
6
8
10
12
 
Fig. 1 Behavior of the LHM of Na as a function of the dose: a) in pure NaCl, b) in NaCl:Br. 
a b 
364 A. V. Sugonyako et al.: Melting properties of radiation-induced Na and Cl2 precipitates in irradiated NaCl 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
Up to 1500 MGy (150 GRad) the production of radiation damage as a function of the dose seems to 
saturate at values of the latent heat of melting (LHM) of Na of ~ 1 J/g, or ~2.3 at% Na and chlorine. In 
the present investigation we will focus our attention at the behavior radiolysis at ultra-high doses. In fig. 
1a we show the results for the LHM of Na in pure NaCl. We note that beyond 1500 MGy the LHM of Na 
for pure NaCl scatters widely, whereas below 1500 MGy the scatter is moderate. At 104 MGy the scatter 
for pure NaCl is one order of magnitude! The scatter is related with specific properties of the pure NaCl 
samples, because for doped samples it is much smaller. 
2.1 NaCl:400 ppm NaBr 
NaBr impurities have a significant effect on the radiation resistance at ultra-high doses. Up to 1500 MGy 
the production of radiation damage is slightly smaller than for pure NaCl. Between 1500 and 104 MGy 
the difference between NaCl:Br and pure NaCl increases drastically with increasing dose. Whereas in the 
ultra-high dose range the resistance of pure NaCl to ionizing radiation decreases rapidly with increasing 
dose, NaCl:Br shows reasonable radiation resistance, although in these materials the amount of damage 
vs the dose does not saturate even at 104 MGy (Fig.1 b). Another feature is that for NaCl:NaBr samples 
the results are, in contrast with pure NaCl, consistent. Comparing NaCl:400ppmNaBr with pure NaCl, 
we observe, that at high doses small bromide concentrations have drastic effects on the damage produc-
tion. The reason why the scatter is small for NaCl:Br compared with pure NaCl might be that the effects 
of other (minority) impurities are not observed. We conclude that the reduction of the damage is due to 
bromide ions. 
2.2 NaCl: 1000 ppm NaF 
In fig. 2a we have plotted the values of the LHM of Na, as a function of the dose. There is a significant 
scatter of the amount of damage in the different samples, which had been irradiated under the same con-
ditions. However, compared with pure NaCl the results for NaCl:F are more consistent. The ‘round’ 
shape of the voids up to 1500 MGy suggests that the stability of these samples is rather high [5]. It is 
possible that the variations for NaCl:F are due to localized back reactions, because the dose is much 
higher than in our previous work. The variations for heavily damaged NaCl:F may also be due to: (i) 
variations of the impurity concentration or (ii) deformations of the samples during preparation, resulting 
in variations in the dislocation density, which affect the radiolysis process. 
 
 
Fig. 2 a) Behavior of the LHM of Na in NaCl:F as a function of the dose, b) latent heat of melting peak of chlorine 
during heating (LHM=9.16 J/g) and cooling (LHM=5.71 J/g). 
phys. stat. sol. (c) 2, No. 1 (2005) / www.pss-c.com 365 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
2.3 Latent heat effects of Cl2 bubbles 
Chlorine precipitates have been detected by their LHM peak at –101 ºC, which coincides with the triple 
point of chlorine. In Fig. 2b we show the chlorine peak in NaCl:KCl during melting and freezing. With 
increasing damage the LHM peak increases and it is proportional to the LHM of Na only if the Cl2 con-
centration is sufficiently high. The ratio of the LHM peaks should be constant, because equal numbers of 
chlorine and sodium atoms are produced. For samples with low damage percentages we often find far 
less melting chlorine than expected. This might be due to two reasons: (i) chlorine precipitates are appre-
ciably smaller than Na nano-particles and (ii) like H and Vk-centers chlorine atoms are bound covalently 
to the nearest chloride ions of the NaCl lattice. By increasing its surface area the precipitates gain en-
ergy; i.e. the precipitates are small and their melting properties might deviate from the bulk properties. 
The discrepancy between the Na and Cl2 results can be reduced by annealing 30 min between 150 and 
180°C, which does not lead to recombination of Na and Cl2. Although the amount of Na remains the 
same, we find a drastic increase of the LHM peak of chlorine. During annealing no extra chlorine atoms 
are produced. I.e. the increase of the LHM peak should be associated with chlorine atoms, which could 
not be observed in the ‘as irradiated’ samples. We assume that during annealing the chlorine molecules 
form larger precipitates, which contribute to the LHM peak. 
For heavily damaged samples we have found large differences between the LHM of chlorine during 
heating or cooling. Although there were differences between details of the results, the total LHM of Na is 
the same for heating and cooling. Systematic experiments have been carried out to determine the ratio R 
of the LHM of chlorine during heating and cooling (Fig. 5). For the selection of samples used to con-
struct Fig. 3 we find R= 1.36. For samples with low LHM values, R does not differ appreciably from 1, 
whereas for some of the most heavily damaged materials the ratio might be 2 or even larger. The reason 
for this behavior is not that a fraction of the chlorine atoms is ‘invisible’, as for low and moderately 
damaged samples. A linear relationship between the LHM of chlorine and Na is observed for samples 
with LHM of Na larger than ~1.5 J/g (or ~3 at% metallic Na) [8]. Below this concentration the LHM of 
chlorine is suppressed. The difference between the values of the LHM observed during heating and cool-
ing is reproducible, which is in contrast with the behavior, expected with the assumption that a fraction 
of the chlorine atoms does not melt [8]. 
 
0 1 2 3 4 5 6 7 8 9
0
2
4
6
8
10
12
14
 
 
1.36319
L
H
M
 C
l 
m
e
lt
in
g
, 
J
/g
LHM Cl freezing, J/g
 
Fig. 3 Ratio R of LHM values during heating (melting) and cooling (freezing) for a selection of comparable sam-
ples. 
 
We have found appreciable differences in the position of the LHM peaks during heating and cooling 
(between 15 and 20 K), which may increase in some cases to 35 K. In the example of Fig. 2b it can be 
366 A. V. Sugonyako et al.: Melting properties of radiation-induced Na and Cl2 precipitates in irradiated NaCl 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
seen, that the difference is about 20 K. The difference in position of the latent heat peak and the ratio of 
the LHM values obtained during heating and cooling seem to be connected with each other. 
Both the different location and the differences of the LHM values during heating and cooling can be 
explained by assuming that there are high pressures in the chlorine bubbles. We assume that during heat-
ing at the melting temperature the surrounding lattice is forced to accommodate the increased volume, 
whereas during cooling the decreased volume is followed by a slow relaxation, which does not contrib-
ute to the LHM. Using the ratio 1.36 (Fig. 3) we find for the internal pressure of the chlorine bubbles a 
few kBar, which is well above the critical pressure of chlorine (76.1 bar). This explains why we have not 
been able to detect the liquid-vapor transition. 
 
Acknowledgements This study has been supported by NWO-NATO NB 67-316 and by STCU (grant # 1761). 
References 
 [1] H. W. den Hartog, D. I. Vainshtein, V. I. Dubinko, and A. A. Turkin, Nucl. Instrum. Methods i Phys. Res. B 
191, 168 (2002). 
 [2] U. Jain and A.B. Lidiard, Philos. Mag. 35, 245 (1977). 
 [3] G. van Opbroek and H.W. den Hartog, J. Phys. C: Solid State Phys. 18, 257-268 (1985). 
 [4] V. I. Dubinko, A. A. Turkin, D. I. Vainshtein, and H. W. den Hartog, J. Nucl. Mater. 289, 86-95 (2001). 
 [5] A. A. Turkin,V. I. Dubinko, D. I. Vainshtein, and H. W. den Hartog, J. Phys. C: Condens. Matter 13, 203 
(2001). 
 [6] D.I. Vainshtein and H.W. den Hartog, Radiat. Eff. Defects Solids 152, 23-37 (2000). 
 [7] J. Seinen, J.R.W. Weerkamp, J.C. Groote, and H.W. den Hartog, Phys. Rev. B 50, 9793 (1994). 
 [8] D.I. Vainshtein, H.W. den Hartog, V.I. Dubinko, and A.A. Turkin, Radiat. Eff. Defects Solids 156, 51-57 
(2001). 
 


Melting properties of radiation-induced Na and Cl2 precipitates in ultra-heavily irradiated NaCl

Exposure of NaCl to high doses of ionizing radiation leads to the formation of nano-particles of metallic Na, very small chlorine precipitates, vacancy voids, and dislocations. A useful way to monitor the stage of the damage formation process is measuring the latent heat of melting of the Na-particles (~100˚C)  and chlorine precipitates (-101˚C). In this paper we will present data, showing that for doses in the range of TRad (1010 Gy) the concentration of radiolytic Na may become very large. Even in pure samples, we have converted more than 20 % of all NaCl molecules into metallic Na and chlorine, but often higher per-centages can be achieved. In this paper we will present new data obtained for ultra-high irradiation doses and a first attempt will be made to understand the results. © 2005 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim 

A. V. Sugonyako

D. I. Vainshtein

H. P. Den Hartog

A. A. Turkin

H. W. Den Hartog

CiteSeerX

Exposure of NaCl to high doses of ionizing radiation leads to the formation of nano-particles of metallic Na, very small chlorine precipitates, vacancy voids, and dislocations. A useful way to monitor the stage of the damage formation process is measuring the latent heat of melting of the Na-particles (~100°C) and chlorine precipitates (-101°C). In this paper we will present data, showing that for doses in the range of TRad (1010 Gy) the concentration of radiolytic Na may become very large. Even in pure samples, we have converted more than 20% of all NaCl molecules into metallic Na and chlorine, but often higher percentages can be achieved. In this paper we will present new data obtained for ultra-high irradiation doses and a first attempt will be made to understand the results

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University of Groningen

University of Groningen Digital Archive

phys. stat. sol. (c) 2, No. 1, 362–366 (2005) / DOI 10.1002/pssc.200460185 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
Melting properties of radiation-induced Na and Cl2 precipitates 
in ultra-heavily irradiated NaCl 
A. V. Sugonyako
1, D. I. Vainshtein
1, H. P. den Hartog
1, A. A. Turkin
2,  
and H. W. den Hartog
*, 1  
1  Solid State Physics Laboratory, University of Groningen, Nijenborgh 4, NL-9747 AG Groningen, 
The Netherlands 
2  National Science Center Kharkov Institute of Physics and Technology, 310108 Kharkov, Ukraine 
Received 11 July 2004, revised 3 September 2004, accepted 17 September 2004 
Published online 20 January 2005 
PACS 61.46.+w, 82.50.Kx 
Exposure of NaCl to high doses of ionizing radiation leads to the formation of nano-particles of metallic 
Na, very small chlorine precipitates, vacancy voids, and dislocations. A useful way to monitor the stage of 
the damage formation process is measuring the latent heat of melting of the Na-particles (~100˚C)  and 
chlorine precipitates (-101˚C). In this paper we will present data, showing that for doses in the range of 
TRad (1010 Gy) the concentration of radiolytic Na may become very large. Even in pure samples, we 
have converted more than 20% of all NaCl molecules into metallic Na and chlorine, but often higher per-
centages can be achieved. In this paper we will present new data obtained for ultra-high irradiation doses 
and a first attempt will be made to understand the results. 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
1 Introduction 
In the earliest stages of the irradiation primary F and H-centers dominate the defect production. The next 
stage is governed by the mobility and reaction rates of the above-mentioned defects, leading to: (i) anni-
hilation of F and H-centers, (ii) production of aggregates (M, R and N centers), chlorine molecules and 
chlorine precipitates, and (iii) formation of defects associated with trace impurities. Subsequently iso-
lated extended defects are formed, which are distributed randomly over the crystal. Extended defects of 
Cl-atoms have not been investigated in detail, because these precipitates can be studied only with a very 
few techniques. This is unfortunate, because chlorine nano-bubbles play a crucial role in advanced stages 
of radiolysis. 
Apart from the above-mentioned defects also dislocations and vacancy voids are created [1]. The efforts 
in the 1970’s and 1980’s were made to explain the formation of damage in NaCl, but vacancy voids were 
not taken into account [2, 3]. The formation of voids is far from trivial, because radiolytic processes in 
NaCl take place in the halide sub-lattice. To create voids it is necessary to displace Na
+ ions and these 
displacements should be incorporated as natural steps in the process. We have proposed a new mecha-
nism describing the formation of neutral vacancy voids [4] to explain the observations for heavily dam-
aged NaCl. 
Heavily irradiated NaCl samples may become unstable, and in some cases they even decompose explo-
sively. This has been explained by interactions between Na-particles, chlorine precipitates and voids [5]. 
In some heavily irradiated NaCl:K appreciable differences may exist between the stored energy and the 
 
 
* Corresponding author: e-mail: h.w.den.hartog@phys.rug.nl, Phone: +3150-363-4789, Fax: +3150-363-4879 phys. stat. sol. (c) 2, No. 1 (2005) / www.pss-c.com  363 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
value expected on the basis of the concentrations of radiolytic Na and chlorine [6]. We assume that the 
lost stored energy is released during instantaneous reactions after measuring the LHM of Na at about 
100 ºC and prior to the stored energy measurement at 300–450 ºC. Depending on the presence of small 
concentrations of impurities (a few 100 ppm) some materials showed a much milder behavior. 
We will present results showing non-saturating behavior of pure and doped NaCl samples up to doses as 
high as 10
10 Gy (10
12 Rad). In these samples new stages of damage formation are reached. It is remark-
able that also in these extremely heavily damaged materials a few 100 ppm impurities affect the radioly-
sis decisively. In agreement with earlier results we have found at ultra-high doses a drastic reduction of 
the rate of production of radiation damage in NaCl samples due to doping with 400 ppm NaBr. On the 
other hand full saturation of the amount of damage in NaCl samples doped with only 400 ppm NaBr as a 
function of the dose, indicated by our earlier observations up to 1500 MGy, could not be confirmed, 
because between 1500 and 10
4 MGy a small but consistent increase of the amount of damage is ob-
served. The reduced production of radiation damage due to NaBr, which has been found earlier, is con-
firmed in this investigation. This might have important applications in the development of radiation re-
sistant overpack materials for storage of high-level nuclear waste. 
The other samples did not show saturation up to 10
4 Mgy. In some samples (including pure NaCl) the 
concentration of metallic Na can be higher than 20 at%. The following question should be dealt with: 
What is the reason why a sample accommodates more than 20% metallic Na plus the same amounts of 
chlorine and large numbers of electro-neutral vacancy voids. The observations for extremely heavily 
damaged samples suggest that in these materials entirely new stages of damage formation develop, 
which are characterized by dense and well-organized, structured ‘fabrics’ of: (i) Na-precipitates, (ii) Cl2 
nano-bubbles, (iii) dislocation loops, and (iv) vacancy voids.  
2  Experimental results and discussion 
The latent heat of melting of Na-precipitates and chlorine bubbles is measured by means of differential 
scanning calorimetry (DSC). We have observed a variety of latent heat peaks due to Na-precipitates. 
Some peaks are well defined (peaks 1 and 2, [7]) while sometimes they vary with the dose, dose rate, 
dopant, irradiation temperature, thermal treatment, etc. The latent heat curves for chlorine are well de-
fined. 
Pure NaCl 
Pure NaCl, latent heat
Irradiation dose (MGy)
0 2000 4000 6000 8000 10000
L
a
t
e
n
t
h
e
a
t
o
f
m
e
l
t
i
n
g
o
f
N
a
(
J
/
g
)
0
2
4
6
8
10
12
NaCl:Br, latent heat
Irradiation dose (MGy)
0 2000 4000 6000 8000 10000
L
a
t
e
n
t
h
e
a
t
o
f
m
e
l
t
i
n
g
o
f
N
a
(
J
/
g
)
0
2
4
6
8
10
12
 
Fig. 1  Behavior of the LHM of Na as a function of the dose: a) in pure NaCl, b) in NaCl:Br. 
a  b 364  A. V. Sugonyako et al.: Melting properties of radiation-induced Na and Cl2 precipitates in irradiated NaCl 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
Up to 1500 MGy (150 GRad) the production of radiation damage as a function of the dose seems to 
saturate at values of the latent heat of melting (LHM) of Na of ~ 1 J/g, or ~2.3 at% Na and chlorine. In 
the present investigation we will focus our attention at the behavior radiolysis at ultra-high doses. In fig. 
1a we show the results for the LHM of Na in pure NaCl. We note that beyond 1500 MGy the LHM of Na 
for pure NaCl scatters widely, whereas below 1500 MGy the scatter is moderate. At 10
4 MGy the scatter 
for pure NaCl is one order of magnitude! The scatter is related with specific properties of the pure NaCl 
samples, because for doped samples it is much smaller. 
2.1 NaCl:400 ppm NaBr 
NaBr impurities have a significant effect on the radiation resistance at ultra-high doses. Up to 1500 MGy 
the production of radiation damage is slightly smaller than for pure NaCl. Between 1500 and 10
4 MGy 
the difference between NaCl:Br and pure NaCl increases drastically with increasing dose. Whereas in the 
ultra-high dose range the resistance of pure NaCl to ionizing radiation decreases rapidly with increasing 
dose, NaCl:Br shows reasonable radiation resistance, although in these materials the amount of damage 
vs the dose does not saturate even at 10
4 MGy (Fig.1 b). Another feature is that for NaCl:NaBr samples 
the results are, in contrast with pure NaCl, consistent. Comparing NaCl:400ppmNaBr with pure NaCl, 
we observe, that at high doses small bromide concentrations have drastic effects on the damage produc-
tion. The reason why the scatter is small for NaCl:Br compared with pure NaCl might be that the effects 
of other (minority) impurities are not observed. We conclude that the reduction of the damage is due to 
bromide ions. 
2.2 NaCl: 1000 ppm NaF 
In fig. 2a we have plotted the values of the LHM of Na, as a function of the dose. There is a significant 
scatter of the amount of damage in the different samples, which had been irradiated under the same con-
ditions. However, compared with pure NaCl the results for NaCl:F are more consistent. The ‘round’ 
shape of the voids up to 1500 MGy suggests that the stability of these samples is rather high [5]. It is 
possible that the variations for NaCl:F are due to localized back reactions, because the dose is much 
higher than in our previous work. The variations for heavily damaged NaCl:F may also be due to: (i) 
variations of the impurity concentration or (ii) deformations of the samples during preparation, resulting 
in variations in the dislocation density, which affect the radiolysis process. 
 
 
Fig. 2  a) Behavior of the LHM of Na in NaCl:F as a function of the dose, b) latent heat of melting peak of chlorine 
during heating (LHM=9.16 J/g) and cooling (LHM=5.71 J/g). phys. stat. sol. (c) 2, No. 1 (2005) / www.pss-c.com  365 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
2.3 Latent heat effects of Cl2 bubbles 
Chlorine precipitates have been detected by their LHM peak at –101 ºC, which coincides with the triple 
point of chlorine. In Fig. 2b we show the chlorine peak in NaCl:KCl during melting and freezing. With 
increasing damage the LHM peak increases and it is proportional to the LHM of Na only if the Cl2 con-
centration is sufficiently high. The ratio of the LHM peaks should be constant, because equal numbers of 
chlorine and sodium atoms are produced. For samples with low damage percentages we often find far 
less melting chlorine than expected. This might be due to two reasons: (i) chlorine precipitates are appre-
ciably smaller than Na nano-particles and (ii) like H and Vk-centers chlorine atoms are bound covalently 
to the nearest chloride ions of the NaCl lattice. By increasing its surface area the precipitates gain en-
ergy; i.e. the precipitates are small and their melting properties might deviate from the bulk properties. 
The discrepancy between the Na and Cl2 results can be reduced by annealing 30 min between 150 and 
180°C, which does not lead to recombination of Na and Cl2. Although the amount of Na remains the 
same, we find a drastic increase of the LHM peak of chlorine. During annealing no extra chlorine atoms 
are produced. I.e. the increase of the LHM peak should be associated with chlorine atoms, which could 
not be observed in the ‘as irradiated’ samples. We assume that during annealing the chlorine molecules 
form larger precipitates, which contribute to the LHM peak. 
For heavily damaged samples we have found large differences between the LHM of chlorine during 
heating or cooling. Although there were differences between details of the results, the total LHM of Na is 
the same for heating and cooling. Systematic experiments have been carried out to determine the ratio R 
of the LHM of chlorine during heating and cooling (Fig. 5). For the selection of samples used to con-
struct Fig. 3 we find R= 1.36. For samples with low LHM values, R does not differ appreciably from 1, 
whereas for some of the most heavily damaged materials the ratio might be 2 or even larger. The reason 
for this behavior is not that a fraction of the chlorine atoms is ‘invisible’, as for low and moderately 
damaged samples. A linear relationship between the LHM of chlorine and Na is observed for samples 
with LHM of Na larger than ~1.5 J/g (or ~3 at% metallic Na) [8]. Below this concentration the LHM of 
chlorine is suppressed. The difference between the values of the LHM observed during heating and cool-
ing is reproducible, which is in contrast with the behavior, expected with the assumption that a fraction 
of the chlorine atoms does not melt [8]. 
 
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LHM Cl freezing, J/g
 
Fig. 3  Ratio R of LHM values during heating (melting) and cooling (freezing) for a selection of comparable sam-
ples. 
 
We have found appreciable differences in the position of the LHM peaks during heating and cooling 
(between 15 and 20 K), which may increase in some cases to 35 K. In the example of Fig. 2b it can be 366  A. V. Sugonyako et al.: Melting properties of radiation-induced Na and Cl2 precipitates in irradiated NaCl 
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 
seen, that the difference is about 20 K. The difference in position of the latent heat peak and the ratio of 
the LHM values obtained during heating and cooling seem to be connected with each other. 
Both the different location and the differences of the LHM values during heating and cooling can be 
explained by assuming that there are high pressures in the chlorine bubbles. We assume that during heat-
ing at the melting temperature the surrounding lattice is forced to accommodate the increased volume, 
whereas during cooling the decreased volume is followed by a slow relaxation, which does not contrib-
ute to the LHM. Using the ratio 1.36 (Fig. 3) we find for the internal pressure of the chlorine bubbles a 
few kBar, which is well above the critical pressure of chlorine (76.1 bar). This explains why we have not 
been able to detect the liquid-vapor transition. 
 
Acknowledgements  This study has been supported by NWO-NATO NB 67-316 and by STCU (grant # 1761). 
References 
 [1] H. W. den Hartog, D. I. Vainshtein, V. I. Dubinko, and A. A. Turkin, Nucl. Instrum. Methods i Phys. Res. B 
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  [2]  U. Jain and A.B. Lidiard, Philos. Mag. 35, 245 (1977). 
  [3]  G. van Opbroek and H.W. den Hartog, J. Phys. C: Solid State Phys. 18, 257-268 (1985). 
  [4]  V. I. Dubinko, A. A. Turkin, D. I. Vainshtein, and H. W. den Hartog, J. Nucl. Mater. 289, 86-95 (2001). 
 [5] A. A. Turkin,V. I. Dubinko, D. I. Vainshtein, and H. W. den Hartog, J. Phys. C: Condens. Matter 13, 203 
(2001). 
  [6]  D.I. Vainshtein and H.W. den Hartog, Radiat. Eff. Defects Solids 152, 23-37 (2000). 
  [7]  J. Seinen, J.R.W. Weerkamp, J.C. Groote, and H.W. den Hartog, Phys. Rev. B 50, 9793 (1994). 
 [8] D.I. Vainshtein, H.W. den Hartog, V.I. Dubinko, and A.A. Turkin, Radiat. Eff. Defects Solids 156, 51-57 
(2001). 
 

Melting properties of radiation-induced Na and Cl2 precipitates in ultra-heavily irradiated NaCl

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