Articles de revista
http://hdl.handle.net/2117/3984
Wed, 16 Aug 2017 22:00:02 GMT2017-08-16T22:00:02ZMonte Carlo simulations for the study of a moderated neutron detector
http://hdl.handle.net/2117/104373
Monte Carlo simulations for the study of a moderated neutron detector
Gómez Hornillos, María Belén; Gorlychev, V.; Caballero Folch, Roger; Cortés Rossell, Guillem Pere; Poch Parés, Agustí; Pretel Sánchez, Carme; Calviño Tavares, Francisco
This work presents the Monte Carlo simulations performed with the MCNPX and GEANT4 codes for the design of a BEta deLayEd Neutron detector, BELEN-20. This detector will be used for the study of beta delayed neutron emission and consists of a block of polyethylene with dimensions 90x90x80 cm3 and 20 cylindrical 3He gas counters. The results of these simulations have been validated experimentally with a 252Cf source in the laboratory at UPC, Barcelona. Also the first experiment with this detector has been carried out in November 2009 in JYFL, Finland. In this experiment the neutron emission probability after beta decay of the fission products 88Br, 94;95Rb, and 138I has been measured; this data is still under analysis. Simulations with MCNPX and GEANT4 have been performed in order to obtain the efficiency of the BELEN-20 detector for each of the above nuclei using the neutron energy distribution corresponding to each nucleus.
Fri, 12 May 2017 12:49:41 GMThttp://hdl.handle.net/2117/1043732017-05-12T12:49:41ZGómez Hornillos, María BelénGorlychev, V.Caballero Folch, RogerCortés Rossell, Guillem PerePoch Parés, AgustíPretel Sánchez, CarmeCalviño Tavares, FranciscoThis work presents the Monte Carlo simulations performed with the MCNPX and GEANT4 codes for the design of a BEta deLayEd Neutron detector, BELEN-20. This detector will be used for the study of beta delayed neutron emission and consists of a block of polyethylene with dimensions 90x90x80 cm3 and 20 cylindrical 3He gas counters. The results of these simulations have been validated experimentally with a 252Cf source in the laboratory at UPC, Barcelona. Also the first experiment with this detector has been carried out in November 2009 in JYFL, Finland. In this experiment the neutron emission probability after beta decay of the fission products 88Br, 94;95Rb, and 138I has been measured; this data is still under analysis. Simulations with MCNPX and GEANT4 have been performed in order to obtain the efficiency of the BELEN-20 detector for each of the above nuclei using the neutron energy distribution corresponding to each nucleus.Development and assessment of fire-related risk unavailability matrices to support the application of the maintenance rule in a PWR nuclear power plant
http://hdl.handle.net/2117/88442
Development and assessment of fire-related risk unavailability matrices to support the application of the maintenance rule in a PWR nuclear power plant
Díaz Bayona, Pedro; Estruch Traveria, Enric; Dies Llovera, Javier; Tapia Fernández, Carlos; Blas Del Hoyo, Alfredo de; Asamoah, Matthew
Two methods are presented which serve to incorporate the fire-related risk into the current practices in nuclear power plants with respect to the assessment of configurations. The development of these methods is restricted to the compulsory use of fire probabilistic safety assessment (PSA) models. The first method is a fire protection systems and key safety functions unavailability matrix which is developed to identify structures, systems, and components significant for fire-related risk. The second method is a fire zones and key safety functions (KSFs) fire risk matrix which is useful to identify fire zones which are candidates for risk management actions. Specific selection and quantification methodologies have been developed to obtain the matrices. The Monte Carlo method has been used to assess the uncertainty of the unavailability matrix. The analysis shows that the uncertainty is sufficiently bounded. The significant fire-related risk is localized in six KSF representative components and one fire protection system which should be included in the maintenance rule. The unavailability of fire protection systems does not significantly affect the risk. The fire risk matrix identifies the fire zones that contribute the most to the fire-related risk. These zones belong to the control building and electric penetrations building.
Fri, 01 Jul 2016 08:11:44 GMThttp://hdl.handle.net/2117/884422016-07-01T08:11:44ZDíaz Bayona, PedroEstruch Traveria, EnricDies Llovera, JavierTapia Fernández, CarlosBlas Del Hoyo, Alfredo deAsamoah, MatthewTwo methods are presented which serve to incorporate the fire-related risk into the current practices in nuclear power plants with respect to the assessment of configurations. The development of these methods is restricted to the compulsory use of fire probabilistic safety assessment (PSA) models. The first method is a fire protection systems and key safety functions unavailability matrix which is developed to identify structures, systems, and components significant for fire-related risk. The second method is a fire zones and key safety functions (KSFs) fire risk matrix which is useful to identify fire zones which are candidates for risk management actions. Specific selection and quantification methodologies have been developed to obtain the matrices. The Monte Carlo method has been used to assess the uncertainty of the unavailability matrix. The analysis shows that the uncertainty is sufficiently bounded. The significant fire-related risk is localized in six KSF representative components and one fire protection system which should be included in the maintenance rule. The unavailability of fire protection systems does not significantly affect the risk. The fire risk matrix identifies the fire zones that contribute the most to the fire-related risk. These zones belong to the control building and electric penetrations building.Characterization of a neutron-beta counting system with beta-delayed neutron emitters
http://hdl.handle.net/2117/85596
Characterization of a neutron-beta counting system with beta-delayed neutron emitters
Agramunt Ros, Jorge; Taín, J.L.; Gómez Hornillos, María Belén; Calviño Tavares, Francisco; Cortés Rossell, Guillem Pere; Pretel Sánchez, Carme; Riego Pérez, Albert; Tarifeño Saldivia, Ariel Esteban
A new detection system for the measurement of beta-delayed neutron emission probabilities has been characterized using fission products with well known ß-delayed neutron emission properties. The setup consists of BELEN-20, a 4p neutron counter with twenty 3He proportional tubes arranged inside a large polyethylene neutron moderator, a thin Si detector for ß counting and a self-triggering digital data acquisition system. The use of delayed-neutron precursors with different neutron emission windows allowed the study of the effect of energy dependency on neutron, ß and ß-neutron rates. The observed effect is well reproduced by Monte Carlo simulations. The impact of this dependency on the accuracy of neutron emission probabilities is discussed. A new accurate value of the neutron emission probability for the important delayed-neutron precursor 137I was obtained, Pn=7.76(14)%.
Wed, 13 Apr 2016 10:59:47 GMThttp://hdl.handle.net/2117/855962016-04-13T10:59:47ZAgramunt Ros, JorgeTaín, J.L.Gómez Hornillos, María BelénCalviño Tavares, FranciscoCortés Rossell, Guillem PerePretel Sánchez, CarmeRiego Pérez, AlbertTarifeño Saldivia, Ariel EstebanA new detection system for the measurement of beta-delayed neutron emission probabilities has been characterized using fission products with well known ß-delayed neutron emission properties. The setup consists of BELEN-20, a 4p neutron counter with twenty 3He proportional tubes arranged inside a large polyethylene neutron moderator, a thin Si detector for ß counting and a self-triggering digital data acquisition system. The use of delayed-neutron precursors with different neutron emission windows allowed the study of the effect of energy dependency on neutron, ß and ß-neutron rates. The observed effect is well reproduced by Monte Carlo simulations. The impact of this dependency on the accuracy of neutron emission probabilities is discussed. A new accurate value of the neutron emission probability for the important delayed-neutron precursor 137I was obtained, Pn=7.76(14)%.Fractal structure and predictive strategy of the daily extreme temperature residuals at Fabra Observatory (NE Spain, years 1917-2005)
http://hdl.handle.net/2117/78472
Fractal structure and predictive strategy of the daily extreme temperature residuals at Fabra Observatory (NE Spain, years 1917-2005)
Lana Pons, Francisco Javier; Burgueño, August; Serra de Larrocha, Carina; Martínez Santafé, Maria Dolors
A compilation of daily extreme temperatures recorded at the Fabra Observatory (Catalonia, NE Spain) since 1917 up to 2005 has permitted an exhaustive analysis of the fractal behaviour of the daily extreme temperature residuals, DTR, defined as the difference between the observed daily extreme temperature and the daily average value. The lacunarity characterises the lag distribution on the residual series for several thresholds. Hurst, H, and Hausdorff, Ha, exponents, together with the exponent beta of the decaying power law, describing the evolution of power spectral density with frequency, permit to characterise the persistence, antipersistence or randomness of the residual series. The self-affine character of DTR series is verified, and additionally, they are simulated by means of fractional Gaussian noise, fGn. The reconstruction theorem leads to the quantification of the complexity (correlation dimension, mu*, and Kolmogorov entropy, kappa.) and predictive instability (Lyapunov exponents, lambda, and Kaplan-Yorke dimension, D-KY) of the residual series. All fractal parameters are computed for consecutive and independent segments of 5-year lengths. This strategy permits to obtain a high enough number of fractal parameter samples to estimate time trends, including their statistical significance. Comparisons are made between results of predictive algorithms based on fGn models and an autoregressive autoregressive integrated moving average (ARIMA) process, with the latter leading to slightly better results than the former. Several dynamic atmospheric mechanisms and local effects, such as local topography and vicinity to the Mediterranean coast, are proposed to explain the complex and instable predictability of DTR series. The memory of the physical system (Kolmogorov entropy) would be attributed to the interaction with the Mediterranean Sea.
Thu, 29 Oct 2015 10:50:38 GMThttp://hdl.handle.net/2117/784722015-10-29T10:50:38ZLana Pons, Francisco JavierBurgueño, AugustSerra de Larrocha, CarinaMartínez Santafé, Maria DolorsA compilation of daily extreme temperatures recorded at the Fabra Observatory (Catalonia, NE Spain) since 1917 up to 2005 has permitted an exhaustive analysis of the fractal behaviour of the daily extreme temperature residuals, DTR, defined as the difference between the observed daily extreme temperature and the daily average value. The lacunarity characterises the lag distribution on the residual series for several thresholds. Hurst, H, and Hausdorff, Ha, exponents, together with the exponent beta of the decaying power law, describing the evolution of power spectral density with frequency, permit to characterise the persistence, antipersistence or randomness of the residual series. The self-affine character of DTR series is verified, and additionally, they are simulated by means of fractional Gaussian noise, fGn. The reconstruction theorem leads to the quantification of the complexity (correlation dimension, mu*, and Kolmogorov entropy, kappa.) and predictive instability (Lyapunov exponents, lambda, and Kaplan-Yorke dimension, D-KY) of the residual series. All fractal parameters are computed for consecutive and independent segments of 5-year lengths. This strategy permits to obtain a high enough number of fractal parameter samples to estimate time trends, including their statistical significance. Comparisons are made between results of predictive algorithms based on fGn models and an autoregressive autoregressive integrated moving average (ARIMA) process, with the latter leading to slightly better results than the former. Several dynamic atmospheric mechanisms and local effects, such as local topography and vicinity to the Mediterranean coast, are proposed to explain the complex and instable predictability of DTR series. The memory of the physical system (Kolmogorov entropy) would be attributed to the interaction with the Mediterranean Sea.Aftershock sequences of three seismic crises at southern California, USA, simulated by a cellular automata model based on self-organized criticality
http://hdl.handle.net/2117/78319
Aftershock sequences of three seismic crises at southern California, USA, simulated by a cellular automata model based on self-organized criticality
Monterrubio Velasco, Marisol; Lana Pons, Francisco Javier; Martínez Santafé, Maria Dolors
Several properties of aftershock series related to the main shocks of Landers, Northridge and Hector Mine (southern California, USA) are reproduced by the Dynamic Fiber Bundle Model, DFBM. Optimum values for the three parameters governing DFBM are determined by searching for the best agreement of real aftershock series properties and those of synthetic sequences generated by this model. The analysis of the model parameter values provides details on the underlying physical mechanism of the aftershock sequence generation. First, the ratio of seismic energy radiated as seismic waves and transferred as stress-strain to adjacent faults; second, the degree of stress heterogeneity reproducing the complex behavior of real aftershock series. Additionally, the results of simulations support the coexistence of two types of relaxation processes. One of them is associated with the well-known modified Omori's (MO) law, which involves elapsed times between consecutive aftershocks monotonically increasing; the other is manifested by episodes of sudden stress release, with inter-event times much shorter than those predicted by MO law. These episodes are assumed to be a consequence of the complex distribution of tectonic stresses and fault geometry. The first process is associated to events designed as leading aftershocks, LA. The second process generates series of events which are designed as cascades, CA. It is worth of mention that several properties concerning CAs can be reasonably related to critical changes on stress field along the simulation process.
Tue, 27 Oct 2015 11:12:24 GMThttp://hdl.handle.net/2117/783192015-10-27T11:12:24ZMonterrubio Velasco, MarisolLana Pons, Francisco JavierMartínez Santafé, Maria DolorsSeveral properties of aftershock series related to the main shocks of Landers, Northridge and Hector Mine (southern California, USA) are reproduced by the Dynamic Fiber Bundle Model, DFBM. Optimum values for the three parameters governing DFBM are determined by searching for the best agreement of real aftershock series properties and those of synthetic sequences generated by this model. The analysis of the model parameter values provides details on the underlying physical mechanism of the aftershock sequence generation. First, the ratio of seismic energy radiated as seismic waves and transferred as stress-strain to adjacent faults; second, the degree of stress heterogeneity reproducing the complex behavior of real aftershock series. Additionally, the results of simulations support the coexistence of two types of relaxation processes. One of them is associated with the well-known modified Omori's (MO) law, which involves elapsed times between consecutive aftershocks monotonically increasing; the other is manifested by episodes of sudden stress release, with inter-event times much shorter than those predicted by MO law. These episodes are assumed to be a consequence of the complex distribution of tectonic stresses and fault geometry. The first process is associated to events designed as leading aftershocks, LA. The second process generates series of events which are designed as cascades, CA. It is worth of mention that several properties concerning CAs can be reasonably related to critical changes on stress field along the simulation process.Criticality in the slowed-down boiling crisis at zero gravity
http://hdl.handle.net/2117/78228
Criticality in the slowed-down boiling crisis at zero gravity
Charignon, Thomas; Lloveras Muntané, Pol Marcel; Chatain, Denis; Truskinovsky, Lev; Vives, Eduard; Beysens, Daniel; Nikolayev, Vadim
Boiling crisis is a transition between nucleate and film boiling. It occurs at a threshold value of the heat flux from the heater called CHF (critical heat flux). Usually, boiling crisis studies are hindered by the high CHF and short transition duration (below 1 ms). Here we report on experiments in hydrogen near its liquid-vapor critical point, in which the CHF is low and the dynamics slow enough to be resolved. As under such conditions the surface tension is very small, the experiments are carried out in the reduced gravity to preserve the conventional bubble geometry. Weightlessness is created artificially in two-phase hydrogen by compensating gravity with magnetic forces. We were able to reveal the fractal structure of the contour of the percolating cluster of the dry areas at the heater that precedes the boiling crisis. We provide a direct statistical analysis of dry spot areas that confirms the boiling crisis at zero gravity as a scale-free phenomenon. It was observed that, in agreement with theoretical predictions, saturated boiling CHF tends to zero (within the precision of our thermal control system) in zero gravity, which suggests that the boiling crisis may be observed at any heat flux provided the experiment lasts long enough.
Mon, 26 Oct 2015 12:12:09 GMThttp://hdl.handle.net/2117/782282015-10-26T12:12:09ZCharignon, ThomasLloveras Muntané, Pol MarcelChatain, DenisTruskinovsky, LevVives, EduardBeysens, DanielNikolayev, VadimBoiling crisis is a transition between nucleate and film boiling. It occurs at a threshold value of the heat flux from the heater called CHF (critical heat flux). Usually, boiling crisis studies are hindered by the high CHF and short transition duration (below 1 ms). Here we report on experiments in hydrogen near its liquid-vapor critical point, in which the CHF is low and the dynamics slow enough to be resolved. As under such conditions the surface tension is very small, the experiments are carried out in the reduced gravity to preserve the conventional bubble geometry. Weightlessness is created artificially in two-phase hydrogen by compensating gravity with magnetic forces. We were able to reveal the fractal structure of the contour of the percolating cluster of the dry areas at the heater that precedes the boiling crisis. We provide a direct statistical analysis of dry spot areas that confirms the boiling crisis at zero gravity as a scale-free phenomenon. It was observed that, in agreement with theoretical predictions, saturated boiling CHF tends to zero (within the precision of our thermal control system) in zero gravity, which suggests that the boiling crisis may be observed at any heat flux provided the experiment lasts long enough.Slow relaxation dynamics and aging in random walks on activity driven temporal networks
http://hdl.handle.net/2117/77865
Slow relaxation dynamics and aging in random walks on activity driven temporal networks
Sousa da Mata, Angelica; Pastor Satorras, Romualdo
We investigate the dynamic relaxation of random walks on temporal networks by focusing in the recently proposed activity driven model [N. Perra, B. Gon, calves, R. Pastor-Satorras, A. Vespignani, Sci. Rep. 2, 469 (2012)]. For realistic activity distributions with a power-law form, we observe the presence of a very slow relaxation dynamics compatible with aging effects. A theoretical description of this processes in achieved by means of a mapping to Bouchaud's trap model. The mapping highlights the profound difference in the dynamics of the random walks according to the value of the exponent gamma in the activity distribution.
Mon, 19 Oct 2015 09:20:07 GMThttp://hdl.handle.net/2117/778652015-10-19T09:20:07ZSousa da Mata, AngelicaPastor Satorras, RomualdoWe investigate the dynamic relaxation of random walks on temporal networks by focusing in the recently proposed activity driven model [N. Perra, B. Gon, calves, R. Pastor-Satorras, A. Vespignani, Sci. Rep. 2, 469 (2012)]. For realistic activity distributions with a power-law form, we observe the presence of a very slow relaxation dynamics compatible with aging effects. A theoretical description of this processes in achieved by means of a mapping to Bouchaud's trap model. The mapping highlights the profound difference in the dynamics of the random walks according to the value of the exponent gamma in the activity distribution.Insights into Q2Qbar2 states from an effective perspective
http://hdl.handle.net/2117/77626
Insights into Q2Qbar2 states from an effective perspective
Ametller Congost, Lluís; Talavera Sánchez, Pedro
We discuss the two-photon coupling of the lightest scalar meson on the basis of an extension of χPT. Using low-energy data on the pion form factor and the γγ→π+π−(π0π0) cross sections as inputs, we find Γ(σ→γγ)≅0.126 keV. The smallness of the result and the relative weight between its components, Γγγ→S1Γγγ→ππ→S1≤1, suggests that the scalar 0++ meson is mainly a Q2Q¯2 state.
Tue, 13 Oct 2015 12:55:24 GMThttp://hdl.handle.net/2117/776262015-10-13T12:55:24ZAmetller Congost, LluísTalavera Sánchez, PedroWe discuss the two-photon coupling of the lightest scalar meson on the basis of an extension of χPT. Using low-energy data on the pion form factor and the γγ→π+π−(π0π0) cross sections as inputs, we find Γ(σ→γγ)≅0.126 keV. The smallness of the result and the relative weight between its components, Γγγ→S1Γγγ→ππ→S1≤1, suggests that the scalar 0++ meson is mainly a Q2Q¯2 state.Is long range transport of pollen in the NW Mediterranean basin influenced by Northern Hemisphere teleconnection patterns?
http://hdl.handle.net/2117/77398
Is long range transport of pollen in the NW Mediterranean basin influenced by Northern Hemisphere teleconnection patterns?
Izquierdo Miguel, Rebeca; Alarcón Jordán, Marta; Periago Oliver, M. Cristina; Belmonte Soler, Jordina
Climatic oscillations triggered by the atmospheric modes of the Northern Hemisphere teleconnection patterns have an important influence on the atmospheric circulation at synoptic scale in Western Mediterranean Basin. Simultaneously, this climate variability could affect a variety of ecological processes. This work provides a first assessment of the effect of North Atlantic Oscillation (NAO), Arctic Oscillation (AO) and Western Mediterranean Oscillation (WeMO) on the atmospheric long-range pollen transport episodes in the North-Eastern Iberian Peninsula for the period 1994–2011. Alnus, Ambrosia, Betula, Corylus and Fagus have been selected as allergenic pollen taxa with potential long-range transport associated to the Northern Hemisphere teleconnection patterns in the Western Mediterranean Basin. The results showed an increase of long range pollen transport episodes of: (1) Alnus, Corylus and Fagus from Western and Central Europe during the negative phase of annual NAO and AO; (2) Ambrosia, Betula and Fagus from Europe during the negative phase of winter WeMO; (3) Corylus and Fagus from Mediterranean area during the positive phase of the annual AO; and (4) Ambrosia from France and Northern Europe during the positive phase of winter WeMO. Conversely, the positive phase of annual NAO and AO are linked with the regional transport of Alnus, Betula and Corylus from Western Iberian Peninsula. The positive phase of annual WeMO was also positively correlated with regional transport of Corylus from this area.
Tue, 06 Oct 2015 17:20:17 GMThttp://hdl.handle.net/2117/773982015-10-06T17:20:17ZIzquierdo Miguel, RebecaAlarcón Jordán, MartaPeriago Oliver, M. CristinaBelmonte Soler, JordinaClimatic oscillations triggered by the atmospheric modes of the Northern Hemisphere teleconnection patterns have an important influence on the atmospheric circulation at synoptic scale in Western Mediterranean Basin. Simultaneously, this climate variability could affect a variety of ecological processes. This work provides a first assessment of the effect of North Atlantic Oscillation (NAO), Arctic Oscillation (AO) and Western Mediterranean Oscillation (WeMO) on the atmospheric long-range pollen transport episodes in the North-Eastern Iberian Peninsula for the period 1994–2011. Alnus, Ambrosia, Betula, Corylus and Fagus have been selected as allergenic pollen taxa with potential long-range transport associated to the Northern Hemisphere teleconnection patterns in the Western Mediterranean Basin. The results showed an increase of long range pollen transport episodes of: (1) Alnus, Corylus and Fagus from Western and Central Europe during the negative phase of annual NAO and AO; (2) Ambrosia, Betula and Fagus from Europe during the negative phase of winter WeMO; (3) Corylus and Fagus from Mediterranean area during the positive phase of the annual AO; and (4) Ambrosia from France and Northern Europe during the positive phase of winter WeMO. Conversely, the positive phase of annual NAO and AO are linked with the regional transport of Alnus, Betula and Corylus from Western Iberian Peninsula. The positive phase of annual WeMO was also positively correlated with regional transport of Corylus from this area.Phase transition in hydrogen-bonded 1-adamantane-methanol
http://hdl.handle.net/2117/77015
Phase transition in hydrogen-bonded 1-adamantane-methanol
Ben Hassine, Bacem; Negrier, Philippe; Barrio Casado, María del; Mondieig, Denise; Massip, Stephane; Tamarit Mur, José Luis
The polymorphism of 1-adarnantane-methanol C11H18O has been investigated by differential thermal analysis and single-crystal and powder X-ray diffraction. Below the melting temperature (389.5 +/- 0.4 K), this compound exhibits an orthorhombic phase (phase I, Pnnm, Z = 12, Z' = 1.5). The melting enthalpy was determined to be 20.5 +/- 0.4 kJ mol(-1), i.e., with an entropy change of (6.34 +/- 0.13)R, which is much higher than the quoted value from Timmermans for the melting orientationally disordered phases (2.5R), thus supporting the orientationally ordered character of phase I. This orthorhombic phase I exhibits a statistical disorder of the hydrogen atom related to the oxygen atom, due to the position of one independent molecule on the mirror. At ca. 272 K, phase I transforms continuously through an order disorder transition to a low-temperature monoclinic phase II (P2(1)/n, Z = 12, Z' = 3). The monoclinic and orthorhombic phases are related by a group subgroup relationship, which perfectly agrees with the continuous character of the II to I transition. Moreover, by a convenient choice of an order parameter related to the continuous tilt of the c-axis, the critical exponent for this transition is found to be close to the theoretical prediction of the three-dimensional Ising model (with a critical exponent of ca. 0.27).
Tue, 22 Sep 2015 11:46:59 GMThttp://hdl.handle.net/2117/770152015-09-22T11:46:59ZBen Hassine, BacemNegrier, PhilippeBarrio Casado, María delMondieig, DeniseMassip, StephaneTamarit Mur, José LuisThe polymorphism of 1-adarnantane-methanol C11H18O has been investigated by differential thermal analysis and single-crystal and powder X-ray diffraction. Below the melting temperature (389.5 +/- 0.4 K), this compound exhibits an orthorhombic phase (phase I, Pnnm, Z = 12, Z' = 1.5). The melting enthalpy was determined to be 20.5 +/- 0.4 kJ mol(-1), i.e., with an entropy change of (6.34 +/- 0.13)R, which is much higher than the quoted value from Timmermans for the melting orientationally disordered phases (2.5R), thus supporting the orientationally ordered character of phase I. This orthorhombic phase I exhibits a statistical disorder of the hydrogen atom related to the oxygen atom, due to the position of one independent molecule on the mirror. At ca. 272 K, phase I transforms continuously through an order disorder transition to a low-temperature monoclinic phase II (P2(1)/n, Z = 12, Z' = 3). The monoclinic and orthorhombic phases are related by a group subgroup relationship, which perfectly agrees with the continuous character of the II to I transition. Moreover, by a convenient choice of an order parameter related to the continuous tilt of the c-axis, the critical exponent for this transition is found to be close to the theoretical prediction of the three-dimensional Ising model (with a critical exponent of ca. 0.27).