Druck-Materie 20b.qxd - JUWEL - Forschungszentrum Jülich

6.2 Condition for thermally stimulated burps
Regularities which were perceived for condition of thermally stimulated burps are as
follows:
• a burp could always be induced by temperature rise of cooling helium if energy accumulated
in a sample exceeded 35-40 J/g (more than 3 hours of irradiation) and size of sample
is ≥0.3 mm.
• stored energy before a burp is close to linear to a temperature difference between temperature
of ignition and some definite temperature Tcl, see Fig. 6. In the figure, data on some
big samples of a segment type (because of the uncertainty of ignition temperature values in
the case) are not presented. Two points (both 120 J/g and 180 J/g) present experiment #7 in
view of impossibility to choose between two methods of restoration of the Qse-value. It is
believed (and this is clear in experiments with water ice, see a relevant section of the report)
that equations of line
Qse = C⋅⋅⋅⋅ (Tcl - Tign) (4)
can’t be absolutely identical for different temperature of irradiation. But, because of low
statistics of experiments, we could derive for the case one averaged curve based on a host
of experimental runs. For the curve, Tcl is equal to 35-36K which is consistent with the
temperature of the threshold of mobility of radicals [13]. The factor C≈ 10 J/g/K.
Stored energy, J/g
200
180
160
140
120
100
80
60
40
20
Q crit (T ign ), experiment
Q crit , approx. by cluster model
Q crit , appr. by Jackson's model
Q max , restored from exper.
0
18 20 22 24 26 28 30 32 34 36
Temperature of methane, K
Figure 6. Critical and maximal quantities of stored energy in solid methane.
• almost no dependence of a size of a sample on ignition condition; only the smallest samples
(≤ 1.0 mm) need slightly higher temperature for ignition of a burp at identical condition
of irradiation.
6.3 Pulse shape and duration of burps
Duration of burps and their temporal structure was deduced from computer processing
of experimental readings of the thermocouples fastened to the copper wall of the sample capsule.
As duration of burps were always much shorter than characteristic time of helium cooling,
a process of heat transfer from a sample to the copper wall is acceptable to be considered
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