Criteria that must be satisfied
for the
Discovery of a New Chemical Element to be Recognized
III. PRODUCTION PROPERTIES
In the following list, we give properties connected with the preparation
of nuclides as just described. The first five concern reaction properties,
the others sample preparation. The note in the last column describes
whether they are "characterization properties" C or "assignment properties"
A(Z) for Z or A(A) for A or A(A,
Z) for both (see section II.3).
| Ei |
Energy of bombarding particles |
C |
| Cs |
Cross section |
C |
| Ey |
Yield curve |
C, A(A, Z) |
| Cb |
Cross bombardments |
C, A(A, Z) |
| Ad |
Angular distribution |
A(Z) |
| As |
Angular selection |
A(Z) |
| Ms |
Mass separation |
A(Z) |
| Vf |
Velocity filter |
A(Z) |
| Tf |
Time of flight selection |
A(Z) |
| Ch |
Chemistry |
A(Z) |
Notations and introduction. Let a new nuclide AZ
be formed in the bombardment of a target AtZt
(it may be a mixture of isotopes) with particle az
energy Ei, with a probability expressed as a cross section
s.
Two types of reactions *
are used to produce transfermium elements:
(i). Hot fusion reactions using ions with 4<z<12 impinging
on actinides produce compound nuclei with high excitation energies
(typically about 40 MeV at projectile energies just above the fusion
threshold)
(ii). Cold fusion reactions using with 17<z<29 impinging
on Bi, Pb or Tl produce compound nuclei with much lower excitation
energies.
Such reactions in which g-rays or neutrons
are emitted accompanied by at most one proton or a-particle
will be called evaporation reactions. So called transfer reactions,
in which the final charge is significantly smaller than Zt+z
, are known to have produced unwanted and confusing backgrounds in measurement
of evaporation reactions.
As to the target, the isotopic composition of the primary material
is always known with sufficient accuracy in the experiments considered
here. The presence of impurities in the target is not expected to be
a source of uncertainty in cold fusion reactions. Impurities of, especially,
Pb in actinide targets have been known to have produced confusion in
the past.
Comments
Ei,Cs (Energy of bombarding particles and reaction
cross section). The minimum necessary and admissible information
about nuclear reactions is the energy of the particles impinging on
a possibly thick target (in which they lose energy before reacting)
and some information on the yield.
Ey (Yield curve). The most complete information is a yield curve:
production cross section as a function of energy of the particle impinging
on a target nucleus.
Yield curves for (az, axn)
reactions tend to have tails and so can be distinguished from the previous
two, if measured with good statistics. (Their maxima occur at higher
energies than those for (az,xn) reactions). For hot
fusion, the maxima for (az,xn) reactions can be considerably
larger than those for (az,xn) reactions, but for cold
fusion they are found to be about two orders of magnitude smaller. Yield
curves for transfer reactions are much broader. At least for hot fusion,
the yield for transfer processes need not be small compared with those
for (az,xn) processes.
The theoretical understanding of reaction cross sections, especially
in the region where fission seriously competes with evaporation, is
insufficient to allow extrapolation to unknown Z cases with the
confidence necessary to establish absolute priorities. Empirical evidence
for the ratios of different kinds of evaporation reactions as a function
of Z and A, which is now available, is, however, a useful
guide.
Cb (Cross bombardments). Comparison of the probability of production
of AZ in different combinations of AtZt
and az can sometimes give valuable assignment criteria.
Ad, As (Angular distribution, Angular selection). The dependence
of the production of AZ in (az,xn)
processes is strongly forward peaked, more than in (az,axn)
or transfer reactions. Thus, determination of angular dependences, or
comparison of yields behind two different collimators, may yield a good
criterion for assigning Z. This property can also be used to
suppress unwanted backgrounds.
Ms (Mass separation). A well calibrated mass spectrometer
with a resolution significantly better than 1/2 mass unit can yield
an excellent criterion for assigning the mass number of the reaction
products. One should, of course, be certain that one does not accidentally
observe molecular fragments with the same SA.
Although in ion sources for mass spectrometry some chemical differentiation
occurs, no useful information concerning Z can be drawn from
A, except of course that an exceptionally high A would
point strongly to a new (high) value for Z. Also, the value from
evidence from other data might be strengthened by combination with mass
spectroscopic evidence (e.g. when a possible daughter was observed.)
Even with limited resolution, a mass ("isotope") separator can be used
to suppress unwanted backgrounds.
Vf (Velocity filter). A velocity filter can give a quite good
(though not complete) separation of evaporation products from the results
of transfer reactions. If combined with the result of an energy determination
(e.g. by measuring the signal in a semiconductor counter catching the
reaction product), it can act as a low resolution mass spectrometer.
Its main use is suppression of unwanted backgrounds.
A variation of the velocity filter is to make use of the differences
in range in matter between evaporation products and those resulting
from transfer reactions.
Tf (Time of flight selection). Measurement of time of flight
of the reaction products can replace or complement the use of a velocity
filter.
Ch (Chemistry). Chemical methods can yield excellent assignment
criteria. Observations of analogies of chemical properties of compounds
involving the elements of unknown Z with those of compounds of
the same chemical type of known elements may suggest specific Z-assignments.
Chemistry can be done with few, or even single atoms of an element.
In these cases, many repeated reactions take place with those few atoms.
This occurs in methods ion exchange (Ci), gas chromatography (Cg), gas
thermochromatography (Ct) or chemical vapour transport (Cv).
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Notes
We here disregard the reported production of a
new element ascribed to secondary particles of unknown energy themselves
produced by the bombardment of a target with an intense beam of high
energy. (Back to text)
