PRODUCTION OF THE SHORT-LIVED ISOTOPES

1
PRODUCTION OF THE SHORT-LIVED ISOTOPES 12N
AND 12B IN THE 14N(?,2n), 14N(?,2p), AND
13C(?,p) REACTIONS L.Z.Dzhilavyan1, A.I.Karev2,
V.D.Laptev1, V.G.Raevsky2 1 Institute for
Nuclear Research, Russian Academy of Sciences,
Moscow, Russia 2 P.N.Lebedev Physical Institute,
Russian Academy of Sciences, Moscow, Russia
2

• The theme of this report is connected with
  12N-nuclei.
• For the first time their production in the
  reaction 12C(p,n) was found by L.W.Alvarez at
  pulsed proton linac.
• Soon W.K.H.Panovsky and D.Reagan also produced
  them at pulsed electron linac in Stanford, using
  the photonuclear reaction
• ???????????n (IA14N-99.63,15N-0.37
  E? thresh?30.6 MeV) (I)

3

• 1. The production and decay features of
  12N-nuclei
• Decay of 12N and ways of their production have a
  remarkable set of features (especially, when 12N
  are produced at electron accelerators)
• 12N-nuclei have a rare short lifetime T1/2?11.0
  ms, what helps to separate 12N-activity from
  another ones and to do it fast.
• 12N-nuclei decay emitting ??-particles. It is
  possible to use coincidence registration of
  pairs of outgoing annihilation ?'-quanta (with
  E?'?0.511 MeV each, flying out in opposite
  directions) and thus to suppress effectively big
  part of background.
• 12N-nuclei decay, emitting ?-particles with the
  very high E? max ?17 MeV.
• 12N are the neutron-deficient nuclei with A ?
  2(Z?1) (while, except for 1H and 3He, for all
  stable nuclei A ? 2Z). This restricts strongly
  choice of reasonable reactions for production of
  12N.
• It is possible to apply photoproduction of 12N
  for rather thick objects (having connection with
  concentrations of nitrogen).
• If an incident beam of ?-quanta has small
  divergence and spot on irradiated objects, then
  we have a common way for localization of
  N-concentrations.
• If for detection of 12N-decays registration of
  (?'-?')-coincidences of annihilation ?-photons
  is used, we may use it also to find localization
  for N-concentrations (together with the common
  method with achieving better accuracy and
  suppressing of background).

4

• 2. The first variant of connected with 12N
  detectors of hidden explosives
• Alvarez suggested the idea, which unfortunately
  was not yet experimentally confirmed, to use for
  a photonuclear detector of hidden explosives
  (DHE) in coincidenceregistration of
  annihilation ?'-quanta from decays of 12N,
  produced on a pulsed electron accelerator in the
  reaction (I) on 14N, contained in big
  concentrations in widely used now explosives. It
  is supposed that this registration takes place
  for 100 ms or even less between accelerators
  pulses. We call such DHE as DHE-1.
• DHE-1 was propose for the external not-destroying
  control of air-passengers baggage. But there is
  also a big interest to DHE for humanitarian
  demining and for testing of cargo.
  Unfortunately, for the latter 2 cases DHE-1 does
  not suit. For landmine searching it is impossible
  to register both annihilation ?'-quanta. For
  cargo containers there is too big absorption of
  outgoing ?'-quanta.

5

• 3. The second variant of connected with 12N
  detectors of hidden explosives
• At the further development of a photonuclear DHE,
  there was suggested to register single ?'-quanta.
  In this case there were made the moderately
  successful, but encouraging experimental tests,
  including our own ones,. We call here such DHE as
  DHE-2. The purpose of DHE-2?the universal DHE for
  all 3 pointed out applications. DHE-2 can work
  at E?' up to several MeV with significant
  decreasing (without taking into account
  advantages from the coincidental technique) of
  the corresponding in energy gamma-background
  level, with essential reduction of absorption of
  required ?'-quanta in irradiated objects and with
  an opportunity to use for ?'-quanta registration
  Cerenkov detectors with "cutting off" the big
  part of gamma-background at small energies. In
  this case for providing with useful signals there
  is some addition from the reaction (II), which
  produces also on 14N radioactive nuclei 12B
  (??-decay E? max?13 MeV T1/2?20.4 ms 1),
  having T1/2 and values of decay E?' for
  registration, close to those for 12N-nuclei
• ????????B??p (E? thresh?25.1 MeV).
  (II)
  At electron accelerators with Ee?55 MeV for
  outgoing electrons (maximum for the pulsed race
  track microtrone, which is now under construction
  in LPI, for these studies), there are few
  reactions, which may produce radioisotopes with 7
  ms ?T1/2 ? 30 ms, and with E?', close to those
  for 12N and 12B. Except for the reactions (I,II)
  such nuclei may be produced in some background
  reactions under bombardment by primary
  bremsstrahlung photons or by secondary neutrons,
  produced before in photoneutron reactions.

6

• Among nuclei-products of these background
  reactions there may be (taking into account
  isotopic abundances, values of reaction
  thresholds, and, to some extents, chemical
  abundances and branching ratios for these
  reactions)
• as 12N and 12B, but produced on some other
  nuclei-targets (not on nitrogen), and among these
  types of reactions there may be
  the photonuclear
  reactions
• ?? 13C???B?p (IA 12C-98.89, 13C-1.11
  E? thresh?17.5 MeV).
  (III)
• ???6O???N?3H?n (IA16O-99.76 E? thresh?45.1
  MeV),
  (IV)
• ???9F??2N???3n (IA 19F-100 E? thresh?45.6
  MeV)
  (V) the reaction under thermal neutrons
• n?11B???B?? (IA 11B-80.2 the released energy
  ?3.4 MeV) (VI) the
  reaction under fast neutrons (En threshneutron
  kinetic energies En at threshold)
• n???C???B?p (En thresh?12.6 MeV)
  
  (VII) as some other nuclei-products, and
  among these reactions there may be
• ???6O??3B?3p (??-decay E? thresh?43.2 MeV
  T1/2?17 ms),
  (VIII)
• ???6O??3O?3n (??-decay E? thresh?52.1 ???
  T1/2?9 ms).
  (IX)
• For DHE-2 there may be serious background in
  several ms after beam, from (n,?')-reactions. On
  the other hand, there may be background from
  giving radioisotopes with 0.1 s?T1/2?1 s
  reactions, in particular, with 3 nucleon escape,
  especially from
• ???2C?9Li?3p (??-decay E? thresh?46.8 MeV
  T1/2?178 ms),
  (X)
• ???2C?9C?3n (??-decay E? thresh?53.1 MeV
  T1/2?126.5 ms).
  (XI)

7
4. The cross sections of the used reactions and
some background reactionsTo optimize DHE, one
needs data on ?, of reactions (I)?(XI). The
measured ? of the reactions (III) and (VII) are
presented on Fig. 1 and 2. Data on ? of the
reaction (VI) under thermal neutrons also may be
found. Unfortunately, we did not find original
experimental data on ? of the reactions (I),
(II), (IV), (V), (VIII)?(XI).
Fig. 1.
Fig.
2.

8
The compiled data on ? of the reactions
(I)?(III), (VIII), and (IX) were reported by
W.P.Trower in 3 works. Data from the last of them
are more complete as well therefore, they are
presented on Fig. 3.
However, these data give rise to
much criticism 1) their sources are not
indicated 2) the data from these studies are
contradictory 3) ? of the reaction (III) from
the last work near their maximum are several
times smaller than the well-consistent
corresponding ? from Fig. 1 4) the claims for
the dynamic range and validity of behavior at
relatively high E? with respect to ? of the
reaction (III), made in this work, are, at least,
surprising (compare if only with the behavior of
? (III) from Fig. 1, measured in the range from
maximum of ? to E??30 MeV). A possible reason for
the latter fact is that not measured ? were
reported, but some predicted ones.
Fig. 3. ?
9

• So, we need experimental data on ? of both
  useful reactions (I) and (II) and the background
  reactions (IV), (V), (VIII)?(XI).
• (For convenience on this and the next pages we
  repeat the list of pointed out before reactions)
• In this technique, some background may be
  eliminated due to the thresholds and
  characteristic E?-ranges in ? of the reactions
  (I) and (II) are much lower than those for the
  background reaction (IV), (V), (VIII)?(XI) and
  another reactions, connected with escape of 3
  nucleons and giving radioisotopes with 0.1 s
  ?T1/2?1 s. On the other hand, some background may
  be significantly reduced, because of the E?
  thresholds and characteristic ranges of ? for the
  reactions (I) and (II) are much higher, than
  those for reactions from the giant resonance
  range, including the (?,p)-reaction (III) and
  reactions, giving the most of photoneutrons.
• ???????????n (IA14N-99.63,15N-0.37
  E? thresh?30.6 MeV)
  (I)
• ????????B??p (E? thresh?25.1 MeV).
  
  (II)
• ?? 13C???B?p (IA 12C-98.89, 13C-1.11
  E? thresh?17.5 MeV).
  (III)
• ???6O???N?3H?n (IA16O-99.76 E? thresh?45.1
  MeV),
  (IV)
• ???9F??2N???3n (IA 19F-100 E? thresh?45.6
  MeV)
  (V) n?11B???B?? (IA 11B-80.2 the released
  energy ?3.4 MeV)
  (VI)
• n???C???B?p (En thresh?12.6 MeV)
  
  (VII) ???6O??3B?3p (??-decay
  E? thresh?43.2 MeV T1/2?17 ms),
  (VIII)
• ???6O??3O?3n (??-decay E? thresh?52.1 ???
  T1/2?9 ms).
  (IX)
• ???2C?9Li?3p (??-decay E? thresh?46.8 MeV
  T1/2?178 ms),
  (X)
• ???2C?9C?3n (??-decay E? thresh?53.1 MeV
  T1/2?126.5 ms).
  (XI)

10

• The small fraction of B in typical tested objects
  permits to believe the background from the
  reaction (VI) to be not very interfering for this
  technique. The background from the reaction (VII)
  should be small because (i) the fraction of
  primary photoneutrons with En?En thresh for this
  reaction is small and (ii) such neutrons with
  overwhelming probability first undergo
  scattering in few of which their energy En
  decreases to values below En thresh. The reaction
  (VIII) is not dangerous due to its small ?. For
  background from the reactions (IV,V) situation
  seems to be the same. In principle, the reaction
  (IX) may give essential contribution, especially
  at searching of land-mines, since soil may
  contain up to 50 of O, but at Ee?55 MeV this
  contribution should be small. All background
  contributions from reactions, giving long-lived
  isotopes (including the reactions (X,XI)) should
  be small too, and their influence may be taken
  into account by usual procedures of background
  subtraction.
• ???????????n (IA14N-99.63,15N-0.37
  E? thresh?30.6 MeV)
  (I)
• ????????B??p (E? thresh?25.1 MeV).
  
  (II)
• ?? 13C???B?p (IA 12C-98.89, 13C-1.11
  E? thresh?17.5 MeV).
  (III)
• ???6O???N?3H?n (IA16O-99.76 E? thresh?45.1
  MeV),
  (IV)
• ???9F??2N???3n (IA 19F-100 E? thresh?45.6
  MeV)
  (V) n?11B???B?? (IA 11B-80.2 the released
  energy ?3.4 MeV)
  (VI)
• n???C???B?p (En thresh?12.6 MeV)
  
  (VII) ???6O??3B?3p (??-decay
  E? thresh?43.2 MeV T1/2?17 ms),
  (VIII)
• ???6O??3O?3n (??-decay E? thresh?52.1 ???
  T1/2?9 ms).
  (IX)
• ???2C?9Li?3p (??-decay E? thresh?46.8 MeV
  T1/2?178 ms),
  (X)
• ???2C?9C?3n (??-decay E? thresh?53.1 MeV
  T1/2?126.5 ms).
  (XI)
• So, it seems that for DHE-2 the most serious
  background may be from the reaction (III), but,
  of course, it is very desirable to check the
  situation experimentally.

11

• 5. The third variant of connected with 12N
  detectors of hidden explosives
• False signalsone of the most important problem
  for all methods of explosive detection, which
  determines methods efficiency, productivity, and
  practical importance. For DHE-1 or DHE-2 it is
  reduced to rejection of signals from another
  substances, containing like explosives N and/or
  C.
• Earlier for DHE-2 there were suggested 2 ways for
  subtraction of connected with the reaction (III)
  background, based on its dependence on ?? and
  ??'. In our opinion these ways have grave
  shortcomings. We suggest our own way.
• N and C the basic elements of widely-used
  explosives, and it is desirable to consider C-
  signals not as background, but as helpful ones in
  detecting and identification of explosives.
• We suggested DHE-3, based on registration and
  analysis of time-distributions of events,
  connected with the reactions (I?III). Relative
  concentrations of 12N- and 12B-isotopes are
  unequivocally connected with relative
  concentrations of N and C in irradiated
  substances and define unique portraits of
  tested substances. Form of ??- event
  time-dependence is determined by values of T1/2
  and by initial ratio of 12N- and 12B- produced
  quantities.

12

• In common case a total number Nt of 12N- and
  12B- nuclei, which leave un-decayed to a moment t
  after an end of irradiation at t?0, is
• Nt ? N0 (N-12) ? exp(??(N-12) ? t) ? N0 (B-12)
  ? exp(??(B-12) ? t), (1)
• where N0 (N-12) , ?(N-12)?(ln 2) / (T1/2)(N-12)
  and N0 (B-12) , ?(B-12)?(ln 2) / (T1/2)(B-12)
  numbers of produced to a moment t ? 0 nuclei, the
  decay constants, and the half-lives for 12N and
  12B, respectively N0 ? ? Nt ? 0 ? N0 (N-12) ?
  N0 (B-12).
• Results of analysis of ?'-radiation
  time-distribution may give initial relative
  concentration of 12N and 12B and identify tested
  substance by parameter k
• k ? k(N-12) ?1 ? k(B-12),
  
  (2)
• where k(N-12)?N0 (N-12) / N0 ?,
  k(B-12)?N0 (B-12) / N0 ? (k-substances
  portrait). A measured in channel number of
  ?'-events from decay of 12N and 12B is
• nt ? ?(N-12)?N0 (N-12)?exp(??(N-12)?t)??t ?
  ?(B-12)?N0 (B-12)?exp(??(B-12)?t)??t, (3)
• where ?t a channel width for a measured
  time-distribution.
• A common trouble for DHE-2 and DHE-3 in several
  ms after irradiation there is high ?'- background
  from reactions (n,?'), initiated by
  photoneutrons. This background may give rise to
  serious distortions of results. There is a sense
  to use not all measured data on distributions
  nt, but only those which are after delay of
  several ms, with refusing from direct
  measurements of N0 ? and from direct calculations
  of k. Instead of that, we suggest to determine
  values of k, proceeding, for example, from two
  measured values n(t), for moments of time ti and
  tj. In this case, according to (3), we get system
  of two equations, which are linear ones with
  respect to N0 (N12) and N0 (B-12)
• ?nti??(N-12)?N0 (N-12)?exp(??(N-12)?ti)??t ?
  ?(B-12)?N0 (B-12)?exp(??(B-12)?ti)??t.
  ?ntj??(N-12)?N0 (N-12)?exp(??(N-12)?tj)??t ?
  ?(B-12)?N0 (B-12)?exp(??(B-12)?tj)??t, (4)

13

• From (4), we get N0 (N-12), N0 (B-12), and
  kN0 (N-12)/N0 ?. Repeating this procedure for
  all values of i and j (i1), we get a sequence
  of values of ki, corresponding to every interval
  of time-distribution.
• To decrease statistical uncertainties at
  determination of k for express-analysis it may be
  useful to turn from the differential form of
  describing of a decay process to the integral
  form. From (1) in this case a difference between
  quantities of un-decayed nuclei to moments of
  time t1 and t2 (t2gtt1)
• Nt1?Nt2?N0 (N-12)?exp(??(N-12)?t1)?exp(??(N-1
  2)?t2)?N0 (B-12)?exp(??(B-12)?t1)?exp(??(B-12)?t
  2)??ni, (5)
• where ?ni a sum of events, registered in
  channels, corresponding to an interval (t2-t1).
  If to take two time intervals, then from
  equations (5) we get a system, analogous to (4),
  from solving of which we get k.
• The suggested method may be practically useful,
  if only k-portraits of explosives and false-
  substances differ essentially. To answer to this
  question we made computer simulation of 12N- and
  12B- production in objects of interest under
  bombardment them by bremsstrahlung, generated by
  incident on radiator electrons with Ee?55 MeV. In
  spite of all criticism with respect to data on ?
  of the reactions (I) and (II), as the first step,
  we decided to use data from Fig. 3. At small
  statistical uncertainties, there were found
  values of N0 (N-12), N0 (B-12), and N0 ? and
  calculated values, called here true ones
  ktrue.

14
Some of found in such a way values of ktrue are
presented in Table 1. We may see, that values of
ktrue for explosives differ essentially from
those for chemical compounds, which may be in
baggage of air-passengers. Values of ktrue for
explosives differ also essentially from those for
objects, which have vegetable origin, what is
very important at operations of humanitarian
demining. Presented in Table 1 values of ktrue
were calculated in approximation with doubtful
data on some ? and without taking into account
some processs details (in particular, dependence
of physical efficiency on E?? and influence of
absorption of ?'-quanta in cover-substances on
E??-spectra). At practice there is possibility
for considerable improving of the situation by
means of experimental calibrations on a real
installation, at which there may be obtained and
written into a base of data values of k for
substances of interest.
Table 1. ?
substance chemical formula ktrue
explosives explosives explosives
trotyl (TNT) CH3C6H2(NO2)3 0.71
PETN C5H8N4O12 0.82
tetryl C6H2(NO2)4NCH3 0.80
hexogen N3(NO2)3(CH2)3 0.92
octogen N4(NO2)4(CH2)4 0.92
false substances false substances false substances
wood --- 0.03
nitron (CH2CHCN)n 0.66
nylon (CO(CH2)4CONH(CH2)6NH)n 0.33
capron (NH(CH2)5CO) n 0.49
soap-(Na) C17H35COONa 0.00
shampoo-(K) C15H31COOK 0.00
15
6. Work simulation for the third variant of
connected with 12N detectors of hidden explosives

• Differences of values of ktrue do not mean that
  at work of real DHE-3 accumulated counts in
  channels will be sufficient for calculating k
  with accuracy, ensured reliable identification of
  substances. That is why there was carried out
  computer simulations of DHE-3 in conditions,
  imitating baggage inspection in airports at the
  following main parameters of the installation

Ee 55 MeV
electron beam current at pulse duration 30 mA 6 ?s
radiator with thickness W 0.35 mm
??-detector surface area 1 m2
distance between tested object and ?'-detector 60 cm
?'-detector physical efficiency 100
16

• The following scenario of work was used
• 1. The ?'-detector is switched on with some delay
  after a beam pulse end, and ?- pulses are
  accumulated in the histogrammic memory of the
  time-digital converter. The obtained histogram is
  written in the computer.
• 2. Obtained according to 1. data are normalized
  and summed up across all channels with receiving
  a number N? . If N? is less than threshold
  N? thresh, it is supposed, that in an
  irradiated region there is no explosives.
• 3. The removed by the scanning device beam from
  the next accelerator pulse irradiates the next
  region of the tested object and initiate new
  accumulation of data and calculation of the new
  number N?. If in this case N? ? N? thresh,
  then the described above procedure of data
  processing for time-distributions is switched on,
  and the previous irradiation signals are used as
  background ones.
• The presented below results of work simulation
  for DHE-3 were obtained at the following
  conditions
• The connected with produced photoneutrons
  background signals have time-distribution,
  described by sum of two exponents. For the first
  exponent (?T1/2?)1?1.5 ms and sum of obtained
  signals (N0)1?700 for the second exponent
  (?T1/2?)2 ? 5 ms and (N0)2 ? 800.
• The useful signals appear from 50 g of trotyl
  (TNT), covered with the water layer with the
  thickness 10 cm. For this registered signals
  N0 ??2100.
• Accumulation of ?'-detector data beguines at 4
  ms after a beam pulse and continues till 19 ms.
  The channel width for the time-digital converter
  1 ms.

17

• Fig. 4. Obtained by simulation distributions
  nt 1background (without explosives)
  2background and effect (i.e. with an explosive)
  3only effect.

18

• Fig. 5. Obtained from results of simulation
  ksimt 1 for 50 g of TNT 2 for 50 g of C
  3for background. It is seen, that while for N-
  and C-containing substances groups of points may
  be in good approximation presented by straight
  lines, parallel to t-axis, for background
  tangents to the approximating curve for the
  corresponding group of points form big angles ?
  with t-axis

19
Table 2. Average values of ksim in comparison
with the obtained before values of ktrue
demonstrate with convincing effectiveness of the
suggested method of substance identification in
the conditions, close to work at the restricted
quantityof usefui signals and the relatively
high levels of background.Big difference of
average values of tg(?) for considered cases is
seen too. Therefore, if even because of some
reasons at an object test a sum of registered
signals receives a sharp increase, but values of
k are strongly dependent on t, then it means that
in this part of an object 12N and 12B were not
produced and its not explosive.
substance ktrue ksim (average) tg(a) (average)
TNT 0.72 0.696 0.062 0.004
carbon 0.0 -0.036 0.070 0.004
background - 2.962 0.62 -0.173
20

• This method permits to construct new devices with
  parameters much better than those for existing
  devices and open perspectives to make detection
  of explosives totally automatic with separation
  them from false objects.
• At the same conditions it was also shown by
  computer simulation, that for DHE-3, installed on
  the moving platform, it is possible to detect
  explosives with weight 40 g in ground at depth 20
  cm and during 8 hours to test 0.01 km2 of
  territory, what is approximately in 500 times
  more than what the commonly used at humanitarian
  demining manual method gives.
• Use of stationary installations of a type in the
  airports permits to get a high efficient and fast
  acting detector of hidden explosives. For
  example, the inspection of one baggage unit in
  airport takes less than 2 s and probability of
  explosive detecting in a case is much higher than
  that for the used nowadays methods (different
  intrascopes, gas analyzers, trained animals and
  so on).

21

• Conclusions
• The algorithm for substance identification, using
  measured time-distributions of 12N- and 12B-
  activities, produced in photonuclear reactions
  and connected with relative concentrations of
  nitrogen and carbon in irradiated substances, was
  considered. It was shown by computer simulation
  that installations for disclosure and
  identification of hidden explosives based on
  photonuclear method can surpass significantly
  used now for these purposes devices on combined
  criterion sensitivity-ability for fast
  acting-trustworthiness. The method is under
  successful development in LPI RAS.