The Charged Current Reaction on Carbon 12

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The electron neutrinos produced by the neutrino source ISIS ineract with the carbon nuclei of the scintillator in the KARMEN detector by interchange of a charged W-boson (thats why it is called charged current). Thereby the carbon nucleus is transformed to a nitrogen nucleus emitting a positron. The positron can be detected in the KARMEN detector as prompt event. The produced nitrogen nucleus with mass number 12 (the same as carbon) is radioactive and decays with a decay time of 15.9 ms back to carbon. The nitrogen nucleus does not move within this short time, so that the electron emitted during the radioactive decay of the nitrogen, can be detected practically at the same positon in the detector as the positron. Thus the neutrino reaction is identified by the detection of a pair of events, strongly correlated in time and space.

During the measuring time from July 1990 until December 1995, the ISIS target was bombarded with protons equivalent to a total charge of 9122 Coulomb. During this bombardment so many neutrinos were produced, that 60,000 Billion neutrinos crossed every square centimetre of the detector cross section. Despite of this, the probability that an interaction of the neutrinos with matter occurs, is so low, that during these five years only about 500 events with the given signature could be clearly identified. The measurement is practically free of background (signal to background ratio is 35:1), i.e. the number of events in the data, which are not neutrino induced, is very low. The following picture shows the time and energy distributions of the prompt and sequential events. The numbers of measured events is plotted in red, a Monte Carlo simulation, representing the expectation for a very long measuring time, is shown in blue. The time distribution of the prompt events (relative to proton beam on ISIS target) shows clearly the decrease as it is expected from the 2.2 microsecond decay time of the muon in the ISIS target, producing the electron neutrinos. The difference time between prompt and sequential event reveals the decay time of the radioactive nitrogen.

The background is (for a neutrino experiment) extremely low, so that one can perform a precision test of the standard model of the weak interaction. If one reconstructs the energy distribution of the electron neutrinos from the energy distribution of the detected electrons, one can compare this distribution with the theoretical expectation. Deviations from the standard model would show up as an increase of the event rate at the high energy tail of the neutrino spectrum.

The Neutral Current Reaction on Carbon-12

If one does not look for the detection of a seqential event, one can analyse several other neutrino reactions beside the charged current reaction introduced above. Here the detection signature is given just by the detection of a single, isolated prompt event, which is not followed by a sequential event. From now on such a signature is called a single prong reaction. The picture below shows an analysis of the energy spectrum of the events measured in the years 1990-95 with KARMEN. The complex spectrum consists of several different neutrino reactions wich will be explained below in more detail.

• The neutral current reaction ¹²C(n,n')¹²C^* causes the prominent peak-strucutre of the energy speactrum at around 14 MeV. electron- (n_e) and muon-neutrinos (n_m) from the muon-decay in the ISIS-Target react via the exchange of Z₀-bosons with the ¹²C-nuclei of the scintillator. These bosons are different from the W^±-bosons exchanged during the charged current reaction, they are electrically neutral. That is where the name neutral current cames from. The neutrines excite mainly one specific nuclear level in carbon with an energy of 15.1 MeV. This excitation energy is released pratically in an instant via the emission of a gamma quant with the same energy of 15.1 MeV. These gamma quanta are detected after their conversion into charged particles in the KARMEN-Detektor and build up the peak structure in the spectrum shown above. KARMEN was the first an up to now only experiment to detect this reaction.

• The charged current reaction to excited states of ¹²C ¹²C(n_e,e^-)¹²C^* does not show a sequential structure, because the excited ¹²C-nuclei decay at once via the emission of protons to the stable ¹¹C. The detection of the protons happens at the same time together with the dections of the electrons. Because of the energy threshold of the carged current reaction the energy spectrum of this reaction ends at 36 MeV.

• The charged current reactions ¹²C(n_e,e^-)¹²C_g.s. in the single prong energy spectrum are events, where the detection of the sequential event did not succeed, because e.g. a to low energy of the positrons. Thus this reaction with a sequential structure also contributes to the single prong energy spectrum. The highest possible energies are 36 MeV.

• Carbon in its natural abundance contains about 1.1% of the isotope ¹³C. The energy threshold for the the charge current reaction on ¹³C ¹³C(n_e,e^-)¹³C is substantially lower than the threshold of the reaction on ¹²C. For this reason the energy of the detected electrons is up to 50 MeV. Moreover the low energy threshold results in a higher cross section, i.e. the probability of a reaction of a n_e is considerably higher. Only this fact allows the detection of this reaction in spite of the very low abundance of ¹³C of 1.1%.

• The n_e and n_m interact via neutrino-electron-scattering mit with electrons in the atoms of the materials used in the KARMEN-detector. This purely leptonic reaction is theoretically well understood and therefore is considered as known during the analysis of the single prong energy spectrum.

• The charged current rection on Iron ⁵⁶Fe(n_e,e^-)⁵⁶Co plays quite a special role. The the n_e interact with the iron of the internal passive shielding. The electrons produced during the reaction an detected indirectly, because they cannot enter directly into the detector. As charged particles they loose their energy quickly within the iron. Moreover the would be deteted by the anti counters and these events would would not be taken into account. In the iron the electrons produce so called bremsstrahlung while they slow down. Bremsstrahlung consists of gamma quante which can escape from the iron and can be detected in the KARMEN-detector after they have been converted back to charged particles. Therefore the energy spectrum has the typical shape of a bremsstrahlung spectrum, which rises steeply to low energies but reaches up to 50 MeV because of the low energy threshold of the reaction.

By the analysis of the single prong energy spectrum of the roughly 2000 neutrino events detected in the years 1990-95 a large number of topics in neutrino-nucleus physics could be addressed. The results can be found in our Publications, especially in a recent overview article .

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