The importance of fine granularity. The Soudan 2 experience and comparison with MINOS Peter Litchfield RAL Soudan 2 is a fine grained drift calorimeter designed to detect proton decay. As well as proton decay we have studied atmospheric neutrinos and cosmic ray muons. What are the lessons we have learned about what is required to study these processes? MINOS has been studied as a detector for atmospheric neutrinos (but not for proton decay) and Soudan 2 as a detector for beam neutrinos. I compare the two to give some idea of how things scale with detector granularity.

Tube matrix Drift tubes encased in a “bandolier” assembly for insulation and easy assembly. The bandolier is fan-folded and stacked between the corrugated steel sheets

Soudan 2 Underground

Particle id  - long non-interacting track single particle p - straight, heavily ionising track Energy measured by range Blue-Green-Red-Black = ionisation  340 MeV p 110 MeV

e - shower, multiple tracks, gaps due to  propagation Energy from counting hits and/or total pulse height e 410 MeV

 +- - interacting track   NC event p  -  0

High energy event with an uncontained .

MINOS beam MC event  cc event E =2.1GeV  GeV  GeV (not seen) p GeV  GeV

MINOS beam event e cc event E GeV e GeV p GeV (not seen)

Proton Decay Advantages of calorimetry Very good vertex and track resolution Observation of ,K,p below Cherenkov threshold Good for high multiplicity channels Detection of protons reduces background Disadvantages Iron is a big nucleus, corrections for intra-nuclear absorption are large MC Data candidate

Proton Decay Analysis

Proton decay results No signal above background Most channels have neutrino background Our best candidate (e  )

Atmospheric Neutrinos What advantages does a fine grain calorimeter have over a water cherenkov? Lower  and  detection thresholds (~100 Mev/c), below cherenkov thresholds. But best oscillation information at high energies. Good particle ID and event reconstruction in high multiplicity events Detection of protons, particularly in quasi-elastic interactions. Addition of the proton much improves the neutrino direction measurement. ~40% of quasi- elastics have an observable proton. Higher density and better track and vertex resolution gives a higher percentage of useful fiducial volume (~70% for Soudan 2, c.f. 40%? for Super-K) Disadvantages? Mass, mass, mass. Particularly the lack of events at high neutrino energy where Super-K sees the biggest effects.

L/E Analysis E measured by full reconstruction of event L measured by the event direction (zenith angle) protons and low energy pions are seen and included in the reconstruction –define a “high resolution” sample with high energy quasi-elastics low energy quasi-elastics with seen proton high energy multiprongs Full Feldman-Cousins analysis of the data in progress

L/E fit   e Red Blue MC 

Results shown at Budapest      

Atmospheric in MINOS We have studied how MINOS can contribute to atmospheric neutrino physics. Much poorer granularity, higher event thresholds and worse flavour discrimination at low energies Good muon identification by range and good resolution in neutrino direction and energy at high energies where Super-K data shows the biggest effects. The magnetic field is a new feature not available in previous experiments; can separate neutrinos and anti-neutrinos and check invariance and flux calculations observe matter effects in any differences? Measure sign of  m 2 ? can measure the momenta of exiting  thus increasing acceptance and improving resolution can determine muon direction by slowing in the magnetic field

Soudan 2-MINOS Soudan 2 quantities are averaged over all directions, MINOS are for normal incidence MINOS - Steel/scintillator sandwich Soudan 2 MINOS Mass 960 tons 5400 tons Density 1.6gm/cc ~3.5gm/cc Steel thickness 1.6mm 25.4mm Mean sampling length ~3.5mm steel >25mm steel Hits/Radiation length ~5 <0.7 Hits/Interaction length ~45 <6 MeV/Shower hit ~ MeV  range ~200cm

High energy  event selection We select high energy events by requiring; A  track >1GeV by range Pulse height > 100 photo-electrons Require 3m length for uncontained  to allow momentum measurement Direction of muon determined by change of curvature in field while slowing vertex activity timing 407 events remain from an 18 kton year MC sample, a four year run. Practically pure sample of  cc events Practically 100% correct direction determination  energy resolution  10% from combination of range and curvature measurements

L/E distributions

MINOS sensitivity region  2 difference between the oscillated and unoscillated distributions calculated on a grid of points and the 90% confidence contour plotted Normalisation assumed known Super-K best fit

Parameter measurement sensitivity  m 2 =10 -3,sin 2 (2  )=1.0 Fixed normalisation

Beam oscillations We have studied the use of Soudan 2 in the MINOS neutrino beam and compared its sensitivity to MINOS. 1 kton of Soudan 2 versus 5kton of MINOS. Again the advantage of Soudan 2 is its good vertex and track resolution and sensitivity to low energy particles. Disappearance reactions with outgoing  For high energy events mass always wins For low energy events where the outgoing muons are contained within the hadron shower the fine granularity improves the event selection and background rejection However the MINOS beam runs out of steam at ~1GeV so the advantages of Soudan 2 are not realised, mass and MINOS always wins.

  e Appearance,   e Characteristics of electron production - short events with a shower profile and large energy deposit near the vertex. Main background is neutral current  0 production Good resolution and pattern recognition is vital to distinguish single from multiple showers Soudan 2 shower identification efficiency higher than MINOS at low energies for the same background.

NC event E GeV GeV n GeV  GeV  GeV,  GeV

Parameter distributions Number of hits Truth Neutrino energy Unoscillated beam e cc

  e NuMI-L- 576 New MINOS CHOOZ 90% C.L. THESEUS THESEUS (energy) 5 ktons of MINOS is better than 1 kton of Soudan 2. Probably 1 kton of Soudan 2  3 ktons of MINOS

    Analysis based on the identification of fast  by their secondary interactions. Soudan 2 has advantages in better separation of outgoing tracks and identification of secondary interactions. Unfortunately, for low  m2 there are very few  formed, particularly in the low energy beams which are best for the other measurements.

Comparison of  efficiencies

Comparison of sensitivities For low  m 2 1kton Soudan 2 is equivalent to 5kton MINOS

Soudan 2 Pluses Uniform honeycomb geometry. Important for proton decay and atmospheric neutrinos which can come from any direction. Not so important for a beam experiment where the predominant direction is known. Very good track and vertex resolution, mostly due to the fine granularity Good particle ID, e, , , proton Particles below cherenkov threshold observed. The higher density and good track resolution gives a much higher fraction of useful volume than a water cherenkov Relatively cheap (though not as cheap as water) and easy to build. Modules were built as a “cottage industry” in the home laboratories. 1 kton of detector cost ~$11M at 1990 prices. Robust and easy to operate. Ran for ~12 years at Soudan with very little trouble, 85-90% of calendar on time. Operated by our mine crew with very little physicist effort required.

Soudan 2 minuses Only 1 kton The ionisation measurement was never as good as had been hoped. Mainly due to the small size of the tube. The transverse position resolution of the track (~3mm) is comparable with the tube radius and thus the length of track in the gas has large errors. However the ionisation measurement is excellent for distinguishing protons from lighter particles. Track directionality for muons is not as good as in a water cherenkov. We find ~85% correct direction determination by eye for single muons using ionisation and coulomb scattering, worse by program. We had hoped that the ionisation rise for stopping particles would be a strong handle but the worse than expected ionisation resolution frustrated this. The directionality improves considerably with energy where coulomb scattering has most weight. Worst below ~200 Mev/c Tracking the particle through the steel to obtain a better energy measurement than assuming a uniform density never much improved the measurement again because of the comparability of the resolution and steel thickness. Not easy to add a magnetic field. Drifting may be affected by a field. Might be able to insert field region between module walls.

Comparison with other techniques Soudan 2 honeycomb v Planar drift chamber Uniformity of response with direction Less demanding tolerances for drift field Better length in gas and thus ionisation and range measurement Magnetic field? Soudan 2 honeycomb v Planar scintillator Uniformity of response with direction Much better granularity per $ Better ionisation measurement in dense material Magnetic field Soudan 2 honeycomb v water cherenkov Better track and vertex resolution Observation of particles below Cherenkov threshold Better fiducial volume fraction Better track directionality Cheaper, therefore more massive

Scaling up Soudan 2 The size of Soudan 2 modules was determined by the capacity of the mine hoist. It has a 5 ton weight limit and the modules essentially filled the cage. With no access limitations one could; increase the drift length, the current modules have about a 30% attenuation in 50cm drift increase the height and width dimensions maybe up to a factor 2 though handling might then become harder. Soudan 2’s granularity was plenty adequate for proton decay. Atmospheric neutrinos are most interesting at the higher energies (>500MeV). The steel thickness could be increased, maybe up to a factor 2 (1.6mm  3mm?). The tube diameter could be increased, helping the ionisation and range measurements and the attenuation with a longer drift length. Electronics has become much cheaper since the mid 1980s. We could either read out the detector with a lot of summing of channels as before but much cheaper or read out each channel separately for little extra cost.

Scaling up Soudan 2 It might be possible to halve the cost of building Soudan 2 per kiloton and maintain the advantages of the fine granularity. A 20 kton detector which would have approximately equal useful event rates but better resolution and event identification than Super-K might cost $ M at today’s prices.