A nitrogen gas laser will be used to create a pulse of photons at 337 nm. These photons will be divided up among approximately 300 optical fibers and delivered to the center of each scintillator element. Inside the scintillator, the 337 nm photons will be converted via fluorescence into an optical distribution centered around 425 nm, the same distribution of light which is created by a passing charged particle. These optical photons will be emitted isotropically from the point of delivery (also roughly similar to those produced by a charged particle), so the exact direction of the fiber at the delivery point is not critical.

The light transmitted in the fibers is ultra-violet, therefore the optical fibers must have a silica core and cladding, otherwise the UV light will fluoresce in the fiber and large losses will occur in transmission. The present system in Hall C does not use such fibers, since the light is converted to optical wavelengths before being transmitted downstairs.

The pulse from the nitrogen laser is originally very narrow in time, only 600 ps wide. This makes the pulse very useful for tracking the timing characteristics of the detector. If a multimode fiber is used, there will be dispersion in the fiber due to different path lengths. Over a long fiber (100m) this can be as large as 15-30 ns, depending on the fiber used. In order to restrict the time spread of the light pulse, it may be necessary to position the laser in the hall, as is done in Hall B.

In order to simulate a real event as much as possible, we may wish not to fire every channel of the spectrometer on every laser firing. In that case, a masking system will be required at the point where the photons are divided up among the many fibers.

If the amount of light delivered to the detectors can be varied in a reproducible way, then the pulse height dependence of the time response can be monitored. Since we will not have ADC's available for every channel, we need a way to monitor the gain: That can be accomplished by lowering the photon pulse height until the discriminator ceases to fire, thus establishing a relation between relative numbers of photons and the discriminator threshold. To accomplish these goals, a variable attentuation system must be employed, also at the point where the photons are divided up among the fibers.

Nitrogen Laser: The existing nitrogen laser in Hall C produces sufficient numbers of photons per pulse for our use, but it may need to be relocated to the Hall as mentioned above. A new laser of this sort costs about $7500.

A constant flow of very pure nitrogen gas needs to be supplied to the laser. Hall B uses a baffled liquid nitrogen tank which warms the boil-off to room temperature before delivery to the laser. This system runs for about two weeks without changing. Hall B has developed an interlocked gas flow sensor which prevents the laser from firing if the gas flow is interrupted. If the laser is positioned in the Hall, then we may wish to copy these systems from Hall B.

There are various methods for distributing the laser light among the optical fibers:

1. Hall B uses a diffusing bar which they obtained from a company as a prototype but which never became a production item; the availability of this bar is therefore in question. Possibly we could develop our own diffusing bar. At any rate, after diffusing the light shines on an array of fiber clusters through a masking system.
2. Another option is to use lenses to produce an expanded parallel (or nearly parallel) beam and let that shine on an array of fiber clusters. The optics for this (from Newport) costs about $3000.
3. Another option is to use a tree of optical splitters. Each splitter takes one fiber as input and divides the light among the outputs more or less equally. Commercial splitters like this come in sizes as large as 1:64. Unfortunately, the equality of the splitting is not very good and the spread in the pulse heights grows as the depth of the tree. These commercial splitters are also very expensive. Our system would require on order of 10 1:32 splitters. The cheapest 1:32 splitter I found costs $1500.

The variable attenuation and masking selectors would presumably be driven by stepping motors or some similar mechanical system. I have not had the opportunity to price out such a system for G0. Three of the diffuser and mask systems used in Hall B were built by William and Mary, and according to their proposal the masking and attenuation mechanical components cost about $4000 (for one system). They used a very expensive continuously variable neutral density filter for the attenuator -- leaving that out drops the price to about $2600.

Two more groups of fibers are needed to complete the delivery system: (1) an individual fiber mounted on the detector for the final delivery, and (2) a fiber which connects the detector fiber to the distribution system patch panel. These fibers can be made by hand using tools which are available from a number of companies. In order to provide the most reliable and uniform delivery, the fiber ends that have connectors need to be polished. These polishing tools and materials cost about $2000 overall.

The total amount of fiber needed depends on the length of the fibers in each stage of the delivery system. Under a reasonable set of assumptions, the total fiber length is almost 4 km, costing about $13k. The connections and couplers involved cost about $8500. (Note that some connectors and fiber will be included in the quote from RoMack, which would therefore reduce these fiber and connector costs by about $2100 if we purchased the fiber cluster assembly from them.)

If you have comments or suggestions, email me at pate@nmsu.edu .