Cs-137 Long Term Irradiator:
Overview:
The CMS collaboration is building a large general purpose high-energy physics detector to explore the energy region that will become accessible with colliding beams of 7 TeV protons.
The detector encircles the collision regions and detects the particles that are produce in the high energy collisions that occur at its center. One unavoidable consequence of this collision is the production of ionizing radiation which effects the performance of all the detector components. Since there are many collisions that will occur in the center of the detector due to the high interaction rate, 4x108/second, with each collision producing some 100 ionizing particles, and since the operational lifetime of the detector is planned for at least 10 years, the radiation resistance of the detector components of a major concern in the design and selection of the detector’s components.
The University of Minnesota group under the leadership of Prof. Rusack has the responsibility ensure the radiation hardness of one electronic component the avalanche photodiode which is used extensively in the detector for the detection of small optical signals. We have been operating a program to qualify on a lot-by-lot basis these APD’s by performing destructive tests with a 252Cf source and now we would like to set up a small test stand using a 1.2 Ci 137Cs source for a long-term stability study.
We will dismount the 137Cs source from its current location and incorporate it into a small system that we have designed. In this system the source will be used to continuously irradiate a matrix of 16 APD’s that will be kept under bias (voltage) for an interval of at least three years unless we observe some unforeseen degradation. We will monitor the APDs’ response to both continuous and pulsed (~ 20 nsec) light with a stabilized light pulser. The electrical response of APD’s will be monitored with custom electronics mounted on the same printed-circuit board as the APD’s and we will monitor electronically the response of the APD’s. Cables from the electronics will be fed out through a hole in the top of the volume.
Figure [1] shows the basic setup for the system. In blue is the source holder in which 137Cssource is housed. It is a C-SO-144 housing from High Voltage Engineering. The source is moved in and out of the housing by a rod attached to it. This is shown in green in the figure. The actual source housing that we will use is shown in Figure [2]. This housing will be connected to a lead-filled collimator block shown in green in figure [1]. The APD array and the monitoring electronics will be mounted at one end of the collimator and the source will be moved to the opposite end of the collimator for the irradiation as shown in the figure. The light from the pulser will be brought to the APD’s on quartz fibers mounted on the side of the APD’s opposite from the source (they are not shown in the figure.)
Figure [1] Schematic of the detector irradiator configuration.
Figure [2] Source assembly in its previous mounting location.
Calculated exposure levels.
Our design goal for radiation protection is to bring the radiation levels to be at or below backgrounds. As protection against gammas can be readily achieved with lead, which is relatively inexpensive, our design has been mostly determined by the size of the flange on the source housing.
In the design of the collimator the minimum thickness of lead for direct gammas to the outside is 10 cm (4 inches), except for the direct (collimated) path to the APD’s. The estimated radiation levels are computed using a formula supplied by RPD, which states that the dose rate from a 1.3 Ci 137Cs source in rem/hr at 30 cm is 6 * 1.3 * 0.662 * 0.946 R/hr. The figure 0.662 is the energy of the gammas in MeV and 0.946 is the branching fraction of Cesium-137 to gammas. This leads to a dose rate of 4 .88 Rem/hr at 30 cm. The lead shielding reduces this flux by a factor of two for 6 mm or 0.25” at these energies. Consequently the dose rate is reduced by a factor of 216 or 65,000 to give a calculated exposure rate of 0.07 mRem/hr, a figure significantly below background.
Lead blocks mounted securely behind the APD array will protect the direct pathway from the collimator. These in turn will be placed against a 2m thick concrete wall.
Source protection:
As the primary purpose of this experiment is to confirm that these APD’s do not change their operational characteristics over long periods of time in a low level radiation environment. To avoid ambiguous events which might be caused by operator error or be a real effect, we will want to leave this setup once operational alone without any interference. Therefore we will enclose the whole assembly underneath a locked cover that will be firmly attached to the table. The keys for this enclosure will remain under the control of the RPD. In the event of that an access is required we will make all interventions under the direct supervision of RPD personnel.
The whole assembly will be inside one of the disused target halls of the Williams laboratory, which is E-45. This room is equipped with an interlocked concrete door that will be part of the same interlock and monitoring system that is used for the Californium source which is located in the opposite W-45 room.
The E-45 room will be protected by the same alarm system that is used to protect the Californium source. This alarm is linked directly to the University of Minnesota police.