We have attempted to measure total cross sections (gr's) for positrons and electrons scattered by potassium and rubidium atoms in the energy range 1 -102 eV using a beam-transmission technique. The measurements are subject to increasing errors as the projectile energy is reduced below 20 eV due to the incomplete discrimination of our apparatus against projectiles scattered through small angles. Our measured positron-K and -Rb gr s each show a peak in the vicinity of 6 eV and decrease substantially as the positron energy is reduced below 6 eV. The observed peaks may be artifacts due to the incompletediscrimination problems referred to above, but careful consideration of those problems does not resolve the marked discrepancies between our low-energy positron-Rb results and those obtained in recent fivestate close-coupling-approximation calculations.PACS number(s): 34.80.i, 34.90.+ q

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Aortic arch injuries following penetrating trauma are typically lethal events with high mortality rates. Traditionally, the standard of care for patients presenting with penetrating injury and aortic involvement has included surgical intervention. We report the case of a 31-year-old man who was managed non-operatively after sustaining multiple stab wounds to the left chest and presenting with mid aortic arch injury. Historically, penetrating injury to the thoracic aorta has been associated with mortality rates greater than 90% and it has one of the poorest prognoses of all penetrating injuries. 1 The majority of patients die at the scene and those who do arrive at hospital are usually in critical condition.Despite advances in trauma management and the development of dedicated trauma centres, morbidity and mortality rates have remained relatively unchanged in patients with penetrating aortic injury (PAI).1 The incidence of penetrating trauma to the aortic arch is not well documented as most patients exsanguinate before receiving treatment. 2 In an autopsy study from Athens, the authors found that traumatic aortic injury was responsible for 12.7% of all injury related fatalities and penetrating trauma was responsible for 13.6% of those deaths. 2 Also reported was the finding that the aortic arch along with the ascending aorta were the most common site of thoracic aortic injury following stab wounds.In patients who present with PAI, operative management is the norm and currently, there are no recommendations for conservative management. This is opposed to blunt aortic injury (BAI), where non-operative management has been explored and guidelines have been proposed by the Society for Vascular Surgery. 3 We present the case of a patient who was managed conservatively following an aortic arch injury secondary to penetrating trauma with strict blood pressure control and outpatient surveillance. Case HistoryA 31-year-old man received via the emergency medical services presented to our emergency department (ED) shortly after sustaining multiple stab wounds to the left anterior chest, left axilla and left upper extremity. En route, the patient's blood pressure was 112/70mmHg and he had a pulse rate of 80bpm. He was diaphoretic at the scene, which improved on arrival to the ED. A standard Advanced Trauma Life Support ® approach was initiated immediately in the trauma bay.The patient was sent for computed tomography (CT) of the chest with contrast in a stable condition. The CT revealed a left upper lobe laceration, a large left haemothorax, a small left pneumothorax and extensive haemorrhages in the mediastinum surrounding the aortic arch with a hint of tiny extrusion at the aortic arch but no obvious source of bleeding. A chest tube placed into the left haemothorax put out 1,800ml serosanguinous drainage in the first 24 hours. The subsequent CT angiography (CTA) revealed a nipple of contrast at the mid aortic arch. A mid aortic arch injury could not be distinguished on the CTA from a laceration of the arterial/ veno...

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